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Downward flow of surface-derived water deep into the upper crust is investigated using 2-D coupled hydrothermal numerical models. In the models, downward flow is driven by either topographic gradients or seismic pumping, while it is facilitated by large episodic variations in fault permeability, intended to mimic fracturing and healing on a fault over repeated seismic cycles. The models show that both forcing scenarios are equally capable of driving surface-derived fluid to the base of faults at 10 km depth in several tens of thousands of years under certain conditions. Downward flow of cold fluid occurs almost exclusively during and shortly after earthquakes, while during the remaining portion of the seismic cycle fluids remain relatively stationary while they undergo thermal relaxation (i.e. heating). Rapid downward flow is favoured by a large coseismic permeability, long permeability healing timescale and large coseismic dilatancy or high topographic relief above the fault at the surface. However, downward fluid flow is completely inhibited if fluid pressures exceeds the hydrostatic gradient, even by modest amounts, which suggests that deep fluid infiltration is unlikely to occur in every region. [ABSTRACT FROM AUTHOR]

SUMMARY: For accurate modelling of groundwater flow and transport processes within an aquifer, precise knowledge about hydraulic conductivity K and its small-scale heterogeneities is fundamental. Methods based on pumping tests, such as hydraulic tomography (HT), allow for retrieving reliable K-estimates, but are limited in their ability to image structural features with high resolution, since the data from time-consuming hydraulic tests are commonly sparse. In contrast, geophysical methods like induced polarization (IP) can potentially yield structural images of much higher resolution, but depend on empirical petrophysical laws that may introduce significant uncertainties to the K-estimation. Therefore, this paper presents a joint inversion procedure for both HT and IP data, which allows for combining the complementary abilities of both methods. Within this approach, a traveltime inversion is applied to the HT data, while the IP inversion is based on a full-decay time-domain forward response, as well as a reparametrization of the Cole–Cole model to invert for K directly. The joint inversion is tested on a synthetic model mimicking horizontally layered sediments, and the results are compared with the individual HT and IP inversions. It is shown that jointly inverting both data sets consistently improves the results by combining the complementary sensitivities of the two methods, and that the inversion is more robust against changes in the experimental setups. Furthermore, we illustrate how a joint inversion approach can correct biases within the petrophysical laws by including reliable K-information from hydraulic tests and still preserving the high-resolution structural information from IP. The different inversion results are compared based on the structural similarity index (SSIM), which underlines the robustness of the joint inversion compared to using the data individually. Hence, the combined application of HT and IP within field surveys and a subsequent joint inversion of both data sets may improve our understanding of hydraulically relevant subsurface structures, and thus the reliability of groundwater modelling results. [ABSTRACT FROM AUTHOR]

The dam of Lampy (Black Mountain, Aude, France) is considered as one of the oldest dams in France. A geophysical survey is performed to better understand the pattern of groundwater flow downstream of this dam in the granitic substratum. Induced polarization is first used to image both electrical conductivity and normalized chargeability. Eight core samples of granite from this site are measured and analysed in the laboratory. Their electrical conductivity and normalized chargeability are expressed as a function of the porosity and cation exchange capacity (CEC). The field data and the petrophysical results are used to image the water content, the CEC and the permeability distribution of the substratum. Then, self-potential is used as a complementary passive geophysical technique, which, in absence of metallic bodies, is directly sensitive to groundwater flow through the so-called streaming potential effect. Indeed, the excess of electrical charges in the vicinity of the solid grains, in the so-called double layer, is dragged by the ground water flow generating in turn an electrical (streaming) current and therefore an electrical field. A map of the resulting self-potential signals is done over the area covered by the induced polarization profiles. This map shows a large positive anomaly with an amplitude of ∼80 mV possibly associated with upwelling groundwater in an area where the soil is water-saturated. A groundwater flow simulation is performed to model this anomaly. This is done in two steps. A preliminary groundwater flow model is built using the permeability and water content distributions obtained from the induced polarization data. Then, this groundwater flow model is updated using the information contained in the self-potential data including the electrical conductivity distribution obtained through resistivity tomography. The algorithm for the inversion of the self-potential data is validated through a 2-D numerical test. This analysis yields a groundwater flow model with the flow being focused through a high permeability zone. This study shows how three geoelectrical methods (self-potential, induced polarization and electrical resistivity) can be efficiently combined to image groundwater flow in the vicinity of a dam. [ABSTRACT FROM AUTHOR]

Faults in the upper crust are sometimes thought to act as self-sealing valves, episodically releasing highly overpressured fluids trapped at greater depth during earthquakes. They are also often considered to be capable of actively pumping fluids into or out of faults in response to coseismic volumetric strain, thermal pressurization or other mechanisms. In this study, we investigate how these different types of behaviour (i.e. valve versus pump) are manifested in earthquakes. We do this using a 2-D plane strain model where frictional sliding on a thrust fault that is fed by a fluid source at its base is coupled to porous flow, thermal pressurization and strong variations in permeability. Our results show that thermal pressurization leads to dramatic dynamic weakening that produces earthquakes that propagate as slip pulses producing large stress drop, large slip and high slip velocities. On the other hand, valve-type behaviour typically produces smaller, less energetic earthquakes that commonly arrest before rupturing the entire fault. In some valve models, we observe complex compound ruptures and swarm activity, which is linked to the ascent of a propagating fluid pressure pulse driven by a large increase in permeability during sliding. Both pump and valve mechanisms can produce anomalously weak faults, though they are each associated with distinctly different fluid pressure and strength evolution over the seismic cycle and during rupture. Our models highlight the complex way in which fluids may interact with earthquakes, especially if valve and pump models coexist. [ABSTRACT FROM AUTHOR]

Understanding the shear-induced dilatancy of rock fractures is important for assessing the permeability evolution and seismic hazard in shale and geothermal reservoirs. The displacement dependence of fracture dilation has been well studied, while the influence of slip velocity is poorly constrained. In this study, we combined displacement- and velocity-dependent aperture models to reproduce the transient shear-induced dilatancy of fractures in sandstone in 16 normal stress unloading tests. Our results show that the combined aperture model can describe the transient fracture aperture evolution during accelerating slip induced by normal stress unloading better than the model dependent only on slip displacement. Slip velocity could enhance the aperture increase on smoother fractures at lower normal stresses and higher slip velocities. Both the dilation factor and characteristic slip distance decrease with increasing normal stress and surface roughness, signifying reduced contribution of slip velocity to transient shear-induced dilatancy at higher normal stresses and surface roughness. The dilation angle increases with the increase of surface roughness, and this increase diminishes at higher normal stresses primarily attributable to more severe asperity wear. These findings highlight the importance of slip velocity in controlling the transient evolution of aperture and permeability of a rock fracture. Our study also provides constraints on the constitutive parameters in the combined aperture model for describing transient shear-induced fracture dilatancy. We suggest that it is crucial to incorporate the velocity-dependent aperture model to simulate the nonlinear evolution of fracture aperture in future analytical and numerical models involving coupled hydromechanical processes in geoenergy systems. [ABSTRACT FROM AUTHOR]

In petrophysics, physical rock properties are typically established through laboratory measurements of individual samples. These measurements predominantly relate to the specific sample and can be challenging to associate with the rock as a whole since the physical attributes are heavily reliant on the microstructure, which can vary significantly in different areas. Thus, the obtained values have limited applicability to the entirety of the original rock mass. To examine the dependence of petrophysical measurements based on the variable microstructure, we generate sets of random 2-D microstructure representations for a sample, taking into account macroscopic parameters such as porosity and mean grain size. For each microstructure produced, we assess the electrical conductivity and evaluate how it is dependent on the microstructure's variability. The developed workflow including microstructure modelling, finite-element simulation of electrical conductivity as well as statistical and petrophysical evaluation of the results is presented. We show that the methodology can adequately mimic the physical behaviour of real rocks, showing consistent emulation of the dependence of electrical conductivity on connected porosity according to Archie's law across different types of pore space (microfracture, intergranular and vuggy, oomoldic pore space). Furthermore, properties such as the internal surface area and its fractal dimension as well as the electrical tortuosity are accessible for the random microstructures and show reasonable behaviour. Finally, the possibilities, challenges and meshing strategies for extending the methodology to 3-D microstructures are discussed. [ABSTRACT FROM AUTHOR]

The effects of pore fluids on the elastic properties of sedimentary rocks can be broadly categorized into two mechanisms: variations in pore compressibility due to the physical properties of the fluids and alterations in the stiffness of the rock frame resulting from rock–fluid interactions. Particularly, as rock–fluid interactions alter the stiffness of contacts between mineral grains, changes in the fluid properties around grain contacts can induce volumetric deformation of the rock as well as in variations of the associated elastic coefficients. Even though many previous studies have explored the influence of swelling of clay minerals, the relationships between changes in elastic moduli and deformation in rocks, which hardly contain swellable minerals, remain to date enigmatic. In this paper, to evaluate quantitatively these effects, drying rates, strains and ultrasonic velocities of small cylindrical Berea sandstone samples were measured as their water saturation was decreased by evaporative drying. The measurements clearly showed drying shrinkage and drastic increases in shear and compressional moduli for all the samples under an almost fully dried condition. Previous studies considered that the alteration in surface energy of grain minerals between wet and dry states affected the contact stiffness between them. Some of them used micromechanical model, uniting the Digby grain contact model with the effective medium theory, to interpret the changes in elastic moduli observed in the non-swellable sandstones during water adsorption on the grain surface. The conventional micromechanical model assumes that a mineral grain is a pure sphere and that the number of contacts between the grains is one. However, grains in a sedimentary rock are generally not purely spherical, and the contact surface is composed of several adhesive asperities. We therefore modified the conventional model by introducing the curvature radius and the number of asperities per contact surface. The modified model well reproduced the shear moduli under wet conditions using the strain and moduli measured under dry conditions. On the other hand, the predictions of the compressional moduli using the model were partially in agreement with the experimental results. Therefore, we attempted to qualitatively interpret the relationship by combining the model with the viscoelastic effect associated with wave-induced fluid flow. The deformation and changes in the elastic moduli of rocks resulting from multiple pore fluids within them can be better understood by the present combined model. [ABSTRACT FROM AUTHOR]

In this work, we revisit the seminal concept of Johnson–Koplik–Schwartz (JKS) length Λ, that is a characteristic length representing an effective pore size which controls various transport-related properties of porous media, such as, the permeability and the electrical conductivity. We present a novel closed-form equation that predicts the behaviour of Λ in partially saturated media, for different saturation states. Using previous models in the literature that predict the intrinsic and relative electrical conductivities under partially saturated conditions, we infer the JKS length Λ and the electrical formation factor F as functions of water saturation and properties associated with the pore-size distribution of the probed porous medium. The proposed method permits to estimate the effective permeability and the relative permeability directly from electrical conductivity measurements, thus opening new-avenues for the remote characterization of partially saturated media. We believe that this new model will prove useful for various characterization and modelling applications from reservoir (CO2 or hydrogen storage) to vadose zone studies. [ABSTRACT FROM AUTHOR]

Earthquakes in fluid-saturated rocks induce sudden changes in pore-pressures that can lead to appreciable postseismic deformation. However, earthquakes can also release fluids from overpressured parts of the crust, which can also contribute to additional postseismic effects. In this study, we use two-dimensional poroelastic models to investigate postseismic deformation and fluid pressures following ruptures on overpressured dip-slip faults that slide according to rate- and state-dependent friction. We show that when the crust prior to rupture has pore pressures in hydrostatic equilibrium, then ruptures induce fluid pressure anomalies according to the field of coseismic elastic volumetric strain. These pore pressure anomalies relax with time to produce characteristic postseismic deformation that depends on the slip magnitude and fault style, as typically observed. However, very different results are obtained if the crust is appreciably overpressured at the time an earthquake is nucleated. In this case, the poroelastic response is overwhelmingly dominated by drainage and fluid pressure drop within the overpressured crust, which is facilitated by an increase in permeability on the fault during rupture. This results in transient postseismic subsidence that occurs irrespective of the faulting style, fault geometry, slip magnitude or details of the coseismic strain field. The time scale and magnitude of this postseismic subsidence are controlled by the magnitude of the fluid overpressure prior to rupture, the maximum coseismic permeability and how rapidly the fault permeability recovers following an earthquake. The poroelastic effects associated with rupturing of overpressured faults may be at least an order of magnitude greater that the classic poroelastic response (i.e., relaxation of pore pressures generated by coseismic elastic volumetric strain). Overall, our results highlight the potentially dramatic effect that the breaching of deep overpressured fluid reservoirs can have on fluid pressures and surface displacements following earthquakes.

We perform seismic and ultrasonic measurements in carbonate and shaley sandstone samples as a function of differential pressure. The velocities show a strong frequency and pressure dependence. The dispersion disappears with increasing pressure and the squirt flow in turn inhibits the pressure dependence. To model these effects, we combine the Gurevich's squirt-flow model with the Mori–Tanaka scheme and the David Zimmerman model, and extend it with third-order elastic constants, to obtain a frequency-dependent acoustoelasticity model. Comparisons between measurements from this study and literature and modelling results show that the P -wave velocity increases non-linearly first and then nearly linearly, dominated by crack closure and acoustoelasticity, respectively. The pressure dependence of wave velocities is reduced by liquid substitution and further by the squirt-flow mechanism. The effects of fluid properties and crack closure on P -wave velocity decrease with differential pressure. The results will feed a new model and help better understanding the wave propagation in pre-stressed rocks at different scales. [ABSTRACT FROM AUTHOR]

In this paper, we propose a fully coupled two-phase poroelastic deformation theory for a spherically layered and self-gravitating Earth. The earth model consists of a solid inner core, a fluid outer core and poroelastic mantle and crust. The boundary-value problems are posed in the Laplace-transformed domain using the spherical system of vector functions, and analytical solutions are obtained in each layer using the dual-variable and position matrix method. As an application, the surface loading problem is considered. The undrained and drained limits are discussed with the corresponding governing equations, which are different from the conventional ones where body force is ignored. Numerical examples show that the poroelastic effect can be significant since the difference between the results for undrained and drained limits are large with obvious time-dependent deformation. This newly derived theory for the coupled boundary-value problem will have broad applications, such as displacement and gravity change due to groundwater depletion, poroelastic rebound after an earthquake and risk evaluation of earthquakes induced by water injection. [ABSTRACT FROM AUTHOR]

Dissolution and precipitation processes modify the structure of the porous media at microscale which significantly affects the macroscopic properties of the media. These variations in the pore geometry result in changes in the hydraulic properties that control the groundwater flow, and also modify the electrokinetic properties associated to the displacement of electrical charges carried by the flow which originates the streaming potential. Under the hypothesis of a uniform dissolution or precipitation of the pores and based on the effective excess charge density approach, we present a physically based theoretical model for estimating the effective excess charge density as a function of time. The model is based on the assumption that the pore structure can be represented by an ensemble of capillary tubes with a smooth periodic variation of their radius and a fractal pore size distribution. The analytical expressions obtained to describe the effective excess charge density depend on the chemical parameters of the fluid and the petrophysical properties of the medium. In addition, the periodic variations assumed in the pore geometry represent a more realistic description of a porous medium than considering the pores as constant radii capillaries. These irregularities allow us to include the hysteresis phenomenon in the electrokinetic properties. The expressions of the proposed model have been tested with experimental data consisting of sets of effective excess charge density-effective saturation, permeability-effective saturation, porosity-time and permeability-time values. In all cases, the model is able to satisfactorily reproduce the behaviour of the data. [ABSTRACT FROM AUTHOR]

Temporal variations in the electromagnetic field generated by seismic waves are accounted for mainly by electrokinetic (EK) and motional induction (MI) effects. Currently, these effects are considered in terms of different sets of equations, even though both mechanisms are assumed in data interpretation. In principle, however, they should be described by a single set of equations, possibly including a new type of coupling effect. This inconsistency suggests that the deviation of conventional equations was somewhat incorrect, with some coupling mechanisms having been overlooked. Here, the derivation of the conventional equations was reconsidered, and necessary revisions were applied. The revised equations describe both EK and MI effects and include a previously unrecognized coupling effect (although its effect is negligible). The solution of the new equations is well approximated by a simple sum of EK and MI effects determined by conventional equations. In conclusion, it was confirmed that the conventional procedure for estimation of seismically induced EM variations in terms of the EK and MI effects is sufficiently accurate, in spite of the conventional governing equations missing some of coupling terms related to the EK and MI effects. [ABSTRACT FROM AUTHOR]

Wave propagation in anelastic rocks is a relevant scientific topic in basic research with applications in exploration geophysics. The classical Biot theory laid the foundation for wave propagation in porous media composed of a solid frame and a saturating fluid, whose constitutive relations are linear. However, reservoir rocks may have a high-viscosity fluid, which exhibits a non-Newtonian (nN) behaviour. We develop a poroelasticity theory, where the fluid stress-strain relation is described with a Kelvin–Voigt mechanical model, thus incorporating viscoelasticity. First, we obtain the differential equations from first principles by defining the strain and kinetic energies and the dissipation function. We perform a plane-wave analysis to obtain the wave velocity and attenuation. The validity of the theory is demonstrated with three examples, namely, considering a porous solid saturated with a nN pore fluid, a nN fluid containing solid inclusions and a pure nN fluid. The analysis shows that the fluid may cause a negative velocity dispersion of the fast P (S)-wave velocities, that is velocity decreases with frequency. In acoustics, velocity increases with frequency (anomalous dispersion in optics). Furthermore, the fluid viscoelasticity has not a relevant effect on the wave responses observed in conventional field and laboratory tests. A comparison with previous theories supports the validity of the theory, which is useful to analyse wave propagation in a porous medium saturated with a fluid of high viscosity. [ABSTRACT FROM AUTHOR]

Porous rocks have long been the focus of intense research driven by their importance in our society as host to our most essential resources (oil, gas, water, geothermal energy, etc), yet their rheology remains poorly understood. With increasing depth, porous rocks transition from being brittle (dilational deformation leading to localized failure) to being ductile (homogeneous compactive flow, no failure). The transition between these two regimes is crucial for reservoir engineering. In fact, brittle, localized deformation of porous rocks is generally accompanied by permeability enhancement but also induced seismicity, while ductile deformation leads to aseismic permeability reduction. Decades of experimental work has shown that this transition is not sharp but rather spans a wide P, T domain, but to this day, no clear boundaries have been established. Here, we subjected pre-faulted samples of Volvic trachyandesite to increasing confining pressure, deforming the samples each pressure step and recording strain partitioning between off-fault bulk deformation and on-fault slip. For the first time, we show that the localized–ductile transition (LDT) in porous rocks is bound by the stress criterion σ y  <   σ f  <   σ flow. Additionally we show that, in this regime, once both fault sliding and bulk flow are active, the partitioning of strain between the two can be described by the empirical ratio: |$(\sigma _\mathrm{f}-\sigma _\mathrm{y})/(\sigma _\mathrm{flow}-\sigma _\mathrm{y}).$| Finally, we propose a critical stress representation that takes into account the existence of the LDT in porous rocks. [ABSTRACT FROM AUTHOR]

PyLith, a community, open-source code for modelling quasi-static and dynamic crustal deformation with an emphasis on earthquake faulting, has recently been updated with a flexible multiphysics implementation. We demonstrate the versatility of the multiphysics implementation by extending the code to model fully coupled continuum poromechanics. We verify the newly incorporated physics using standard benchmarks for a porous medium saturated with a slightly compressible fluid. The benchmarks include the 1-D consolidation problem as outlined by Terzaghi, Mandel's problem for the 2-D case, and Cryer's problem for the 3-D case. All three benchmarks have been added to the PyLith continuous integration test suite. We compare the closed form analytical solution for each benchmark against solutions generated by our updated code, and lastly, demonstrate that the poroelastic material formulation may be used alongside the existing fault implementation in PyLith. [ABSTRACT FROM AUTHOR]

A new set of complex dynamic localized response coefficients (LRC-NEW), related to the fast P waves, slow P waves, S waves, and electromagnetic (EM) waves, respectively, is derived to overcome the numerical inaccuracy occurring while computing the seismoelectric (SE) wave fields in stratified low-permeability porous media based on the reflectivity method. In addition, the dynamic electrokinetic coupling coefficient is modified to make it applicable for the electrical double layer of arbitrary thickness. After a thorough validation, we find that the EM waves simulated by using the known LRC expressions differ significantly with the analytical solutions when the static permeability is below 10–15 m2, while those simulated by using LRC-NEW achieve excellent fits with the analytical solutions even when the static permeability is as low as 10–20 m2. A sensitivity study demonstrates that an optimum static permeability exists where the amplitude of evanescent SE responses reaches its maximum. The normalized root-mean-square amplitude of the evanescent SE signals in low-permeability rocks such as shales is more sensitive to permeability than seismic waves, even when the interdependence of porosity and static permeability is considered. Sensitivity studies also manifest that evanescent SE responses are sensitive to salinity and water saturation even in low-permeability shales. Notably, the amplitude of the evanescent SE conversions increases monotonically with water saturation, which is much simpler than the relationship between the seismic amplitude and water saturation. Therefore, the SE method has the potential to characterize the permeability, salinity, and water saturation of low-permeability reservoirs. [ABSTRACT FROM AUTHOR]

Plenty of studies have suggested that pore fluid may play an important role in earthquake rupture processes. Establishing numerical models can provide great insight into how pore fluid may affect earthquake rupture processes. However, numerical simulation of 3-D spontaneous ruptures in poroelastic mediums is still a challenging task. In this paper, it is found that a closed-form time-domain Green's function of Biot's poroelastodynamic model can be constructed when the source frequency and source-field distance are within a certain range. The time-domain Green's function is validated by being transformed into the frequency domain and comparing with the frequency-domain Green's functions obtained by former papers. Poroelastic wave propagation phase diagrams for various two-phase poroelastic mediums are then plotted to show the applicable range of frequency and source-field distance for the new time-domain Green's function. It is shown that the applicable range not only include the frequency and spatial range of concern in seismology but also overlap that in acoustics. Based on the time-domain Green's function, the boundary integral equations (BIEs) for modelling dynamic ruptures in elastic mediums are extended to fluid-saturated mediums. In the meantime, a functional relationship between the effective stress tensor and the total stress tensor in fluid-saturated mediums is also obtained, which allows us to directly obtain the effective stress by BIEs. The spontaneous rupture processes controlled by the slip-weakening friction law on faults in elastic mediums and in fluid-saturated mediums are compared. It is found that under the same conditions, fluid-saturated rocks are more prone to supershear rupture than dry rocks. This result suggests that pore fluid may promote the excitation of supershear rupture. The poroelastic wave propagation phase diagrams also suggest that simulating a coseismic phase in the real scale requires a certain sample length in laboratories. They also suggest that an undrained governing equation is suitable for seismic wave propagation simulation in poroelastic media. [ABSTRACT FROM AUTHOR]

The petrophysical properties of 41 volcanic samples from La Soufrière volcanoe (Guadeloupe Island, Eastern Caribbean, France) are investigated. We first measure the complex conductivity spectra of these rock samples at 4 salinities (NaCl) at laboratory conditions (∼20 °C). For each rock sample, we determine the (intrinsic) formation factor, the surface conductivity and the Cole–Cole normalized chargeability. We also measure the compressional wave velocity (dry and saturated), the shear wave velocity in saturated conditions, the (dry and saturated) thermal conductivity, the dry specific heat capacity and the permeability of the rock samples as well as their cation exchange capacity (CEC) and connected porosity. The formation factor versus porosity obeys Archie's law with a cementation exponent of 2.16 ± 0.10. The surface conductivity and the normalized chargeability are proportional to each other and to the CEC divided by the tortuosity of the material (product of the formation factor by the connected porosity) as predicted by the dynamic Stern layer model. Permeability can be predicted from the normalized chargeability and the formation factor inside one order of magnitude. The thermal conductivity and the seismic properties can be evaluated from the connected porosity of the core samples formation factors. A non-linear relationship is established between the shear wave velocity and the compressional wave velocity for the present data set and other data from the literature. Finally, we show on a specific example, how to convert an induced polarization survey on a stratovolcano into a seismic velocity model (P - and S -waves velocity distributions). We perform a specific application to Papandayan Volcano, a stratovolcano located in Java Island (Indonesia). This work paves the way to the joint inversion problem of seismic and induced polarization surveys for volcanic unrest monitoring. [ABSTRACT FROM AUTHOR]

The dynamics of accretionary prisms and the processes that take place along subduction interfaces are controlled, in part, by the porosity and fluid overpressure of both the forearc wedge and the sediments transported to the system by the subducting plate. The Hikurangi Margin, located offshore the North Island of New Zealand, is a particularly relevant area to investigate the interplay between the consolidation state of incoming plate sediments, dewatering and fluid flow in the accretionary wedge and observed geodetic coupling and megathrust slip behaviour along the plate interface. In its short geographic extent, the margin hosts a diversity of properties that impact subduction processes and that transition from north to south. Its southernmost limit is characterized by frontal accretion, thick sediment subduction, the absence of seafloor roughness, strong interseismic coupling and deep slow slip events. Here we use seafloor magnetotelluric (MT) and controlled-source electromagnetic (CSEM) data collected along a profile through the southern Hikurangi Margin to image the electrical resistivity of the forearc and incoming plate. Resistive anomalies in the shallow forearc likely indicate the presence of gas hydrates, and we relate deeper forerarc resistors to thrust faulting imaged in colocated seismic reflection data. Because MT and CSEM data are highly sensitive to fluid phases in the pore spaces of seafloor sediments and oceanic crust, we convert resistivity to porosity to obtain a representation of fluid distribution along the profile. We show that porosity predicted by the resistivity data can be well fit by an exponential sediment compaction model. By removing this compaction trend from the porosity model, we are able to evaluate the second-order, lateral changes in porosity, an approach that can be applied to EM data sets from other sedimentary basins. Using this porosity anomaly model, we examine the consolidation state of the incoming plate and accretionary wedge sediments. A decrease in porosity observed in the sediments approaching the trench suggests that a protothrust zone is developing ∼25 km seaward of the frontal thrust. Our data also imply that sediments deeper in the accretionary wedge are slightly underconsolidated, which may indicate incomplete drainage and elevated fluid overpressures of the deep wedge. [ABSTRACT FROM AUTHOR]

Impermeable barriers are abundant in rock formations and their delineation via seismic reflections are of prime importance. However, such barriers have received little attention in the poroelasticity literature. For this reason, we revisit the reflection–transmission problem for two porous half-spaces which are in welded contact and across which no fluid exchange is possible. We carry out this analysis in the poroelastic theory initiated by de la Cruz and Spanos (1985), which describes four waves: the fast P , fast S -, slow P and slow S waves. The fast P and S waves account for the in-phase motion in compression and shear nature, whereas the slow P and S waves are the out-of-phase motion. We find that the often times suggested notion of effective elastic scattering is not quite correct because of the generation of slow waves at the impermeable contact. The energy flux the slow waves carry is negligibly small at low frequencies when the fluid is viscously coupled to the solid frame. In contrast, slow waves carry substantial energy flux at high frequencies when the fluid is viscously decoupled. Moreover, the reflection–transmission behaviour at an impermeable contact is distinctively different when compared to that of a permeable contact, which means that, in principle, amplitudes of scattered waves can be used to identify the kind of contact. A seemingly unnoted but important result is the strong conversion scattering from a fast S wave into the slow P wave at high frequencies, which may be an alternative way to observe slow P waves in the laboratory as well as a means to characterize impermeable contacts. [ABSTRACT FROM AUTHOR]

Determining the properties that control fluid flow and pressure migration through rocks is essential for understanding groundwater, energy reservoirs and fault zones. However, direct measurements of these properties underground generally require expensive and invasive processes such as pumping large volumes of water in or out of the ground. These conventional methods may change the structure that they are trying to measure and do not resolve variations in space without complex, multiple experiments. Here, we capitalize on the existing, natural fluctuations of pore pressure, flow and temperature that are always present underground, likely driven by tides, seismic waves and other noise in the Earth. We develop the theoretical framework to utilize these noises to determine hydrogeologic structure. We begin with prior work demonstrating a connection between cross-correlation and Green's functions for diffusive processes and illustrate how the existing theory could be used specifically for the case of inferring hydraulic diffusivity from closely spaced observations of pore pressure. We extend the current theory to include a capability for inferring Green's functions from closely spaced flow rate measurements, rather than pore pressure data. We then note that closely spaced temperature measurements are much more practical as an observational system, however, diffusive transport is likely not the dominant cause of high-frequency fluctuations of temperature in the Earth. Advective transport is a more plausible controlling factor and thus we need to incorporate advection into the theory and model the corresponding response functions. We therefore develop a semi-analytical method that includes advection and numerically compute the corresponding response function. We use these synthetics to illustrate how cross-correlation of ambient noise in subsurface temperature measurements carries information about hydraulic structure and show that it is possible to constrain hydraulic diffusivity values from cross-correlation of closely spaced continuous temperature measurements. This new method opens up the possibility of passive determination of reservoir properties with high spatial resolution from closely spaced, continuous temperature data, which is a realistic deployment strategy that could be used in a wide variety of settings. [ABSTRACT FROM AUTHOR]

SUMMARY: Geophysical exploration technologies are developed not only to detect and quantify the geofluid in a reservoir but also with the expectation to identify the reservoir's permeability that determines its exploitability. However, the knowledge about whether there is a geophysically invertible pore parameter that determines the permeability of a rock, the key to the geophysical identification of permeability, remains poorly understood. We address this knowledge gap by establishing, through dedicated laboratory measurements and theoretical modelling, the correlations between permeability and the porosity and aspect ratios of the dual-pore structure in artificial sandstones with approximately the same porosity but with different permeability. Permeability is found to increase with increasing compliant porosity but show no apparent relations with the dual-pore aspect ratios, indicating compliant porosity is the key parameter determining the permeability of rocks with the same porosity. The results reveal the key factor that influences permeability in sandstones with the same porosity, and provide a potential way to simply identify the permeability to indicate the exploitability of a geofluid reservoir. [ABSTRACT FROM AUTHOR]

SUMMARY: Due to the strong heterogeneity of the reservoir, it is difficult to extend the classical reservoir parameter prediction model to the new work area. Traditional geological methods rely on experience, while machine learning algorithms lack consideration of reservoir heterogeneity. This makes it impossible for these methods to achieve good predictive performance in new work areas. In this paper, we discuss designing a multilayer deep transfer learning network (MDTL). The network enables accurate prediction of reservoir parameters in new areas based on core and logging data from mature areas. For the architectural design, we establish a network structure that realizes data interaction in different work areas. For network parameter optimization, we design a multilayer maximum mean discrepancy loss function. It guides network training to learn feature knowledge about data in different work areas. For logging data processing, we explore a support vector machine method to remove abnormal noise in logging data. We apply MDTL to predict shale gas porosity and total organic carbon content in southern Sichuan. Compared to state-of-the-art reservoir parameter prediction models, MDTL achieves the best performance in new well areas. The network integrates the idea of multilayer transfer learning, reduces the influence of reservoir heterogeneity and effectively ensures the prediction accuracy. [ABSTRACT FROM AUTHOR]

Our limited knowledge of the relationship between changes in the state of an aquifer or reservoir and the corresponding changes in the elastic moduli, that is the rock physics model, hampers the effective use of time-lapse seismic observations for estimating flow properties within the Earth. A central problem is the complicated dependence of the magnitude of time-lapse changes on the saturation, pressure, and temperature changes within an aquifer or reservoir. We describe an inversion methodology for reservoir characterization that uses onset times, the calendar time of the change in seismic attributes, rather than the magnitude of the changes. We find that onset times are much less sensitive than magnitudes to the rock physics model used to relate time-lapse observations to changes in saturation, temperature and fluid pressure. We apply the inversion scheme to observations from daily monitoring of enhanced oil recovery at the Peace River field in Canada. An array of 1492 buried hydrophones record seismic signals from 49 buried sources. Time-shifts for elastic waves traversing the reservoir are extracted from the daily time-lapse cubes. In our analysis 175 images of time-shifts are transformed into a single map of onset times, leading to a substantial reduction in the volume of data. These observations are used in conjunction with bottom hole pressure data to infer the initial conditions prior to the injection, and to update the reservoir permeability model. The combination of a global and local inversion scheme produces a collection of reservoir models that are best described by three clusters. The updated model leads to a nearly 70 percent reduction in seismic data misfit. The final set of solutions successfully predict the observed normalized pressure history during the soak and flow-back into the wells between 82 and 175 days into the cyclic steaming operation.

Natural fractures in shale and sandstone that formed under diagenetic and metamorphic conditions are frequently filled with mineral cement that lack residual fracture porosity visible under the petrographic microscope and that are generally interpreted to be impermeable. Scanning electron microscopy of calcite, dolomite, quartz, and barite fracture cement from low‐permeability, diagenetically altered shale and sandstone formations provides evidence of open and variably connected elongate pores or gaps with apertures of 10–600 nm, referred to as nanoscale grain boundary channels (NGBC). Electron backscatter diffraction of samples prepared using broad ion‐beam milling shows that NGBCs occur along grain boundaries of blocky or columnar fracture cement. Grain boundaries are either faceted or display undulation at the nanometer scale. NGBCs tend to increase in aperture with increasing maximum formation burial temperature, indicating dissolution‐precipitation kinetics influences grain boundary structure. Transmission electron microscopy reveals heterogeneous crystallographic domains with possible amorphous regions bridging across grain boundary channels. We propose a model of dynamic concurrent dissolution‐precipitation along grain boundaries that preserves NGBCs in carbonate and quartz fracture cements that have experienced diagenetic to low‐grade metamorphic conditions. While partially healed, NGBCs may be sufficiently connected to increase permeability of low‐permeability formations containing cemented fractures. Plain Language Summary: Using high‐resolution electron images, we observe pores at the nanometer scale in minerals that fill naturally occurring fractures in deeply buried shale and sandstone formations. These pores are below the resolution of conventional optical microscopes and were not observed in earlier studies. Mineral‐filled fractures were therefore considered completely occluded with mineral cement and impermeable to naturally migrating fluids in the Earth's crust, in particular oil and gas. We found these nanoscale pore structures occur in all the samples we analyzed, regardless of the composition of the fracture‐filling mineral cement present in them (calcite, dolomite, quartz, barite), and the depth at which the fractures occurred in the crust. We inferred that these nano‐scale pore structures were caused by the concurrent precipitation and dissolution of minerals along the surface of adjacent mineral grains, thus preventing the complete occlusion of the grain boundaries. These pores partially explain the general observation that rock formations in deep wells have frequently greater permeability than the values predicted from laboratory measurements of rock samples collected from these boreholes. Key Points: Nanoscale grain boundary channels (NGBC) are ubiquitous in fracture cement preserving residual porosity in otherwise fully cemented fracturesFormation of NGBC is attributed to dynamic solution‐precipitation of grain‐bridging mineral domainsNGBC may create preferred flow pathways in low‐permeability formations [ABSTRACT FROM AUTHOR]

During the excavation of tunnel construction, there are often delays or threats to personal safety due to water inflow. Detecting permeability distribution in front of a tunnel face in advance can provide a basis for preventing water inflow, thereby ensuring safety in tunnel construction. Based on the complex petrophysical relationship between the geoelectric properties and the permeability, we have compared the existing induced polarization (IP) models for permeability prediction. The relaxation time, which is one of the induced polarization parameters, is the most critical parameter for predicting the permeability coefficient based on the induced polarization. Many researchers have proposed predictive models applicable to surface hydrological investigations; however, the tunnel environment is impacted by excavation and tectonic movements, and the permeability coefficient varies over a wider range than the surface strata. Therefore, we selected 50 artificial sandstone samples with the permeability varying over a wide range for common tunnels to measure the IP attenuation curve, and we obtained multiple IP parameters, such as the relaxation time and chargeability, to construct a predictive model based on induced polarization parameters suitable for the tunnel environment. The nature of the correlation between IP parameters and permeability is that both are affected by pore structure and fluid ion concentration. Considering this, we introduced formation factors and ion concentrations into the prediction model to improve prediction accuracy. [ABSTRACT FROM AUTHOR]

In this study, seismic inversion is carried out using a genetic algorithm (GA) as well as a hybrid genetic algorithm (HGA) approach to optimize the objective function designed for the inversion. An HGA is a two steps coupled process, where a local optimization algorithm is applied to the best model obtained from each generation of the GA. The study aims to compare the qualitative as well as the quantitative performance of both methods to delineate the reservoir zone from the non-reservoir zone. Initially, the developed algorithm is tested on synthetic data followed by its application to real data. It is found that the HGA for synthetic data is providing more accurate and high-resolution subsurface information as compared with the conventional GA although the time taken later is less as compared with the former methods. The application to real data also shows very high-resolution subsurface acoustic impedance information. The interpretation of the impedance section shows a low impedance anomaly zone at (1055–1070) ms time interval with impedance ranging from (7500 to 9500) m s−1*g cc−1. The correlation between seismic and well data shows that the low impedance zone is characterized as a clastic glauconitic sand channel (reservoir zone). In seismic inversion using an HGA, one can delineate the areal extent of the reservoir zone from the non-reservoir zone more specifically as compared to the GA-derived impedance. The convergence time of HGA is 4.4 per cent more than GA and can be even more for larger seismic reflection data sets. Further, for a more detailed analysis of the reservoir zone and to cross-validate inverted results, an artificial neural network (ANN) is applied to data, and porosity volume is predicted. The analysis shows that the low impedance zone interpreted in inversion results are correlating with the high porosity zone found in ANN methods and confirm the presence of the glauconitic sand channel. This study is important in the aspect of qualitative as well as quantitative comparison of the performance of the GA and HGA to delineate sand channels. [ABSTRACT FROM AUTHOR]

Full-waveform inversion (FWI) is considered as a high-resolution imaging technique to recover the geophysical parameters of the elastic subsurface from the entire content of the seismic signals. However, the subsurface material properties are less well estimated with elastic constraints, especially for the near-surface structure, which usually contains fluid contents. Since Biot theory has provided a framework to describe seismic wave propagations in the poroelastic media, in this work, we propose an algorithm for the 2-D time-domain (TD) poroelastic FWI (PFWI) when the fluid-saturated poroelastic equations are applied to carve the physical mechanism in the shallow subsurface. To detect the contribution of the poroelastic parameters to shallow seismic wavefields, the scattered P - SV & SH wavefields corresponding to a single model parameter are derived explicitly by Born approximation and shown numerically afterward. The Fréchet kernels are also derived and exhibited in P - SV & SH schemes to analyse the sensitivities of the objective function to different poroelastic parameters. Furthermore, we verify the accuracy of the derivations through model parameter reconstructions. We perform a series of numerical tests on gradients with respect to different model parameters to further evaluate inter-parameter trade-offs. PFWI holds potential possibilities to directly invert fluid-related physical parameters of the shallow subsurface. [ABSTRACT FROM AUTHOR]

To improve our understanding of wave dispersion and adsorption signature of organic shales, we select a set of vertical shale samples cored from different formations and conduct the forced-oscillation measurements in seismic frequencies of 2–800 Hz at the oven-dried conditions. The results demonstrated that both Young's modulus and Poisson's ratio of immature Barnett and Eagle Ford shales and mature Mancos shale remain roughly constant and the extensional attenuations are also insignificant at the entire frequency band. However, the mature Jungar shale exhibits strong dispersions with the magnitude of 23.4 per cent for Young's modulus at the confining pressure of 3.4 MPa. The dispersion and magnitude of Jungar shale overall exhibit a decreasing trend with increasing pressure, but still maintain strong attenuation at the confining pressure of 20.8 MPa. It is found that the single parameter of TOC, thermal maturity and clay content are not the controlling factors of wave dispersion and adsorption signatures. Instead, the geochemical index |${\rm{TOC}} \times {S_1}/({S_1}{\rm{ + }}{S_2})$| is positively correlated with dispersion and attenuation magnitude, suggesting that organic richness and maturity level jointly control the wave adsorption characteristics. Geochemical analysis and phenomenological modelling imply that such kinds of energy dissipation in organic shales are possibly caused by the viscous friction at interfaces between inorganic grains and the surface of liquefied or bitumen-filled organic matter. Besides, a modified standard linear solid model containing multiple sets of a characteristic frequency is proposed and well captures the continuous dispersion signatures considering that the Jungar shale contains various maturity-induced viscous organic matter. [ABSTRACT FROM AUTHOR]

Free pressure oscillations during pumping operations in boreholes may potentially constrain hydraulic characteristics of the surrounding material. These damped oscillations occur when flow rate is suddenly changed, and their period and decay rate depend on the hydraulic properties of the entire hydraulic system: the porous medium, a section of the borehole, and/or the injection line, depending on test set-up. There have been previous attempts to estimate transmissivity values from free pressure oscillations that occurred during slug tests in open boreholes. The analysis used did not account for viscous losses due to the fluid interacting with the borehole wall. In contrast, dispersion relations of flow waves in a tight borehole (i.e. a cylindrical hole in an impermeable medium) account for wall friction. We extend a previous analytical treatment of flow waves by changing the boundary condition of the fluid velocity at the borehole wall to include fluid exchange between borehole and porous medium. In addition, we performed numerical modelling of waves propagating in boreholes with impermeable and permeable walls to assess the effect of the assumptions behind the analytical solution. We established how to distinguish cases in which the flow into the porous medium affects the oscillation characteristics (suitable for a hydraulic analysis) from those in which the equipment properties dominate the observations. Applying our methods to a range of field observations yielded plausible hydraulic property values of the rock volume surrounding the borehole. [ABSTRACT FROM AUTHOR]

Abnormal seismic activities near reservoirs usually show a strong spatiotemporal correlation with the water filling history. Reservoir-induced seismicity is thought to be related to crustal pore pressure and stress changes caused by water impounded behind the dams. Though the Coulomb-type stress analysis helps illuminate areas that would be under risk following reservoir impoundment, it lacks the ability to explain the temporal characteristics of reservoir-induced seismicity. We present a numerical investigation of the seismicity rate evolution of reservoir-induced earthquakes. Our modelling employs a fully coupled 2-D poroelastic model to calculate the pore pressure and stress changes caused by water impoundment and incorporates the rate- and state-dependent friction law to investigate the seismicity rate. We demonstrate that shallow earthquakes are mainly caused by pore pressure increase, while poroelastic stress transfer takes the dominant role at depth. Whether a fault would be brought close to failure depends on its geometrical properties and its relative location to the reservoir. The temporal evolution of reservoir-induced earthquakes is primarily controlled by tectonic environment instead of the diffusion of pore pressure. [ABSTRACT FROM AUTHOR]

Finding an accurate method for estimating permeability aside from well logs has been a difficult task for many years. The most commonly used methods targeted towards regression technique to understand the correlation between pore throat radii, porosity and permeability are Winland and Pittman equation approaches. While these methods are very common among petrophysicists, they do not give a good prediction in certain cases. Consequently, this paper investigates the relationship among porosity, permeability, and pore throat radii using three methods such as multiple regression analysis, artificial neural network (ANN), and adaptive neuro-fuzzy inference system (ANFIS) for application in transition zone permeability modeling. Firstly, a comprehensive mercury injection capillary pressure (MICP) test was conducted using 228 transition zone carbonate core samples from a field located in the Middle-East region. Multiple regression analysis was later performed to estimate the permeability using pore throat and porosity measurement. For the ANN, a two-layer feed-forward neural network with sigmoid hidden neurons and a linear output neuron was used. The technique involves training, validation, and testing of input/output data. However, for the ANFIS method, a hybrid optimization consisting of least-square and backpropagation gradient descent methods with a subtractive clustering technique was used. The ANFIS combines both the artificial neural network and fuzzy logic inference system (FIS) for the training, validation, and testing of input/output data. The results show that the best correlation for the multiple regression technique is achieved for pore throat radii with 35% mercury saturation (R35). However, for both the ANN and ANFIS techniques, pore throat radii with 55% mercury saturation (R55) gives the best result. Both the ANN and ANFIS are later found to be more effective and efficient and thus recommended as compared with the multiple regression technique commonly used in the industry.

In volcanic regions ascending magma is subject to depressurization and is generally accompanied by exsolution of volatiles. We assume a process in which these volatiles propagate upward across newly fractured and permeable rock layers, bringing a sharp increase of pore pressure and temperature within a thin disc-shaped region (inclusion). Thermo-poro-elastic (TPE) inclusion models provide a mechanism to explain seismicity and deformation induced by p and T changes in absence of new magma emplacement in volcanic contexts. They are also suitable to represent the mechanical effects due to fluid extraction and re-injection in geothermal fields. In the present work analytic solutions are provided for the displacement, strain and stress fields assuming a TPE unbounded medium. Significant deviatoric stress is generated by positive increments of pore pressure and temperature: the stress field is fully deviatoric outside the TPE inclusion, but a strong isotropic stress component is present within, leading to highly heterogeneous faulting mechanisms: if the disc plane is horizontal, thrust faulting mechanisms are favoured within the TPE disc over optimally oriented faults and normal mechanisms above. The model is easily generalized to a vertically thick disc with variable temperature and pore-pressure changes: then, an extensional environment can be obtained even within the TPE inclusion assuming upward decreasing of pore pressure and temperature changes. The supplied analytical solution may be used to model near-field TPE inclusion effects and to validate more complex numerical modelling. [ABSTRACT FROM AUTHOR]

Patterns in nature are often interpreted as a product of reaction-diffusion processes which result in dissipative structures. Thermodynamic constraints allow prediction of the final state but the dynamic evolution of the microprocesses is hidden. We introduce a new microphysics-based approach that couples the microscale cross-constituent interactions to the large-scale dynamic behaviour, which leads to the discovery of a family of soliton-like excitation waves. These waves can appear in hydromechanically coupled porous media as a reaction to external stimuli. They arise, for instance, when mechanical forcing of the porous skeleton releases internal energy through a phase change, leading to tight coupling of the pressure in the solid matrix with the dissipation of the pore fluid pressure. In order to describe these complex multiscale interactions in a thermodynamic consistent framework, we consider a dual-continuum system, where the large-scale continuum properties of the matrix–fluid interaction are described by a reaction-self diffusion formulation, and the small-scale dissipation of internal energy by a reaction-cross diffusion formulation that spells out the macroscale reaction and relaxes the adiabatic constraint on the local reaction term in the conventional reaction-diffusion formalism. Using this approach, we recover the familiar Turing bifurcations (e.g. rhythmic metamorphic banding), Hopf bifurcations (e.g. Episodic Tremor and Slip) and present the new excitation wave phenomenon. The parametric space is investigated numerically and compared to serpentinite deformation in subduction zones. [ABSTRACT FROM AUTHOR]

Large earthquakes can alter regional groundwater pressure, resulting in fluid flow, and the process of restoring hydrostatic equilibrium would in turn lead to observable surface deformation, termed poroelastic rebound, which is one of the most important post-seismic mechanisms for stress transfer and triggering. To constrain the poroelastic contributions to the early post-seismic deformation, we model the hydrologic response within 1.5 months following the 2015 M w 8.3 Illapel earthquake and remove its effects from the observed geodetic signals. Results demonstrate the post-seismic fluid-flow patterns from the co-seismic high-slip region to the north and south sides, and the northern poroelastic effects are remarkably stronger than those on the south side, verified by northern liquefaction phenomena. Therefore, previous pure afterslip models overestimate the asperities on both flanks of the co-seismic rupture zone and underestimate the middle region, with local errors of more than 50 per cent. It highlights the importance of considering the poroelastic effects, when modelling the transient post-seismic deformation. [ABSTRACT FROM AUTHOR]

Quantification understanding, and prediction of physical rock properties rely so far predominantly on laboratory analyses of cores and plugs. Based on such data, petrophysical models are found that relate both microstructural properties and environmental conditions to geophysically accessible quantities. When considering reactive rock–fluid–gas systems, for example in geothermal energy, enhanced oil recovery or carbon dioxide sequestration, especially with carbonatic rock matrix, this approach is costly and time-consuming at best, or impossible to implement at worst. This is based on the two following reasons: First, porosity, permeability and accessible internal surface area in solid rock plugs are often so low that experimental time duration of many months or even years would be required to achieve chemical equilibrium. Secondly, plugs are single specimens of their — generally heterogeneous — original rock formation, which strongly questions the representativeness of single-plug data. To overcome these shortcomings, we present a new methodology based on the combination of systematic crushing, multimethod laboratory measurements and model-based computational evaluation with solving an inverse problem. As a first step, a large amount of undisturbed rock is intentionally crushed and divided in several particle size classes. Then, petrophysical laboratory measurements are carried out on all particle size classes. The resulting data set is finally inverted for the intended properties of the undisturbed rock. This inverse problem entails a three-level forward model, which parametrizes the undisturbed rock properties, particle characteristics and particle packings, but can also be freely adapted to other tasks by any suitable model representation. The three-level model is designed to enforce the petrophysical correlation of all properties at all levels while using a minimal set of model parameters, thus keeping the inverse problem overdetermined. For the inversion, we have developed a publicly available software tool (AnyPetro) based on a Gauss–Newton inversion scheme to minimize a damped least-squares objective function. To demonstrate and validate the proposed methodology, we present a study using five rock types — four carbonates and one sandstone as a reference. Laboratory measurements of complex electrical conductivity (from spectral induced polarization), specific surface (from nitrogen adsorption) and intraparticle porosity (from mercury intrusion) have been carried out on eight particle size classes and on plugs of each rock for comparison. Supportive and complementary analyses include, for example particle geometry, nuclear magnetic resonance, scanning electron microscopy, computer tomography, uniaxial compression strength and mineralogical composition. We show that our new methodology is highly capable of robustly recovering the complex electrical conductivity, specific surface and porosity of the undisturbed rocks from the measured data. The resulting sets of model parameters are petrophysically reasonable and verifiable. The presented methodology can further be applied to the use of drill cuttings as sample material, which is often the only available rock material from deep wells. Our findings also represent a methodological advance for laboratory experiments on reactive systems and both the interpretation and prediction of petrophysical rock properties in such systems. [ABSTRACT FROM AUTHOR]

The seismic history of the Mosha fault, the largest and most active fault of Eastern Tehran metropolis, and its relation to the Damavand active volcano, the highest mountain in the Middle East, is investigated. We deduce that the central Mosha, near the Damavand, has a higher seismicity than either its western or eastern segments. On 7 May 2020, an Mw 5.1 earthquake occurred on the central Mosha, about 40 km east of Tehran and 10 km southwest of the Damavand crest, and it was felt intensely in Tehran. Its rupture is imaged and located in a region that presented a relative seismic quiescence compared to its eastern and western parts, during the last 14 years, suggesting its partial locking and heterogeneous distribution of fault frictional strength on this segment of Mosha. Its significant directivity to the west is confirmed by the mainshock rupture model, its PGA distribution, and distribution of early aftershocks. The rupture model suggests a relatively small stress drop of 2.6 bar, which is consistent with the comparatively high rupture dimension of 9 km for a Mw 5.1 earthquake, and indicates the easy rupture expansion on the central Mosha near the Damavand Volcano. The central Mosha experienced earthquakes in 1930, 1955, and 1983, as well as high microseismic activity and the 2020 seismic sequence, all of which strongly point to a possible influence of the Damavand Volcano on the seismicity of the central Mosha. This is corroborated by the observation of hydrothermal zones on the Mosha fault and the extension of a sill-like Damavand young magma chamber until central Mosha in tomography studies. We propose that the existing heat may increase the pore pressure on the fault, which lowers the effective normal stress, facilitates the nucleation-expansion of the rupture, and unclamps the fault. Damavand could act as a fuse and nucleate earthquakes, and if the rupture extends toward the west, it could have a significant directivity effect on low-frequency seismic waves that reach Tehran without attenuation and affect tall structures. In addition, high site amplification for frequencies up to 16 Hz due to the deep sedimentary basin, mainly in the mid-city of Tehran, will be remarkable for short buildings.

We examined the effects of the petrophysical and mineralogical variables on the acoustic properties of Upper Assam sandstones. All parameters were evaluated in the laboratory using recognized standard laboratory methods. Compressional wave velocity was measured in the laboratory using piezoelectric transducers of 54 kHz by using the ultrasonic-through transmission technique. Single parameter correlations among bulk density, porosity, permeability and mineralogy with compressional wave velocity showed that the compressional wave velocity correlated inversely with porosity, permeability and feldspar content and directly with bulk density and quartz content. For instance, a plug with a higher amount of feldspar content showed a corresponding decrease in compressional wave velocity. Similarly, higher quartz content showed a higher compressional wave velocity. Nevertheless, to depict a clear correlation among different parameters, a multiparameter analysis was performed. It was observed that the coefficient of determination improved from 0.596 to 0.899 when compressional wave velocity was modelled in terms of bulk density, porosity, quartz and feldspar content collectively, rather than when compressional wave velocity was modelled as a function of porosity alone. Hence, our study suggests that multiple parameters exhibit a coherent influence on compressional wave velocity. Therefore, a multivariate statistical approach involving petrophysical and mineralogical influences would be more realistic. Furthermore, major mineral fractions of sandstone rock type, that is quartz and feldspar fractions were varied for a range of porosity to determine the variations in compressional wave velocity for different saturation conditions. Capturing the fluctuations in compressional wave velocity within the geological constraints will aid in possible reservoir characterization away from the well-control or beyond the available data range. The integrated approach can result in more accurate and precise rock physics models that may help to infer key reservoir parameters. The developed models can be used in quantitative seismic interpretation for reservoir characterization and to identify prospective reservoirs. [ABSTRACT FROM AUTHOR]

© The Authors 2017. When pH and alkalinity increase, calcite frequently precipitates and hence modifies the petrophysical properties of porous media. The complex conductivity method can be used to directly monitor calcite precipitation in porous media because it is sensitive to the evolution of the mineralogy, pore structure and its connectivity. We have developed a mechanistic grain polarization model considering the electrochemical polarization of the Stern and diffuse layers surrounding calcite particles. Our complex conductivity model depends on the surface charge density of the Stern layer and on the electrical potential at the onset of the diffuse layer, which are computed using a basic Stern model of the calcite/water interface. The complex conductivity measurements of Wu et al. on a column packed with glass beads where calcite precipitation occurs are reproduced by our surface complexation and complex conductivity models. The evolution of the size and shape of calcite particles during the calcite precipitation experiment is estimated by our complex conductivity model. At the early stage of the calcite precipitation experiment, modelled particles sizes increase and calcite particles flatten with time because calcite crystals nucleate at the surface of glass beads and grow into larger calcite grains. At the later stage of the calcite precipitation experiment, modelled sizes and cementation exponents of calcite particles decrease with time because large calcite grains aggregate over multiple glass beads and only small calcite crystals polarize.

One of the challenges associated with subsurface high‐pressure fluid injections is the estimation of the seismic hazard and its spatial footprint. Field data have shown that the spatial footprint typically varies significantly between injections into the basement and injections above basement. Here, we show that varying degrees of spatial correlations in porosity or log(permeability) can explain this observation. Using high‐resolution well‐log data, we first show that porosity within the basement tends to follow a power‐law scaling, S(k) ∼ 1/kβ, with β ≈ 0.9, while above basement β > 1.4. Using this in a novel conceptual model, we show that β controls the spatial footprint of fluid‐induced seismicity such that large values of β lead to more seismic activity at large distances and a higher variability in the spatio‐temporal migration of seismic events, hence, explaining the field observations. Our findings indicate that correlations in log(permeability) need to be incorporated in the seismic hazard assessment. Plain Language Summary: Fluid‐induced earthquakes are a side effect of industrial operations such as hydraulic fracturing and enhanced geothermal systems, where high‐pressure fluids are pumped into the earth's crust to increase oil and/or gas flow to a well from petroleum‐bearing rock formations or to improve permeability in underground geothermal reservoirs. The induced seismicity has become a concern for industry and nearby residents. One of the challenges associated with this seismic hazard is answering the question of how far away from the injection site seismicity can be induced. Previous observations showed that if the fluids are pumped into the crystalline basement, the occurrence of the seismicity is typically very localized in space, while otherwise it is not. Here, we introduce a novel model that explains this behavior as a consequence of a varying degree of correlations in permeability. By analyzing high‐quality permeability data, we directly show that the degree of correlations in the crystalline basement is indeed significantly different from formations above the basement. Our findings can be directly incorporated in any seismic hazard assessment to improve the safety of workers and residents. Key Points: Spatial correlations in permeability can control the spatial localization of fluid‐induced seismicitySpatial correlations in permeability tend to be different within the basement and above basementThe variability in the migration profile of fluid‐induced seismicity is controlled by the permeability field [ABSTRACT FROM AUTHOR]

We consider lithological tomography in which the posterior distribution of (hydro)geological parameters of interest is inferred from geophysical data by treating the intermediate geophysical properties as latent variables. In such a latent variable model, one needs to estimate the intractable likelihood of the (hydro)geological parameters given the geophysical data. The pseudo-marginal (PM) method is an adaptation of the Metropolis–Hastings algorithm in which an unbiased approximation of this likelihood is obtained by Monte Carlo averaging over samples from, in this setting, the noisy petrophysical relationship linking (hydro)geological and geophysical properties. To make the method practical in data-rich geophysical settings with low noise levels, we demonstrate that the Monte Carlo sampling must rely on importance sampling distributions that well approximate the posterior distribution of petrophysical scatter around the sampled (hydro)geological parameter field. To achieve a suitable acceptance rate, we rely both on (1) the correlated PM (CPM) method, which correlates the samples used in the proposed and current states of the Markov chain and (2) a model proposal scheme that preserves the prior distribution. As a synthetic test example, we infer porosity fields using crosshole ground-penetrating radar (GPR) first-arrival traveltimes. We use a (50 × 50)-dimensional pixel-based parametrization of the multi-Gaussian porosity field with known statistical parameters, resulting in a parameter space of high dimension. We demonstrate that the CPM method with our proposed importance sampling and prior-preserving proposal scheme outperforms current state-of-the-art methods in both linear and non-linear settings by greatly enhancing the posterior exploration. [ABSTRACT FROM AUTHOR]

In this work, we propose a hydro-mechanical simulation model to study the strong interaction of fluid flow and fracture deformation under in situ stress conditions. The general model is reduced under physics-based assumptions to provide an efficient numerical approach for inverse analysis of experimental studies and is applied to experimental field data obtained from hydraulic tests conducted at the Grimsel Test Site, Switzerland. The present set of hydro-mechanical measurement data provides not only valuable information about the transient pressure and flow evolution but also the transient change of fracture deformation. We aim to introduce a strongly coupled hydro-mechanical model to numerically characterize the fractured reservoir based on experimental data below the limit of hydraulically induced irreversible changes of the reservoir's properties. Insights into the leading mechanisms of flow processes throughout hydraulic testing under in situ conditions are then gained by best numerical fits of the measurement data. Based on the experimental and numerical findings, this study emphasizes the importance of a consistent consideration of local and non-local fracture deformation throughout inverse analysis of hydraulic testing data to (a) better understand hydro-mechanical flow processes in fractured reservoirs and (b) increase the estimation quality of hydraulic properties of tested fractures. [ABSTRACT FROM AUTHOR]

Natural gas can be temporarily stored in a variety of underground facilities, such as depleted gas and oil fields, natural aquifers and caverns in salt rocks. Being extensively monitored during operations, these systems provide a favourable opportunity to investigate how pressure varies in time and space and possibly induces/triggers earthquakes on nearby faults. Elaborate and detailed numerical modelling techniques are often applied to study gas reservoirs. Here we show the possibilities and discuss the limitations of a flexible and easily formulated tool that can be straightforwardly applied to simulate temporal pore-pressure variations and study the relation with recorded microseismic events. We use the software POEL (POroELastic diffusion and deformation) which computes the poroelastic response to fluid injection/extraction in a horizontally layered poroelastic structure. We further develop its application to address the presence of vertical impermeable faults bounding the reservoir and of multiple injection/extraction sources. Exploiting available information on the reservoir geometry and physical parameters, and records of injection/extraction rates for a gas reservoir in southern Europe, we perform an extensive parametric study considering different model configurations. Comparing modelled spatiotemporal pore-pressure variations with in situ measurements, we show that the inclusion of vertical impermeable faults provides an improvement in reproducing the observations and results in pore-pressure accumulation near the faults and in a variation of the temporal pore-pressure diffusion pattern. To study the relation between gas storage activity and recorded local microseismicity, we applied different seismicity models based on the estimated pore-pressure distribution. This analysis helps to understand the spatial distribution of seismicity and its temporal modulation. The results show that the observed microseismicity could be partly linked to the storage activity, but the contribution of tectonic background seismicity cannot be excluded. [ABSTRACT FROM AUTHOR]

Fault shear slip potential is analysed in the area where induced earthquakes (up to 3.9 M w) occurred in May–June 2015 approximately 30 km south of Fox Creek, Western Canada Sedimentary Basin, Canada. The induced earthquakes were generated by the hydraulic fracturing of the Upper Devonian Duvernay Formation. Interpretation of a 3-D seismic survey and analysis of the ant tracking attribute identifies a linear discontinuity that likely represents a subvertical fault with strike length of 1.4 km, which is aligned with the zone of induced earthquake hypocentres. 1-D–3-D geomechanical modelling is conducted to characterize mechanical rock properties, initial reservoir pressure and stress field. Hydraulic fracture propagation and reservoir pressure buildup simulations are run to analyse lateral fluid pressure diffusion during well treatment. The interaction of natural fractures introduced as Discrete Fracture Network and hydraulic fractures is tested. 3-D poroelastic reservoir geomechanical modelling is completed to simulate slip reactivation of the identified fault zone. The obtained results support that additional pressure buildup of 20 MPa in treatment wells can propagate laterally along hydraulic fractures (and potentially natural fracture network) for about 550 m and reach the fault zone. The increase of fluid pressure by 20 MPa in the fault zone results in dextral slip along the fault, mostly in the interval of the Duvernay and overlying Ireton Formations, corroborating prior focal mechanism results and hypocentral depths. The simulations indicate that lateral transmission of additional fluid pressure from the fracturing stimulation area to the fault zone could happen in a few days after the treatment of lateral wells that is supported by the observed induced earthquakes. This study helps to quantify changes in fluid pressure and stresses that may result in fault shear slip during hydraulic fracturing and predict the potential of induced seismicity connected to hydrocarbon production from the Duvernay Play. [ABSTRACT FROM AUTHOR]

The magnetic anisotropy exhibited by ferrofluid-impregnated samples serves as a proxy for their pore fabrics, and is therefore known as magnetic pore fabric (MPF). Empirically, the orientation of the maximum susceptibility indicates the average pore elongation direction, and predicts the preferred flow direction. Further, correlations exist between the degree and shape of magnetic anisotropy and the pores' axial ratio and shape, and between the degrees of magnetic and permeability anisotropies. Despite its potential, the method has been rarely used, likely because the large variability in reported empirical relationships compromises interpretation. Recent work identified an additional contribution of distribution anisotropy, related to the arrangement of the pores, and a strong dependence of anisotropy parameters on the ferrofluid type and concentration, partly explaining the variability. Here, an additional effect is shown; the effective susceptibility of the ferrofluid depends on the measurement frequency, so that the resulting anisotropy depends on measurement conditions. Using synthetic samples with known void geometry and ferrofluids with known susceptibility (4.04 SI and 1.38 SI for EMG705 and EMG909, respectively), magnetic measurements at frequencies from 500 to 512 kHz are compared to numerical predictions. Measurements show a strong frequency-dependence, especially for EMG705, leading to large discrepancies between measured and calculated anisotropy degrees. We also observe artefacts related to the interaction of ferrofluid with its seal, and the aggregation of particles over time. The results presented here provide the basis for a robust and quantitative interpretation of MPFs in future studies, and allow for re-interpretation of previous results provided that the ferrofluid properties and measurement conditions are known. We recommend that experimental settings are selected to ensure a high intrinsic susceptibility of the fluid, and that the effective susceptibility of the fluid at measurement conditions is reported in future studies. [ABSTRACT FROM AUTHOR]

Accurate modelling of the frequency-dependence of seismic wave velocity related to fracture system and fluid content is crucial to the quantitative interpretation of seismic data in fractured reservoirs. Both mesoscale fractures and patchy saturation effects can cause significant velocity dispersion and attenuation in the seismic frequency band due to wave-induced fluid flow (WIFF) mechanism. Considering the coupled impact of 'mesoscale fractures' and 'patchy saturation', we derive expressions for the frequency-dependent anisotropy in partially saturated porous rock containing two fracture sets with different orientations, sizes and connectivities. Especially, we simplify the rock-physics model as an orthorhombic (ORT) media by assuming the mesoscale fractures to be orthogonal and give the explicit expressions for frequency-dependent elastic constants. Finally, we give the expressions for the frequency-dependent phase velocity in patchy saturated and fractured ORT media and investigate the effect of patchy saturation on P-wave velocity at different polar and azimuth angles. In this paper, we investigate the effects of fluid saturation and fluid pressure on frequency-dependent velocities and Thomsen anisotropy parameters. Also, the effect of the relative permeability is very noticeable. The relaxation frequency can be lower in partially saturated fractured rocks compared with the fully saturated case, which makes the rock have a larger stiffness. The non-monotonic relationships between frequency-dependent anisotropy and fluid saturation add complexity to seismic forward modelling and inversion in reservoirs with complex fracture patterns. [ABSTRACT FROM AUTHOR]

We have developed a new method for introducing the effects of pore water distribution into the interpretation of P -wave velocity (VP) dispersion in a partially saturated rock. Pore water in an unsaturated rock is distributed as patches, and mesoscopic-scale wave-induced fluid flow (WIFF) from saturated patches to empty patches is known as a key mechanism for interpreting P -wave dispersion. Mesoscopic WIFF depends on permeability and the scale of the saturated patch, thus evaluating their influences is important for estimating the relationship between water saturation (⁠|${S_{\rm{w}}}$|⁠) and VP. The simplest and conventional law for simulating the |${S_{\rm{w}}} - {V_{{P}}}$| relation considering the mesoscopic WIFF is White's model, which describes the patch size changes proportional to the saturation. However, White's model assumes homogeneity in the size and distribution of pores, and does not consider the effects of pore water distribution in a partially saturated rock. In the rock, pore water is distributed heterogeneously according to the pore radius, causing non-linear changes in the permeability and size of the saturated patches against the water saturation. In this study, we modified White's model by introducing relative permeability and a new coefficient describing the nonlinear change of the patch size into the conventional version, and applied the modified model to better interpret our experimental |${S_{\rm{w}}} - {V_{{P}}}$| relations, which were measured by an ultrasonic technique at 200 and 500 kHz for two types of Berea sandstone samples with different permeabilities as their water saturation was decreased by evaporative drying. The relative permeabilities were determined by applying the Mualem–van Genuchten model into the capillary pressure curves from mercury intrusion porosimetry. Furthermore, we proposed a calculation method for the new coefficient using tortuosity, which corresponds to the pore water connection. The modified White's model could reproduce the experimental |${S_{\rm{w}}} - {V_{{P}}}$| relations better than other conventional models. Consequently, our modification, considering the effect of pore water distribution, would be useful for more quantitative interpretation of P -wave velocity and attenuation under coexisting multifluid conditions. [ABSTRACT FROM AUTHOR]

A strategy is presented to incorporate prior information from conceptual geological models in probabilistic inversion of geophysical data. The conceptual geological models are represented by multiple-point statistics training images (TIs) featuring the expected lithological units and structural patterns. Information from an ensemble of TI realizations is used in two different ways. First, dominant modes are identified by analysis of the frequency content in the realizations, which drastically reduces the model parameter space in the frequency-amplitude domain. Second, the distributions of global, summary metrics (e.g. model roughness) are used to formulate a prior probability density function. The inverse problem is formulated in a Bayesian framework and the posterior pdf is sampled using Markov chain Monte Carlo simulation. The usefulness and applicability of this method is demonstrated on two case studies in which synthetic crosshole ground-penetrating radar traveltime data are inverted to recover 2-D porosity fields. The use of prior information from TIs significantly enhances the reliability of the posterior models by removing inversion artefacts and improving individual parameter estimates. The proposed methodology reduces the ambiguity inherent in the inversion of high-dimensional parameter spaces, accommodates a wide range of summary statistics and geophysical forward problems.

We investigated the induced seismicity, source mechanisms and mechanical responses of a decametre-scale hydraulic stimulation of a pre-existing shear zone in crystalline rock, at the Grimsel Test Site, Switzerland. The analysis reveals the metre-scale complexity of hydraulic stimulation, which remains hidden at the reservoir-scale. High earthquake location accuracy allowed the separation of four distinct clusters, of which three were attributed to the stimulation of fractures in the damage zone of the shear zone. The source mechanism of the larger-magnitude seismicity varied by cluster, and suggests a heterogeneous stress field already prevailing before stimulation, which is further modified during stimulation. In the course of the experiment, stress redistribution led to the aseismic initiation of a tensile-dominated fracture, which induced seismicity in the fourth of the identified seismic clusters. The streaky pattern of seismicity separated by zones without seismicity suggests fluid flow in conduits along existing fracture planes. The observed submetre scale complexity questions the forecasting ability of induced seismic hazard at the reservoir scale from small-scale experiments. [ABSTRACT FROM AUTHOR]