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Subsurface natural hydrogen accumulations that can be economically and safely recovered are a major target of the present roadmap towards the use of hydrogen as an alternative and C-free energy resource. The existence of H 2 -rich reservoirs in ophiolites, where H 2 is produced via serpentinization, was theoretically predicted based on the intensity of gas seeps in Turkey and Albania. Here, we examine the geological bases supporting the potential existence of H 2 reservoirs within ophiolites in general, also considering available data on ophiolitic petroleum reservoirs, and we document a direct observation of a H 2 -rich accumulation within an ophiolite. We studied the case of a borehole, drilled in the 1970s for metal mining exploration purposes within the Tișovița–Iuți ophiolite, in Romania, which accidentally encountered pressurized gas with ∼29 vol% H 2 at a depth of about 800 m. We reconstructed the geometry of the ophiolite sequence in correspondence with the well and suggested that the reservoir is within cataclastic dunite near the tectonic contact between layered dunite and the harzburgite-dunite peridotites; the upper, non deformed portion of serpentinized layered dunite, likely with the contribution of tonalite veins, acted as a fluid trap. Although plugged, we detected H 2 leakage from this well and from a second well in the same area, with H 2 concentrations exceeding 100 ppmv in the air and 0.1 mg H 2 /L in the water within the wellhead open casing. We propose a broad conceptual model in which deformed, fractured, and non-deformed, impermeable sections of serpentinized peridotites can determine ophiolitic H 2 -rich reservoirs and traps, respectively, as factors that shall be considered in the guidelines for H 2 reservoir exploration. • Direct observation of pressurized H 2 -rich reservoir in ophiolite (Romania). • Potential reservoirs and cap rocks in ophiolites. • H 2 leakage observed in the abandoned wells. [ABSTRACT FROM AUTHOR]

Gas chimneys and gas clouds in the subsurface media are known as one of the indications of possible petroleum reservoirs. Investigations of their properties are mostly initiated by seismic attribute interpretation on reflection seismic data. However, due to the complexity of their behavior and their difficult interpretation of seismic attributes, state-of-the-art methods are mostly required to be applied on the seismic data to prevent any misinterpretation. This is mostly done through attribute integration and multi-attribute analysis. This research presents a study on seismic attributes and integration on several 2D seismic reflection lines from the Gorgan Plain. It is located in Northeast Iran, on the western border of the region's well-known Kopeh-Dagh fold and thrust belt, and southeastern border of the South Caspian Basin. Hydrocarbon systems of the Gorgan Plain are poorly known and have not been widely studied, but according to preliminary investigations, this region has the potential for hydrocarbon occurrences. The aim of this study is to investigate presence and then delaminate the affected area of possible gas chimneys that are related to possible hydrocarbon reservoirs. Gas chimneys are assumed to be created due to the routes, mostly made by faults, that provoke light hydrocarbons components to migrate toward the surface. Preliminary interpretations of seismic reflection data in this study revealed that at least two gas chimneys occurred within the Gorgan Plain. As it was mentioned, since they are mostly due to the faulting above the hydrocarbon reservoir, gas chimney and heavy faulting might exhibit the same effects on the seismic data and then on its attributes, which are amplitude reduction and high damping on energies, distortion of the waveshape and seismic velocity reduction. Thus, care should be taken in separation of these two different geologic phenomena on seismic attributes. This also was done in this study through utilized integration of the most relevant seismic attributes such as Instantaneous-phase, Chaos, Variance and Remove-bias attributes. Based on the result of interpretations and according to the evolution of the basin and its structural reconstruction on other studies, gas chimneys of the Gorgan Plain, are in relation to the operation of fault zones in Cenozoic erathem in the region. These fault zones which cut the entire Cenozoic erathem, create the pathway for vertical migration of hydrocarbons through Cheleken formation (reservoir rock) and its overburden sedimentary sequences. In other words, operation of fault zones within Cenozoic sedimentary sequence, is the main reason for gas seepage in the Gorgan Plain, which is also shown in seismic data. [ABSTRACT FROM AUTHOR]

To understand the fluid hydraulic behavior change in a fracture under different shear displacements, fluid flow simulations at different flow velocities are carried out based on the Navier–Stokes equation. We characterized and conducted statistics on the change and distribution of the fracture aperture under different shear displacements, especially the measurement of the contact area under shear displacement, which was helpful for accurately studying the effect of shear displacement on fracture fluid flow. The results showed that the shear displacement of a fracture could be regarded as dislocation between the upper and lower fracture surfaces. The fluid flow velocity became larger near the contact sites and the streamlines became denser. The resulting hydraulic aperture exhibited a negative correlation with the contact area. The relationship between the volumetric flow rate and pressure gradient during the flow field simulation could be well-fitted by the Forchheimer equation, and the steepness of the curves was related to the contact area. The linear coefficient a and the nonlinear coefficient b in the Forchheimer equation were positively correlated with the contact area. Based on the measured flow velocity, the mechanical aperture was usually an order of magnitude larger than the hydraulic aperture. The linear relationship between the mechanical aperture and hydraulic aperture was described. The positive correlation between the permeability and mechanical aperture profiled during shear explained the hydraulic behavior in the flow field. [ABSTRACT FROM AUTHOR]

Pockmarks are morphological depressions commonly observed in ocean and lake floors. Pockmarks form by fluid (typically gas) seepage thorough a sealing sedimentary layer, deforming and breaching the layer. The seepage‐induced sediment deformation mechanisms, and their links to the resulting pockmarks morphology, are not well understood. To bridge this gap, we conduct laboratory experiments in which gas seeps through a granular (sand) reservoir, overlaid by a (clay) seal, both submerged under water. We find that gas rises through the reservoir and accumulates at the seal base. Once sufficient gas over‐pressure is achieved, gas deforms the seal, and finally escapes via either: (a) doming of the seal followed by dome breaching via fracturing; (b) brittle faulting, delineating a plug, which is lifted by the gas seeping through the bounding faults; or (c) plastic deformation by bubbles ascending through the seal. The preferred mechanism is found to depend on the seal thickness and stiffness: in stiff seals, a transition from doming and fracturing to brittle faulting occurs as the thickness increases, whereas bubble rise is preferred in the most compliant, thickest seals. Seepage can also occur by mixed modes, such as bubbles rising in faults. Repeated seepage events suspend the sediment at the surface and create pockmarks. We present a quantitative analysis that explains the tendency for the various modes of deformation observed experimentally. Finally, we connect simple theoretical arguments with field observations, highlighting similarities and differences that bound the applicability of laboratory experiments to natural pockmarks. Plain Language Summary: Pockmarks are pit‐like depressions common in ocean and lake floors, formed by gas seepage through underlying sediments. Despite relevance to both fossil fuel exploration and global warming, the mechanisms by which pockmarks evolve remain elusive. We conduct simple laboratory experiments in which we inject air into a layer of glass beads ("reservoir") overlain by a layer of clay ("seal"), all submerged underwater in a transparent box. We find that gas rises through the sand and accumulates at the base of the clay. Then, gas pressure rises until it suffices to deform the clay and escape, forming a pockmark. This occurs by one of three mechanisms, depending on clay thickness and stiffness: (a) heaving of a dome which then fractures in thin clay layers; (b) faulting in thick, stiff clays; and (c) bubbles ascend in thick, soft clays. Pipe‐like focused gas conduits connecting the clay bottom to the pockmark are created by the rise of a "trains" of bubbles that weaken their path. These pathways can also initiate in faults. Repeated seepage events push the clay particles, suspending them in water to create a pockmark. Our findings agree with field observations, improving our understanding of natural pockmark formation. Key Points: Sandbox experiments link pockmark morphology (irregular vs. conical) to gas seepage‐induced deformation of the host (seal) layerExperiments and theory show seal thickness and consolidation control deformation mechanism: doming, brittle (faults), or plastic (bubbles)Theoretical calculations predict that under field conditions, the preferred mechanism for gas escape will be bubbles rising in faults [ABSTRACT FROM AUTHOR]

The study presents a novel setup for measuring the flow regime of hydrate particles in a gas-dominated flow, which is of interest for applications such as natural gas transportation. A closed-flow loop, driven by a novel internal fan, enables continuous observation of hydrate particle behavior in a gas flow. The experimental setup allows the production and insertion of HFC134a gas hydrate particles with diameters of 10–50 μm into the gas flow loop via a bypass loop. The performance curve of the internal fan is validated, and its suitability for achieving the required flow speed (5 m/s) is demonstrated. Through an observation window using camera systems, the flow regime of glass beads is successfully visualized and analyzed. To validate the experimental data, a coupled computational fluid dynamics–discrete element method model is used to simulate the particle flow density distribution. The study findings demonstrate the effectiveness of the experimental setup in characterizing the flow regime of hydrate particles in a gas-dominated flow. [ABSTRACT FROM AUTHOR]

The technology of CO2 geological storage and CH4 intensive mining (CO2-ECBM) in coal seams integrates greenhouse gas emission reduction and new fossil energy development and has great development prospects. The CO2 injection, CO2 sequestration mechanism and storage capacity, and CH4 stimulation effect constitute the core content of the effectiveness of CO2-ECBM, among which CO2 injection is the most critical. Traditional seepage analysis methods often struggle to tackle flow-related issues influenced by microscale effects and intricate channels. This paper highlights the advantages of employing lattice Boltzmann (LBM) numerical simulations to study CO2 seepage behaviors when teaching a Rock Mass Seepage Mechanics Course. This course primarily covers topics such as the pore structure of rock, unstable liquid seepage, gas seepage theory and related subjects. Its goal is to provide students with a solid theoretical foundation to address the complexities of fluid seepage in pours media encountered in practical scenarios. A novel LBM-based methodology was employed to estimate the CO2 seepage capacity by incorporating the effects of different concentrations of [Bmin]Cl solution (0 wt%, 1 wt%, 3 wt%, and 5 wt%). The CO2 velocity distribution cloud map of each coal sample was simulated; the average velocity distribution curve of each coal sample was obtained; and the velocity profile of the seepage channel of each coal sample was described. This study can provide theoretical guidance for the technology of CO2 geological storage and CH4 intensive mining in coal seams. [ABSTRACT FROM AUTHOR]

Continuous monitoring of volcanic gas emissions is crucial for understanding volcanic activity and potential eruptions. However, emissions of volcanic gases underwater are infrequently studied or quantified. This study explores the potential of Distributed Acoustic Sensing (DAS) technology to monitor underwater volcanic degassing. DAS converts fiber-optic cables into high-resolution vibration recording arrays, providing measurements at unprecedented spatio-temporal resolution. We conducted an experiment at Laacher See volcano in Germany, immersing a fiber-optic cable in the lake and interrogating it with a DAS system. We detected and analyzed numerous acoustic signals that we associated with bubble emissions in different lake areas. Three types of text-book bubbles exhibiting characteristic waveforms are all found from our detections, indicating different nucleation processes and bubble sizes. Using clustering algorithms, we classified bubble events into four distinct clusters based on their temporal and spectral characteristics. The temporal distribution of the events provided insights into the evolution of gas seepage patterns. This technology has the potential to revolutionize underwater degassing monitoring and provide valuable information for studying volcanic processes and estimating gas emissions. Furthermore, DAS can be applied to other applications, such as monitoring underwater carbon capture and storage operations or methane leaks associated with climate change. [ABSTRACT FROM AUTHOR]

Coal bed methane, a crucial clean energy source, has attracted extensive research attention. Characterized by intricate and rough fracture systems, coal seam is vital for gas migration, which will be influenced by the in situ stress, coal temperature, adsorption–desorption effect, solid deformation, and gas pressure. This paper introduces an innovative, interdisciplinary fractal model that addresses the limitations of current computational models in accurately representing the complex fractures under the coupled multi-field effects. Four novel fractal micro-parameters are introduced to capture the dynamics of rough networks. And rigorous validation against field extraction data reveals that the proposed micro-parameters outperform existing methods in analytical efficacy. Notably, those micro-parameters significantly influence fracture behavior and gas seepage. For instance, a DT increase from 1.2 to 1.8 and an ε rise from 0.06 to 0.18 lead to a respective 29.8% and 22.7% increase in gas pressure. Moreover, alterations in these fractal micro-parameters under coupled multi-field effects markedly impact coal bed stress, raising safety concerns in engineering projects, with a potential increase in coal stress by up to 2.62%. This research offers innovative insights into the complex coupled mechanisms governing rough fractures and significantly advances the understanding of the efficiency and safety in clean energy extraction processes. [ABSTRACT FROM AUTHOR]

In unconventional reservoir engineering, such as coalbed methane and shale gas extraction, fracture behavior is pivotal in gas accumulation, migration, and extraction, acting as a primary channel for gas flow. Current research inadequately addresses the quantitative impact of fracture roughness on gas extraction. In this study, we introduce a novel interdisciplinary model that quantitatively characterizes shale fracture roughness and correlates it with shale permeability. This model comprehensively considers factors affecting shale extraction, including fracture roughness, in situ stress, reservoir deformation, and adsorption-desorption dynamics. It provides a thorough analysis of how fracture roughness influences gas seepage, extraction efficiency, and reservoir stability under various physical conditions. Our validated findings reveal that fracture roughness significantly affects shale permeability, stress responses, and displacement. As the fracture roughness coefficient χ increases from 0.2 to 1.0, the maximum reduction in gas pressure within the hydraulically fractured region is 4.7%, while the maximum increase in reservoir stress is 2.8%. In shale reservoirs near extraction well, particularly in hydraulically fractured zones, the maximum decrease in shale gas pressure is 11.1%, and the maximum increase in stress is 3.2%, which offer a groundbreaking approach for optimizing extraction rates and ensuring project safety in the industry. [ABSTRACT FROM AUTHOR]

Coal is a natural and complex porous medium, which contains pore-fracture structures of different scales. The pore-fracture structures of coal directly affect the characteristics of gas storage and migration. In this paper, high-resolution X-ray CT scanning technology is used to quantitatively characterize the pore-fracture structure of medium and high-rank coals. First, the porosity inversion method is used to determine the optimal threshold segmentation of pores and fracture in medium and high-rank coals. Then, the pore and fracture model of representative element volume (REV) was extracted and established to characterize the area porosity, pore surface area, and shape factor of the two coals, and the relationship between pore surface area and shape factor was investigated. Finally, an equivalent pore network model (PNM) of connected pore topology was established, and pore-throat parameters are statistically analyzed, including pore volume, pore radius, throat radius, throat length, and coordination number. The results show that the average pore diameter and average surface area of macropores in medium-rank coals are larger than those of high-rank coals, and high-rank coals have fewer isolated pores. With the increase of coal rank, the length and radius of the throat gradually decrease, which indicates that the throat of the medium-rank coal has a higher degree of development, and the gas seepage ability in the throat is stronger. The outcome of this study is of great significance for comprehensively understanding the pore-fracture structure of coal in micro-scale. [ABSTRACT FROM AUTHOR]

The increasing demand for natural gas energy will promote unconventional natural gas, such as coal seam gas and shale gas, to play a key role in future energy development. The mechanical properties of coal seams are weaker compared with conventional natural gas reservoirs. The fluid–solid coupling phenomenon exists widely at the pore scale and macro scale of coal seams, and runs through the whole process of coalbed gas exploitation. The objective of this study is to establish a microscale gas–water flow model for coalbed methane considering fluid structure coupling. Frist, this study used scanning electron microscopy (SEM) to obtain microscopic pore images of coal rocks. Then, we constructed a numerical model to simulate the movement of coalbed methane and water within the scale of coal cleats based on the Navier–Stokes equation, phase field method, and solid mechanics theory. Finally, we analyzed the effects of injection pressure and wettability on the microscopic two-phase seepage characteristics and displacement efficiency of coal. Our research shows that when the injection pressure is increased from 60 kPa to 120 kPa, the displacement completion time is shortened from 1.3 × 10−4 s to 7 × 10−5 s, and the time is doubled, resulting in a final gas saturation of 98%. The contact angle increases from 45° to 120°, and the final gas saturation increases from 0.871 to 0.992, an increase of 12.2%. [ABSTRACT FROM AUTHOR]

Anomalously intensive gas seeps on the sea surface due to damage to the Nord Stream 1 and Nord Stream 2 subsea gas pipelines in the Baltic Sea have been analyzed using optical (Kanopus-V and Sentinel-2B) and radar (Sentinel-1A) satellite images. Positive contrasts of NRCS (up to 7.5 dB) registered by Sentinel-1A radar and of spectral reflectance (up to 0.73 units) registered by Kanopus-V and Sentinel-2B optical sensors have been detected in the area of the gas seeps. Specific features of the seeps were revealed on the sea surface in the form of dome-shaped swells, fountains, vortices, foam, wave breaking, disturbance of the surface wave structure, and wind shadows. Taking into consideration the volume and density of gas located in the damaged gas pipelines, it was shown that the total volume of methane emission was no more than ~0.51 Tg, i.e., less than 0.1% of the annual global methane emissions into the atmosphere. [ABSTRACT FROM AUTHOR]

The data of gas-geochemical studies of bottom sediments of the Laptev—Siberian Sea transition zone are presented. The established isotope–geochemical parameters indicate the domination of epigenetic hydrocarbon gases of various gas sources in the sediments. Methane concentrations up to 8.3047 cm3/kg and the sum of homologues up to 0.0260 cm3/kg indicate the formation of hydrocarbon anomalies exceeding the anomaly criteria for Arctic shelf sediments by a factor of 166 and 26. The main geological factors for the formation and distribution of anomalies in the bottom sediments of the Laptev–Siberian Sea transition zone are the high gas content of the underlying sediments and gas sources, the fold and fault tectonics, the geostructural position, the seismic activity, and the depth of occurrence of hydrocarbon gas sources. [ABSTRACT FROM AUTHOR]

Pockmarks are formed as a result of gas (methane) or/and groundwater outflow from the sea bottom. Methane, the second most important (after CO2) greenhouse gas, has a significant impact on biogeochemical processes in the bottom sediments by affecting the cycling of some elements, e.g. C, Fe, and S. Active pockmarks may also lead to changes in water column conditions by causing nutrients release from sediments. In the present study, we have focused on the impact of biogeochemical processes in pockmarks (methanogenesis, anaerobic methane oxidation, and groundwater seepage) on the transformation of iron (Fe) and the mineral composition of the sediment. In pore water, concentrations of hydrogen sulfide, phosphate, ammonia, sulfate, chloride, dissolved inorganic carbon, iron, and methane were analyzed. In the sediment, Fe speciation was performed using sequential extraction. The mineral composition was determined using powder X-Ray diffraction and scanning electron microscopy. The results from two pockmarks (with active gas seepage and groundwater infiltration) and two reference stations in the southern Baltic Sea show that geochemical conditions in pockmark sediments are significantly different from those in the typical muddy sea bottom. Pore water in pockmarks is characterized by lower sulfate and higher dissolved carbon concentrations as compared to areas of the seafloor where such structures are absent. This is due to the outflow of groundwater, which was confirmed by lower chloride concentration. In addition, sulfate is used to oxidize methane diffusing from deeper layers. Sediments in pockmarks are enriched in Fe(II) carbonates and depleted in Fe(III) (oxy)hydroxides, resulting from the anaerobic oxidation of methane with Fe(III) (Fe-AOM). Ferrous iron produced in large quantities during Fe-AOM is precipitated with carbonates. [ABSTRACT FROM AUTHOR]

The possibilities offered by the use of variational mode decomposition–based Hilbert marginal spectrum and the differential cepstrum for gas‐bearing detection are studied in this paper. We propose a novel variational mode decomposition–based Hilbert marginal differential cepstrum for hydrocarbon detection. Variational mode decomposition–based Hilbert marginal spectrum is first computed. Then discrete cosine transform is carried out to the differential logarithmic variational mode decomposition–based Hilbert marginal spectrum to obtain the variational mode decomposition–based Hilbert marginal differential cepstrum. For hydrocarbon detection, the seismic amplitude anomaly section is generated by extracting the first and second common quefrency sections. Compared with the traditional Fourier‐based cepstrum, the wavelet‐based cepstrum and the Berthil cepstrum, it has the ability to effectively reveal more detailed frequency‐dependent amplitude anomalies with high accuracy and resolution. Model tests and field data applications from a carbonate reservoir in China show that the variational mode decomposition‐based Hilbert marginal differential cepstrum can provide a better gas‐prone interpretation. The proposed method can be a complementary approach to current cepstrum‐based hydrocarbon detection methods and the spectrum decomposition methods. [ABSTRACT FROM AUTHOR]

Three-dimensional seismic exploration has been widely used to produce subsurface images for oil and gas. As one of the most commonly used multiple attenuation methods, 2D Radon transform cannot describe the three-dimensional wave-fields in realistic subsurface conditions. Conventional 3D Radon transform assumes that the properties of the medium are the same, or approximately the same, in all directions. The time slice of seismic data will be a standard ellipse, and its focal points are located at survey lines. However, In the case of complex geology, the medium may have different property in different directions. And the time slice of acquired 3D seismic data becomes a deflected ellipse, and its focal points are no longer located on the survey lines. Conventional 3D Radon transform based on standard ellipse can no longer describe seismic data accurately. A rotating ellipse model parameter is introduced to 3D Radon transform to describe seismic data in complex areas. The three-parameter 3D Radon transform based on rotating ellipse model is derived in detail. However, the operator matrix in the new formula is huge due to the introduced variable, and it cannot be decomposed into small matrices, leading the computation cost to be considerably high. The frequency and curvature are merged into one new parameter to deal with the low computational efficiency problem. The corresponding fast algorithm is also derived. The application of synthetic and real data examples shows that the proposed method can describe complex seismic data more accurately, and the method can attenuate multiples in complex cases much better. [ABSTRACT FROM AUTHOR]

The distribution characteristics and main controlling factors of hydrocarbon phases in deep strata from petroliferous basins are important for the evaluation of oil–gas resources and decision-making regarding exploration. The distribution characteristics and controlling factors of the Ordovician hydrocarbon phases are systematically analyzed in the Tazhong I block, Tarim Basin, NW China. The results show that the Ordovician reservoirs in the Tazhong I block are characterized as multi-phase reservoirs with a lateral co-existence of condensates, normal oil reservoirs, and heavy oil reservoirs. From east to west, gas-rich in the fault belt and oil-rich in the platform area are presented. Meanwhile, there are regular variations in the geochemical characteristics of the Ordovician hydrocarbon, showing decreasing trends in the gas/oil ratio (GOR), wax contents, dryness coefficients, methane contents, and methane carbonate isotope values (δ13C1) and an increasing trend in oil densities. Because the same Cambrian–Lower Ordovician source for the Ordovician hydrocarbon is observed in the Tazhong I block, the regular variations in the hydrocarbon phases and geochemical characteristics can be interpreted as records of gas invasion, biodegradation, multi-stage filling, thermal cracking, and thermochemical sulfate reduction (TSR) rather than controlled by the source rock organofacies. This indicated that different kinds of secondary processes for a diversity of the hydrocarbon phase can appear in one region. Our re-construction of the Ordovician hydrocarbon accumulation model in the Tazhong I block encourages future exploration to target gas reservoirs in the fault belt and oil reservoirs in the platform area. [ABSTRACT FROM AUTHOR]

As a typical unconventional energy reservoir, natural gas hydrate is believed to be the most promising alternative for conventional resources in future energy patterns. The exploitation process of natural gas hydrate comprises a hydrate phase state, heat and mass transfer, and multi-phase seepage. Therefore, the study of heat transfer characteristics of gas hydrate is of great significance for an efficient exploitation of gas hydrate. In this paper, the research methods and research progress of gas hydrate heat transfer are reviewed from four aspects: measurement methods of heat transfer characteristics, influencing factors of heat transfer in a hydrate system and hydrate-containing porous media systems, predictive models for effective thermal conductivity, and heat transfer mechanisms of hydrate. Advanced measurement techniques and theoretical methods that can be adopted for the heat transfer characteristics of gas hydrate in the future are discussed. [ABSTRACT FROM AUTHOR]

The key area of the Arctic Ocean for atmospheric venting of CH4 is the East Siberian Arctic Shelf (ESAS). Leakage of methane through shallow ESAS waters needs to be considered in interactions between the biogeosphere and a warming Arctic climate. The development of remote sensing techniques for gas seepage detection and mapping is crucially needed for further applications in the ESAS and other areas of interest. Given the extent of the seepage areas and the magnitude of current and potential future emissions, new approaches are required to effectively, rapidly, and quantitatively survey the large seepage areas. Here, we consider the main features of gas seep detection on the sea surface in the characteristics of wind waves and radar signals. The kinematics of wave packets based on the kinetic equation for the spectral density of the wave action of surface waves is described. The results of a full-scale experiment on the remote radar observation of a model gas seep to the sea surface in the radar equipment signals are considered. The characteristic radar signatures of the gas seep in a wide range of hydrometeorological conditions, the parameters of which were recorded synchronously with the radar mapping, were determined. The results of the first radar observations of natural methane seeps on the ESAS are presented, and their radar contrasts are evaluated. The theoretical conclusions are in good qualitative agreement with the results of the model experiment and field studies and can be used for further research in aquatic areas with potential gas seepage, both of natural or anthropogenic origin, such as bubbling release from broken underwater gas pipelines. [ABSTRACT FROM AUTHOR]

The study of fluid inclusions in petroliferous basins is an effective method to understand hydrocarbon migration and accumulation. In this paper, the fluid inclusions in the Ordovician carbonate rock samples taken from the TS3, TP18, YQ8 and YJ2-3 wells in Tahe Oilfield are analyzed by experiments, the purpose is to explore the accumulation period of the Ordovician oil and gas reservoirs in the Tahe area and the reasons for the differences between different blocks.The results show that the Ordovician Yingshan Formation and the Yifangfang Formation in the Tahe area are rich in fluid inclusions, and there are only a single phase of oil, gas and brine in the phase. There are also two phases of oil, gas and water mixed with each other. According to the fluorescence characteristics and homogenization temperature of hydrocarbon inclusions, combined with the burial history-thermal evolution history of the study area, it is determined that the Tahe oil and gas reservoir is filled in the fourth stage, in the middle of the Caledonian period (454-446 Ma), and in the late Hercynian-Indosinian period (255-217 Ma), late Yanshanian period(143-99 Ma), Himalayan period (25-5 Ma).Among them, the middle of Caledon is mainly filled with low-mature oil, with a small amount of mature oil; the late high-mature oil in the late Hercynian is filled with some mature oil; the late Yanshan is mainly filled with high mature oil; during the Himalayan period, as the depth of burial continues to increase, the cracking of the accumulated hydrocarbons has occurred, mainly the migration of gas hydrocarbons. The four wells selected in this study belong to different tectonic units, after analysis, the author believes that the difference between single wells is on the one hand the influence of hydrocarbon thermal evolution and the other is influenced by tectonic movement. [ABSTRACT FROM AUTHOR]

The advancement of plasma technology is intricately linked with the utilization of computational fluid dynamics (CFD) models, which play a pivotal role in the design and optimization of industrial-scale plasma reactors. This comprehensive compilation encapsulates the evolving landscape of plasma reactor design, encompassing fluid dynamics, chemical kinetics, heat transfer, and radiation energy. By employing diverse tools such as FLUENT, Python, MATLAB, and Abaqus, CFD techniques unravel the complexities of turbulence, multiphase flow, and species transport. The spectrum of plasma behavior equations, including ion and electron densities, electric fields, and recombination reactions, is presented in a holistic manner. The modeling of non-thermal plasma reactors, underpinned by precise mathematical formulations and computational strategies, is further empowered by the integration of machine learning algorithms for predictive modeling and optimization. From biomass gasification to intricate chemical reactions, this work underscores the versatile potential of plasma hybrid modeling in reshaping various industrial processes. Within the sphere of plasma catalysis, modeling and simulation methodologies have paved the way for transformative progress. Encompassing reactor configurations, kinetic pathways, hydrogen production, waste valorization, and beyond, this compilation offers a panoramic view of the multifaceted dimensions of plasma catalysis. Microkinetic modeling and catalyst design emerge as focal points for optimizing CO2 conversion, while the intricate interplay between plasma and catalysts illuminates insights into ammonia synthesis, methane reforming, and hydrocarbon conversion. Leveraging neural networks and advanced modeling techniques enables predictive prowess in the optimization of plasma-catalytic processes. The integration of plasma and catalysts for diverse applications, from waste valorization to syngas production and direct CO2/CH4 conversion, exemplifies the wide-reaching potential of plasma catalysis in sustainable practices. Ultimately, this anthology underscores the transformative influence of modeling and simulation in shaping the forefront of plasma-catalytic processes, fostering innovation and sustainable applications. [ABSTRACT FROM AUTHOR]

Enhancing natural gas storage capacity by utilizing the pore space of sediments in cavern bottoms holds significant potential, particularly in high impurity salt formations. This study presents comprehensive insights into the pore structures of sediment samples through the application of X-ray computed tomography imaging. Three-dimensional reconstruction models were employed to extract and quantitatively analysed the pore network characteristics. Fluid flow simulations were conducted to investigate the non-uniform distribution of the brine velocity field, influenced significantly by particle size. Probability and cumulative distribution curves of pore equivalent radius, pore coordination number, pore throat equivalent radius, and throat length were effectively fitted using the log-normal and Boltzmann functions, respectively. Notably, the permeability of partial packings exhibited a positive correlation with porosity and displayed an upward trend in correlation with particle size. Moreover, during the debrining process, sediments comprising finer particles exhibited a higher susceptibility to blockage, thereby escalating the risk of particle clogging. These findings offer valuable insights for the development of anti-clogging strategies during debrining operations. Highlights: Expanding storage capacity: Storing natural gas in the pore space of sediments in cavern bottoms offers a significant increase in storage capacity in high impurity salt formations. X-ray computed tomography analysis: The use of X-ray computed tomography allowed for the reconstruction and quantitative analysis of the pore structures and pore network models of the sediment samples. Fluid flow simulations: Three-dimensional simulations of fluid flow in the packing samples provided insights into the non-uniform distribution of the brine velocity field and its dependence on particle size. Permeability and particle blockage: The study revealed that permeability of the partial packings exhibited a positive correlation with particle size. Fine particles were found to pose a higher risk of blockage during debrining, highlighting the importance of anti-clogging methods in the process. [ABSTRACT FROM AUTHOR]

The performance of chemical and biological additives in the methane hydrates formation and dissociation processes is of relevance for the development of gas-transport and gas-storage systems. The effect of sodium surfactin, rhamnolipids, and sodium dodecyl sulfate (SDS) on the methane hydrate formation process was assessed in this work at different temperatures and a fixed pressure of 50 bar. The studied parameters were induction time, methane uptake, period to reach 90 percent of the consumed gas, water-to-hydrate conversion, and formation rate. Concentrations for sodium surfactin were 3, 150, 750, 1500, 2000, and 2500 ppm, while rhamnolipids and SDS solutions were analyzed at 1500, 2000, and 2500 ppm. Performance testing of these additives was carried out by means of the isochoric–isothermal method. The experimental setup consisted of an isochoric three-cell array with 300 mL of capacity and magnetic stirring. According to the results, the sodium surfactin promoted the methane hydrate formation since the kinetics were higher and the water-to-hydrate conversion averaged 24.3%; meanwhile, the gas uptake increased as concentration was rising, and the induction time was reduced even at a temperature of 276.15 K. [ABSTRACT FROM AUTHOR]

To study the influence of gas pressure on coal permeability evolution, we conducted experiments on coal samples from the No. 9 coal seam in Tangshan Coal Mine, Hebei Province, China. Different gas pressures (helium and nitrogen) were applied, and nitrogen-induced deformations were measured. We also analyzed the coal samples' pore structure using mercury injection porosimetry, obtaining pore surface fractal dimensions. The increase in nitrogen pressure from 0.3 MPa to 3 MPa resulted in an elevation of adsorption strain from 0.168 × 10−3 to 1.076 × 10−3, with a gradual decrease observed in the extent of this increase. However, the permeability of coal samples initially decreased from 16.05 × 10−18 m2 to 4.91 × 10−18 m2 and subsequently rose to 5.69 × 10−18 m2. Helium showed similar trends to nitrogen, with average permeability 1.42–1.88 times higher under the same pressure. The lowest permeability occurred at 1.5 MPa for helium and 2.5 MPa for nitrogen. Gas absorptivity plays a crucial role in coal permeability evolution. Additionally, we observed coal's compressibility to be 7.2 × 10−11 m2/N and corrected porosity to be 53.8%, considering matrix compression. Seepage pores larger than 100 nm accounted for 80.4% of the total pore volume, facilitating gas seepage. Surface fractal dimension Ds1 correlated positively with micropore volume, while Ds2 and Ds3 correlated negatively with pore volume and gas permeability. [ABSTRACT FROM AUTHOR]

The flow regime of gas migration and production in a low-permeability coal seam includes desorption, diffusion, low-velocity nonlinear seepage, and linear seepage. However, the current field test methods of coal seam permeability in China are all based on linear seepage theory, leaving out the low-velocity nonlinear seepage with gas threshold pressure gradient. Therefore, a field test method for gas threshold pressure gradient and permeability is established in this paper. First, the two upward cross seam boreholes (the piezometric borehole and gas extraction borehole) are constructed in the untapped area of the floor drainage roadway with the low-permeability coal seam. After sealing the holes, the pressure measurement and gas extraction are carried out, respectively. Then, a radial flow mathematical model considering the gas threshold pressure gradient was constructed, and the model was solved by the COMSOL Multiphysics software. Finally, the gas threshold pressure gradient and permeability of the coal seam were obtained by fitting the simulated data of gas pressure and flow to the measured data in the field. The method was applied to the test of gas threshold pressure gradient and permeability before and after the hydraulic punching. Field test results show that this method can more accurately characterize the permeability in the gas extraction process of low-permeability coal seam than the traditional method. [ABSTRACT FROM AUTHOR]

High-quality seismic data is key to determining the success or failure of oil and gas exploration in complex mountain fronts. Observation systems are important factors affecting the quality of the seismic collection. Currently, the seismic collection design of piedmont zones is mainly based on optimizing and analyzing a single collection parameter. The multiparameter optimization is not performed as a whole. Moreover, the collection parameters that significantly impact the imaging quality of piedmont zones are not deeply excavated. This paper first uses the relationship between the acquisition parameters and the signal-to-noise ratio of a previous seismic data analysis to solve this problem and then obtains the prestack offset profiles of different acquisition parameters through forward modeling and prestack offset methods, further analyzes the acquisition parameters that have a more significant impact on imaging quality, uses the degenerative treatment method to process and analyze the newly collected seismic data, and delves into the most sensitive acquisition parameters for imaging quality. The results show that the influence of the gun line distance is the largest, and the influence of the receiving line distance, the number of receiving lines, and the number of receiving channels is reduced. Therefore, the effect of each acquisition parameter on the imaging quality is different, and different acquisition parameters should be selected according to the depth of the target layer. [ABSTRACT FROM AUTHOR]

Understanding hydrocarbon microseepage mechanism is important for interpreting hydrocarbon anomalies in the surface geochemical exploration. An experiment was developed to simulate the vertical microseepage of hydrocarbons with multiple lithological layers. The results revealed that the microseepage below the water table was dominated by buoyancy of microbubbles with diffusion as a secondary mechanism, however, it was dominated by diffusion with little effect by gas convection above the water table. The micro-fractures were the dominant channels for microseepage. A phenomenological model was developed to quantify the vertical microseepage in different media and provided a better tool in a field environment. [ABSTRACT FROM AUTHOR]

The fluid flow behavior, generally referred to as seepage, could determine the hydrocarbon and brine movement behavior. Movable fluid property, as one of the vital parameters for seepage characteristic evaluation, was generally used for tight oil reservoirs' fluid flow ability assessment. The nuclear magnetic resonance technique was used to experiment with movable fluid percentage and movable fluid porosity, which can provide a realistic assessment of the amount of fluid that can flow in the porous media. Other techniques were also used to analyze the main factors in regulating the differences in movable fluid parameters. However, the research about fluid flow behavior was generally based on traditional methods, while the seepage characteristics from the pore-scale view are still a myth. To promote this process, in this study, core samples obtained from the Chang 7 reservoir of the Triassic Yanchang Formation in the Longdong region of Ordos Basin, China, were tested. The results show that the average movable fluid percentage and average movable fluid porosity of the total 16 core samples are 36.01% and 2.77%, respectively. The movable fluid exists mainly in the midlarge pores with the corresponding T 2 relaxation time over 10 ms. T 2 distributions mainly present four typical patterns: (1) bimodal distribution with similar amplitudes of the two peaks (occupying 6.25%), (2) bimodal distribution with higher right peak and lower left peak (occupying 18.75%), (3) bimodal distribution with higher left peak and lower right peak (occupying 56.25%), and (4) unimodal distribution (occupying 18.75%). Pore structure heterogeneity is closely related to the movable fluid parameters; the movable fluid parameters exhibit a relatively good correlation with core throat radius as well as permeability. There is an obvious difference between the movable fluid parameters and the microscopic characteristic factors in tight oil reservoirs due to the difference in physical properties, clay mineral content, microcracks, and pore structure characteristics. This research has provided a new perspective for the movable fluid property evaluation, and the relevant results can give some advice for the oil field development. [ABSTRACT FROM AUTHOR]

Focused fluid flow through sub-seafloor pipes and chimneys, and their seafloor manifestations as pockmarks, are ubiquitous. However, the dynamics of flow localization and evolution of fluid escape structures remain poorly understood. Models based on geomechanical mechanisms like hydro-fracturing and porosity wave propagation offer some useful insights into fluid flow and escape dynamics, but face limitations in capturing features like mobilized granular matter, especially in the upper sediment layers where the link between fracture and pockmark is not always clear. Here, we propose a mathematical model based on the multiphase theory of porous media, where changes in subsurface and seafloor morphology are resolved through seepage-induced erosion, fluidization, transport, and re-deposition of granular material. Through simulation of an idealized scenario of gas escape from overpressured shallow gas reservoir, we demonstrate that our model can capture flow localization and formation of pipes, chimneys, and pockmarks. Our simulations show (1) formation of conical focused-flow conduits with a brecciated core and annular gas channels; (2) pockmarks of W and ring shapes; and (3) pulsed release of gas. Sediment erodibility and flow anisotropy control the morphology of focused fluid flow and escape structures, while permeability shows negligible impact. While the geological setting for this study is theoretical, we show that our results have real-world analogs. [ABSTRACT FROM AUTHOR]

Understanding reservoir, source, seal, migration pathways and trap characterization is the first step towards better energy resource exploitation. Several studies have been conducted in recent decades on the reservoir characteristics of conventional resources. Unconventional resources, on the other hand, have received more attention as alternative future energy solutions to depleted conventional resources. Drilling, exploration, development, and production of conventional and unconventional resources pose technical challenges to the energy industry. This special issue attempts to address the most pressing technical challenges in developing conventional reservoirs (such as sandstone and carbonate reservoirs) as well as unconventional energy sources (e.g., shale gas, shale oil, tight gas sand, coal bed methane, geothermal and gas hydrates). The special issue is composed of 21 papers that are classified into six different groups related to the physical and mechanical properties of reservoirs, petroleum geochemistry, petroleum geology, geological modelling, unconventional resource evaluation and conventional reservoir evaluation. [ABSTRACT FROM AUTHOR]

Based on the latest interpretation results of 2D seismic data, combined with drilling, logging, and geochemical data, the reservoir‐forming conditions of the exclusive economic zone in the Cuba's sector of the Gulf of Mexico (Hereinafter refer to GOM) are studied systematically, and the oil and gas accumulation model in the study area is predicted. The favourable exploration zones are evaluated and optimized. The results show that: ① High quality Upper Jurassic‐Lower Cretaceous and potential Middle‐Lower Jurassic source rocks are developed in the study area. ② A set of Upper Jurassic‐Lower Cretaceous petroleum system and a potential Middle‐Lower Jurassic petroleum system are mainly developed in the study area. ③ At present, the proved petroleum system are found in the Upper Jurassic‐Lower Cretaceous strata. The oil and gas mainly comes from the Upper Jurassic‐Lower Cretaceous source rocks, and the reservoirs are mainly Upper Jurassic‐Lower Cretaceous fractured carbonate. The main seals are inter‐reservoir tight limestone, marl, and Palaeocene mudstone. ④ The thrust structural traps in the footwall of the fold thrust belt on the south side of the Florida platform, the margin zones of the Florida platform and Yucatan platform, and the Middle‐Lower Jurassic structural traps on the south side of the Yucatan platform have superior source‐reservoir‐cap configuration conditions. It is suggested that these areas have good oil and gas exploration potential in GOM. [ABSTRACT FROM AUTHOR]

An experimental system based on the fluid–structure coupling method is established to link the fracture surface with the change of seepage field, and the seepage mechanism of bifurcation fractures under confining pressure is expounded. It is found that the relationship between hydraulic slope (J) and flow volume (Q) appears as an upward convex function as the effective permeability (ke) increases gradually with J. As inlet flow gradually increases, the rising rate of hydraulic slope gradually weakens. Under the action of confining pressure at all levels, there will be a greater closing effect on the side of the bifurcation fracture with a larger bifurcation angle, thereby promoting seepage of the deflection fluid in the other side of the fracture. The load and contact area of the bifurcation fracture surfaces under confining pressure, measured by pressure film, hinder the fluid flow, which is the main reason for the deflection flow behavior of the fluid. The mechanical behavior of the fracture surfaces under confining pressure is such that the microconvex contacts occur first and then the contact process radiates to the surrounding media, forming contact areas with different loads. Deflection flow behavior caused by the fracture contact under confining pressure has positive significance for research on seepage characteristics. [ABSTRACT FROM AUTHOR]

Permeability enhancement of low permeability coal seams is a key tool in coal mine gas extraction and utilization. Numerical investigations are used to analyze the geometric parameters of the drilled gas cavity and the effect of initial pressure on explosion parameters to explore the potential of original gas blasting in enhancing crack penetration in coal seams. On the basis of these analyses, the changes in characteristic parameters of the coal body under blasting pressure are examined. The results show that the maximum explosion pressure tends to be close to the theoretical explosion maximum when the L/Φ is 1.67–2.5, and the mechanical effect of the explosion pressure and surge velocity on the wall reaches the optimum in the experimental group when the L/Φ is 1.67. The initial pressure and the maximum explosion pressure demonstrate a linear positive correlation, and increasing the former can effectively enhance the force applied to the hole wall. During the gas explosion pressurization stage, the cracking range of coal under pressure is 0.05 m, but the duration of peak pressure can extend the range of cracking. The combined effect of the stress and pressure fields causes elastic energy storage in the coal body to fail in the radial region of the hole wall, but regains elastic energy storage as the pressure increases. The effect of fracturing on the radial coal seam permeability of the borehole wall before and after the fracturing effect is expanded by 19.6 times, proving that in situ gas explosion in boreholes can effectively improve the gas seepage characteristics of coal seams and increase the gas recovery rate. The simulations indicate that the application of in situ gas explosion fracturing and permeation technology is limited by the increase in maximum explosion pressure and the cumulative effect time of the peak pressure. This provides a theoretical basis for understanding the constraints of gas explosion fracturing and permeation technology. [ABSTRACT FROM AUTHOR]

Mechanisms related to sub‐seabed fluid flow processes are complex and inadequately understood. Petrophysical properties, availability of gases, topography, stress directions, and various geological parameters determine the location and intensity of leakage which change over time. From tens of seafloor pockmarks mapped along Vestnesa Ridge on the west‐Svalbard margin, only six show persistent present‐day seepage activity in sonar data. To investigate the causes of such restricted gas seepage, we conducted a study of anisotropy within the conduit feeding one of these active pockmarks (i.e., Lunde Pockmark). Lunde is ∼400–500 m in diameter, and atop a ∼300–400 m wide seismic chimney structure. We study seismic anisotropy using converted S‐wave data from 22 ocean‐bottom seismometers (OBSs) located in and around the pockmark. We investigate differences in symmetry plane directions in anisotropic media using null energy symmetries in transverse components. Subsurface stress distribution affects fault/fracture orientations and seismic anisotropy, and we use S‐wave and high‐resolution 3D seismic data to infer stress regimes in and around the active seep site and study the effect of stresses on seepage. We observe the occurrence of changes in dominant fault/fracture and horizontal stress orientations in and around Lunde Pockmark and conclude minimum (NE‐SW) and maximum (SE‐NW) horizontal stress directions. Our analysis indicates a potential correlation between hydrofractures and horizontal stresses, with up to a ∼32% higher probability of alignment of hydrofractures and faults perpendicular to the inferred minimum horizontal stress direction beneath the Lunde Pockmark area. Plain Language Summary: Hydrocarbon gases, mostly methane, leak from the seabed into the ocean at different locations worldwide. Methane is a potent greenhouse gas, therefore understanding the natural processes of methane release has important implications for climate research. We investigate the role of regional and local stresses in controlling an ongoing gas leakage at Vestnesa Ridge, offshore west‐Svalbard. For this, we study the behavior of shear (S)‐waves, recorded by 22 sensors on the seabed, around a geological feature (pockmark) that releases methane at the present day. S‐waves propagate in relation to the orientation of subsurface faults, fractures and stresses. S‐wave data analysis indicates a change of stress in shallow (<150–200 m below the seafloor) sediments across the pockmark. We observe that the preferred orientation of fractures releasing methane into the ocean, matches the orientation of the stresses predicted from the wave analysis. This study advances our understanding of the influence of local and regional geological processes on the release of methane from the seabed. Key Points: The S‐wave analysis using ocean‐bottom seismic (OBS) data indicates seismic anisotropy around a seeping pockmark on the W‐Svalbard MarginThe occurrence and orientation of symmetry planes in shallow anisotropic sediments vary across the pockmarkCombined analyses using S‐wave and 3‐D seismic data suggest that preferred fault and fracture orientations follow local stress conditions [ABSTRACT FROM AUTHOR]

Coal spontaneous combustion is determined by a variety of factors. Testing can describe the changes incoal spontaneous combustion with various factors, however, the dependence of spontaneous combustion on various factors is unclear. Therefore, reliability theory was used to deduce the functional relationship of the dependence of coal spontaneous combustion on various factors, and construct a model algorithm for predicting the probability of occurrence of coal spontaneous combustion, which is adopted to evaluate and rank the degree of influence of various factors on coal spontaneous combustion. Effective prevention methods are proposed by strengthening the role of the most important factors. The results show that, by taking the duration of coal heating to 150°C as the measurement standard of coal spontaneous combustion, the duration of the initial stage of coal heating increases linearly with the increase of specific heat capacity, thermal conductivity, and moisture content of coal. With the increase of oxygen concentration, oxidation rate, and initial temperature of the coal, the duration of the initial stage of coal heating decreases exponentially. With the increase of gas flow seepage velocity in the coal body, the duration of the initial heating stage changes in a parabolic manner. At the same time, the probability of spontaneous combustion decreases exponentially with the increase of specific heat capacity and moisture content of the coal. The probability of coal spontaneous combustion increases linearly with the increase of coal thermal conductivity, oxygen concentration, gas seepage velocity, and rate of oxidation. The dependence of coal spontaneous combustion probability on different factors can be expressed as follows: coal temperature > gas seepage velocity > specific heat capacity > oxidation rate > oxygen concentration > moisture content > thermal conductivity. [ABSTRACT FROM AUTHOR]

In the process of working face mining, the permeability of the coal seam and the crack evolution characteristics of overlying strata are very important for efficient gas drainage. In this study, the distribution characteristics of the stress field and crack field in the working face and their relations are analyzed mainly by 3DEC numerical simulation. Furthermore, combined with the on-site measurement of coal seam stress, gas pressure, and gas seepage in front of the working face and the gas seepage in overlying strata before and after mining, the coupling effect of stress field and gas field and the law of gas migration and distribution in the working face are deeply explored. The results show that the changing trend of gas seepage and gas pressure is controlled by the stress change of the working face, and with the increase of stress, gas pressure and gas seepage also increase. The peak position of gas pressure is the farthest from the coal wall, about 22.5~25 m, followed by the peak of stress and gas seepage. When the permeability of coal and rock mass increases, the gas seepage increases and the gas pressure decreases. The coal seam stress and gas seepage in the working face and gas seepage in the overlying strata fracture zone along the tailgate side are generally greater than those on the headgate side, but the gas pressure is the opposite. Mining cracks and strata separation provide a good channel and space for gas migration and accumulation. Along the strike and tendency of the working face, gas is mainly concentrated in the overlying strata crack space above the separation zone and the roof and overlying strata crack space on the side of the tailgate, respectively. Based on this, the directional borehole gas drainage technology and borehole layout scheme in the fractured zone are put forward, which effectively reduce the gas concentration in the working face by 30~36%. [ABSTRACT FROM AUTHOR]

The nonlinear behaviors in deep shale gas seepage are investigated, involving the non-Darcy effect, desorption, and viscoelasticity. The seepage model accounts for the nonlinear compressibility factor and gas viscosity due to their stronger non-linearity at a high pressure and temperature. The viscoelastic behavior in deep shales, including matrix deformation and proppant embedment, is quantified, and the evolution of the time-varying and pressure-dependent porosity and permeability is derived. A semi-analytical approach with explicit iteration schemes is developed to solve the pressure field. The proposed model and method are verified by comparing the simulation results with the field data. The results show that the gas production contributed by the non-Darcy effect and desorption is much higher in deep shale than in shallow shale. However, Darcy flow contributes 85% of the total gas production of deep shales. If the effect of viscoelastic behavior is neglected, the accumulative gas production would be overestimated by 18.2% when the confining pressure is 80 MPa. Due to the higher pressure and temperature, the accumulative gas production in deep shale is 150% higher than that in shallow shale. This investigation helps to clarify the performance of the non-Darcy effect, desorption, and viscoelastic behavior in deep shales, and the proposed model and approach can facilitate the optimization simulations for hydraulic fracturing strategy and production system due to its high efficiency. [ABSTRACT FROM AUTHOR]

The porous media in tight reservoirs are mainly composed of micro- and nanopores, gas seepage through which is complex, making it difficult to study. Physical simulation using micron tubes is an intuitive and effective method to study the seepage mechanism of tight gas. The lattice Boltzmann method (LBM) is the most effective method for the tight gas seepage simulation, and it has been widely used. Microscale gas seepage simulation experiments and LBM simulations of micron tubes with different inner diameters were performed. The results showed that in micron tubes, the gas flow increases nonlinearly with an increasing pressure gradient. Influenced by compression and rarefaction effects, the degree of the nonlinearity of pressure distribution in series micron tubes increases with inlet pressure. The existence of a connecting channel between parallel micron tubes breaks the linear distribution of pressure in the original micron tubes, and the gas forms a raised relative high-pressure area at the connection of the two micron tubes; the wider the channel, the greater the bulge. The average gas flow rate in the whole micron tube increases with the channel width, and the seepage capacity increases instead of decreases. The diameter change of one micron tube has no effect on the gas flow in the other micron tube. Although the two micron tubes are connected, they are still relatively independent individuals. These research results lay a foundation for the correct understanding of the characteristics and laws of tight gas seepage in the pores of reservoirs at the micro- and nanoscales, and they have important theoretical significance for the study of seepage mechanisms in tight gas reservoirs. [ABSTRACT FROM AUTHOR]

A simplified method of determining lattice Boltzmann boundary conditions based on self-affine microchannels with an inherent roughness in a tight reservoir is presented in this paper to address nonlinear efficiency problems in fluid simulation. This approach effectively combines the influence of rough surfaces in the simulation of the flow field, the description of L-fractal theory applied to rough surfaces, and a generalized lattice Boltzmann method with equivalent composite slip boundary conditions for inherent roughness. The numerical simulations of gas slippage in a two-dimensional plate model and rough surfaces to induce gas vortex reflux flow are also successfully carried out, and the results are in good agreement with the simulation results, which establishes the reliability and flexibility of the proposed simplified method of rough surfaces. The effects of relative average height and fractal dimensions of the rough surfaces under exact boundary conditions and equivalent coarsened ones are investigated from three perspectives, namely those of the average lattice velocity, the lattice velocity at average height position at the outlet, and the coefficient of variation for lattice velocity at average height position. It was found that the roughness effect on gas flow behavior was more obvious when it was associated with the enhanced rarefaction effect. In addition, the area of gas seepage was reduced, and the gas flow resistance was increased. When the fractal dimension of the wall was about 1.20, it has the greatest impact on the fluid flow law. In addition, excessive roughness of the wall surface tends to lead to vortex backflow of the gas in the region adjacent to the wall, which greatly reduces its flow velocity. For gas flow in the nanoscale seepage space, wall roughness hindered gas migration rate by 84.7%. For pores larger than 200 nm, the effects of wall roughness on gas flow are generally negligible. [ABSTRACT FROM AUTHOR]

Seepage gas hydrate systems are abundant around the world. However, it is very challenging to characterize this type of gas hydrate because the internal structure of a seepage structure is difficult to image by the conventional seismic exploration method. Here, a novel approach is proposed to achieve high-resolution seismic imaging of a seepage gas hydrate system related to a pipe structure. Raw seismic data with small-size bins are acquired with a harrow-like acquisition geometry that uses one long cable (up to 2000 m) and multiple short cables (less than 1000 m). A high-frequency GI gun is used as a source. During processing, velocity analysis and footprint attenuation are more focused. Seismic imaging resolution has been greatly improved. A small-scale carbonate layer that is hard to identify on the conventional profile is well revealed on the new profile. The new profiles also better depict the boundary and internal structure of the pipe structure. These improve our understanding of seepage gas hydrate systems. Our approach provides an important complement to conventional offshore seismic exploration. [ABSTRACT FROM AUTHOR]

The Sciacca basin extends in the southwestern part of Sicily and hosts an important geothermal field (the Sciacca Geothermal Field) characterized by hot springs containing mantle gasses. Newly acquired high-resolution seismic profiles (Boomer data) integrated with a multichannel seismic reflection profile in close proximity to the Sciacca Geothermal Field have documented the presence of numerous active and shallow fluid-related features (pipes, bright spots, buried and outcropping mud volcanoes, zones of acoustic blanking, and seafloor fluid seeps) in the nearshore sector between Capo San Marco and Sciacca (NW Sicilian Channel) and revealed its deep tectonic structure. The Sciacca Geothermal Field and the diffuse submarine fluid-related features probably form a single onshore–offshore field covering an area of at least 70 km2. This field has developed in a tectonically active zone dominated by a left-lateral transpressive regime associated with the lithospheric, NNE-striking Sciacca Fault System. This structure probably favored the rising of magma and fluids from the mantle in the offshore area, leading to the formation of a geothermal resource hosted in the Triassic carbonate succession that outcrops onshore at Monte San Calogero. This field has been active since the lower Pleistocene, when fluid emissions were likely greater than today and were associated with greater tectonic activity along the Sciacca Fault System. [ABSTRACT FROM AUTHOR]

After the shale reservoir is hydraulically fractured, the shale gas is transported to the horizontal well through various media. Hydraulic fracturing produces hydraulic fractures and generates secondary fractures. The hydraulic fractures and fracture induction areas are filled with gas and water phases. In this work, using power-law fracture distribution and porous media fractal theory, as well as accounting for seepage mechanisms in both the water and gas phases, a shale multi-scale mining model is constructed. The results of the study demonstrated that: (1) The degree of hydraulic fracturing is determined by a number of factors. An appropriate degree of hydraulic fracturing is more conducive to improving gas production. There is a competitive relationship between shale gas seepage and water reflux. Either above or below the moderate interval may lead to excessive water reflux and affect gas production. (2) Various properties of the water phase also influence the production results and pore structure evolution. A moderate maximum relative permeability of water and non-wetting phase entry pressure can contribute to a good pore structure progression and a more desirable gas production. (3) The contribution of hydraulic fracturing porosity to pore structure evolution is greater than the initial relative permeability of water and non-wetting phase entry pressure. [ABSTRACT FROM AUTHOR]

The study evaluates the geothermal energy potential of two depleted oil and gas reservoirs representing two different lithostratigraphic formations—the carbonate formation of the Visean age from the basement of the Carpathian Flysch and the Rotliegend sandstone formation from the Eastern part of the Foresudetic Monocline, Poland. Advanced modeling techniques were employed to analyze the studied formations' heat, storage, and transport properties. The obtained results were then used to calculate the heat in place (HIP) and evaluate the recoverable heat (Hrec) for both water and CO2 as working fluids, considering a geothermal system lifetime of 50 years. The petrophysical parameters and Hrec were subsequently utilized in the generalized c-means (GFCM) clustering analysis, which helped to identify plays with the greatest geothermal potential within the studied formations. The central block emerged as the most promising area for the studied carbonate formation with Hrec values of ~1.12 and 0.26 MW when H2O and CO2 were used as working fluids, respectively. The central block has three wells that can be easily adapted for geothermal production. The area, however, may require permeability enhancement techniques to increase reservoir permeability. Two prospective zones were determined for the analyzed Rotliegend sandstone formation: one in the NW region and the other in the SE region. In the NW region, the estimated Hrec was 23.16 MW and 4.36 MW, while in the SE region, it was 19.76 MW and 3.51 MW, using H2O and CO2 as working fluids, respectively. Both areas have high porosity and permeability, providing good storage and transport properties for the working fluid, and abundant wells that can be configured for multiple injection-production systems. When comparing the efficiency of geothermal systems, the water-driven system in the Visean carbonate formation turned out to be over four times more efficient than the CO2-driven one. Furthermore, in the case of the Rotliegend sandstone formation, it was possible to access over five times more heat using water-driven system. [ABSTRACT FROM AUTHOR]

The structure of caprocks is often greatly altered by different scales of faults or fissures in long-term geological tectonic evolution, and the sealing performance may be deteriorated. In this paper, a simplified geological model characterized as multilayered sandstone and fault-bearing caprocks extracted from the Shuang 6 underground gas storage located in the Liaohe oilfield was established. Different fault geometry (e.g., fault length, fault dip angle, and fault type) and seepage attributes (porosity and permeability) were considered to illustrate their impacts on natural gas migration during the cyclic high rate of injection and production of natural gas. The results showed that the seepage anisotropy and the natural gas front are strongly affected by the formation properties and, especially, are hindered by the low permeability sandstone layers. The difference in the lateral migration distance of natural gas in different layers can reach 110 m at the end of the injection period, with an annual injection volume of 108 m3. The migration of natural gas along the fault zone is mainly controlled by the permeability of faults, followed by fault scale, fault dip angle, and fault type. The sealing failure of caprocks in the fault zone does not occur based on the simulated gas migration distribution, showing that a very limited amount of natural gas migrates into the caprocks. [ABSTRACT FROM AUTHOR]

For horizontal well in low-permeability gas reservoir, the effects of threshold pressure gradient, stress sensitivity, and gas slippage have significant impacts on the well productivity. At present, there are few productivity formulas for horizontal gas well considering all these factors. In this paper, based on the flow analysis of horizontal well in low-permeability gas reservoir, the whole flow field was divided into two parts, namely, far wellbore region and near wellbore region, among which the far wellbore region is composed of plane linear flow region and plane radial flow region and the near wellbore region is composed of vertical plane radial flow region and spherical plane central flow region. Then, a new productivity formula is established based on the steady-state seepage theory and the equivalent seepage resistance method, with the consideration of threshold pressure gradient, stress sensitivity, and gas slippage effect. The accuracy of the formula in this paper is verified through comparing with other classical models, and the influence of various factors on the well productivity is analyzed. The analysis results show that stress sensitivity has the most significant effect on horizontal gas well production, followed by threshold pressure gradient, and the gas slippage has the least effect. With the consideration of all influencing factors, the higher the formation pressure and reservoir thickness, the higher the productivity, and the increase of productivity increases with the increase of flow pressure difference. The increasing trend of gas productivity index per meter with the increase of reservoir permeability is first fast and then slow. When the reservoir permeability is greater than 1.2 mD, the increment of gas productivity index per meter (MGPI) decreases. When the length of horizontal well is greater than 1400 m, the increment of gas productivity index per meter decreases with the increase of gas reservoir thickness. Therefore, it is recommended to control the horizontal well length within 1400 m in low-permeability gas reservoir. In addition, the absolute open flow charts corresponding to reservoir thickness and horizontal well length under different reservoir permeability conditions were also given, which can provide theoretical guidance for the selection of horizontal well length during the development of low-permeability gas reservoirs. [ABSTRACT FROM AUTHOR]

Submarine methane seepage can potentially be promoted by the dissociation of the marine hydrates surrounding the continental margins due to oceanic warming since the Last Glacial Maximum. This seepage could be archived by authigenic carbonates at seeping sites, but the time lag caused by heat transmission through the sediment column leads to an inconsistency between the ages of the carbonate and the period of bottom water warming. Here we present the records of the authigenic carbonate crust from drilling site D1 in the Mid‐Okinawa Trough. Uranium–thorium dating results show that the carbonate crust mainly grew downwards during 14–6 ka. Gas hydrates hosted in the relatively thin stability zone dissociated in a rapid response to bottom water warming and intensified the methane seepage. Our study better supports the hypothesis that a considerable amount of methane can be released from marine hydrates due to global climatic changes. Plain Language Summary: Methane hydrates are ice‐like crystalline compounds and often found in deep‐ocean marine sediments. Ocean warming in the past could have destabilized methane hydrates and led to a rapid discharge of free methane gas. Geological information on this methane escape could be preserved by carbonate rocks. To date, the hypothesis of the control of ocean warming on hydrate melting since the Last Glacial Maximum (19,000–26,000 years ago) has not been fully tested. To examine this hypothesis, we used the rock samples of seep‐carbonate retrieved by seafloor drilling in the Mid‐Okinawa Trough. Uranium–thorium dating results show that the seep carbonates formed between 6,000 and 14,000 years ago. It took less time for heat to diffuse downwards from warming seawater to trigger hydrate melting at this location than in most of other oceans at similar water depths. The smaller time lag makes the carbonate rock age close to the period of ocean warming. Our results better support that global ocean warming could potentially release methane from gas hydrates during glacial–interglacial transitions. The consequent methane transport into the oceans and likely also the atmosphere might impact ocean acidification and climatic warming. Key Points: Presentation of the chronology of methane seepage by dating authigenic carbonates collected from drilling core D1 in the Okinawa TroughOcean warming–induced hydrate dissociation led to the downward growth of authigenic carbonate for 14–6 ka after the Last Glacial MaximumThis carbonate record better supports the link of hydrate dissociation with ocean warming due to a smaller thermal lag effect [ABSTRACT FROM AUTHOR]

Sub‐seabed fluid flow, gas hydrate accumulation and seafloor methane seepage are tightly interwoven processes with implications for marine biodiversity, ocean chemistry and seafloor stability. We combine long‐offset seismic reflection data with high‐resolution seismic data to investigate shallow structural deformation and its relationship to focused gas migration and hydrate accumulation in the southern Hikurangi subduction wedge. Anticlines, effective traps for focusing free gas, are characterized by both normal faults and vertical zones of hydraulic fracturing within the hydrate stability zone. The normal faults form as a result of sediment layer folding and gravitational collapse of ridges during uplift. We document both longitudinal (ridge‐parallel) and transverse (ridge‐perpendicular) extensional structures (normal faults and elongated hydraulic fracture zones) in the sub‐seafloor of anticlinal ridges. Intriguingly, gas flow through ridges close to the deformation front of the wedge exploits longitudinal structures, while ridges further inboard are characterized by gas flow along transverse structures. This highlights pronounced changes in the shallow deformation of ridges in different parts of the wedge, associated with a switching of the least and intermediate principal stress directions. It is critical to understand these shallow stress fields because they control fluid flow patterns and methane seepage out of the seafloor. Plain Language Summary: Methane gas is released naturally from the seafloor at locations known as "gas seeps." Gas seeps often occur on seafloor ridges, because gas tends to flow into folded sediments beneath ridges. In this study, we explored how geological structures (faults and fractures) beneath seafloor ridges control the flow of gas toward the seafloor. We observed that gas flow beneath ridges is aligned along one of two main orientations: either approximately parallel or perpendicular to the long‐axis of the ridge. Structures aligned parallel with a ridge axis occur beneath ridges near the toe of the accretionary wedge—that is, at the seaward end of the large wedge of deformed sediments that has formed as a result of one tectonic plate sliding beneath another. Structures aligned perpendicular to the ridge occur further landward within the accretionary wedge. The change in the orientation of structures was unexpected, and shows us there are significant differences in the way ridges deform as they develop through geological time. The results are important because geological structures control how and where gas migrates to the seafloor, where it then acts as a food source for diverse biological communities, while also influencing the chemistry of the ocean. Key Points: Gas migration through ridges occurs along both longitudinal (ridge‐parallel) and transverse (ridge‐perpendicular) zones of fracturingShallow stress fields differ significantly between ridges, reflecting differences in ridge evolution and deformationSeismic reflection images of the base of gas hydrate stability and gas‐water contacts are strongly affected by seismic frequency content [ABSTRACT FROM AUTHOR]