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Fracture injection/shut-in tests are commonly used to measure the state of stress in the subsurface. Injection creates a hydraulic fracture (or in some cases, opens a preexisting fracture), and then the pressure after shut-in is monitored to identify fracture closure. Different interpretation procedures have been proposed for estimating closure, and the procedures sometimes yield significantly different results. In this study, direct, in-situ strain measurements are used to observe fracture reopening and closure. The tests were performed as part of the EGS Collab project, a mesoscale project performed at 1.25 and 1.5 km depth at the Sanford Underground Research Facility. The tests were instrumented with the SIMFIP tool, a double-packer probe with a high-resolution three-dimensional borehole displacement sensor. The measurements provide a direct observation of the fracture closure signature, enabling a high-fidelity estimate of the fracture closure stress (ie, the normal stress on the fracture). In two of the four tests, injection created an opening mode fracture, and so the closure stress can be interpreted as the minimum principal stress. In the other two tests, injection probably opened preexisting natural fractures, and so the closure stress can be interpreted as the normal stress on the fractures. The strain measurements are compared against different proposed methods for estimating closure stress from pressure transients. The shut-in transients are analyzed with two techniques that are widely used in the field of petroleum engineering – the ‘tangent’ method and the ‘compliance’ method. In three of the four tests, the tangent method significantly underestimates the closure stress. The compliance method is reasonably accurate in all four tests. Closure stress is also interpreted using two other commonly-used methods – ‘first deviation from linearity’ and the method of (Hayashi and Haimson, 1991). In comparison with the SIMFIP data, these methods tend to overestimate the closure stress, evidently because they identify closure from early-time transient effects, such as near-wellbore tortuosity. In two of the tests, microseismic imaging provides an independent estimate of the size of the fracture created by injection. When combined with a simple mass balance calculation, the SIMFIP stress measurements yield predictions of fracture size that are reasonably consistent with the estimates from microseismic. The calculations imply an apparent fracture toughness 2-3x higher than typical laboratory-derived values.

The EGS Collab project is performing well-monitored rock stimulation and flow tests at the 10-m scale in an underground research laboratory to inform challenges in implementing enhanced geothermal systems (EGS). This project, supported by the US Department of Energy, is gathering data and observations from the field tests and comparing these to simulation results to understand processes and to build confidence in numerical modeling of the processes. Experiment 1 (now complete) examined hydraulic fracturing in an underground test bed at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, at a depth of approximately 1.5 km in a well-characterized phyllite. Geophysical monitoring instrumentation in six of eight sub-horizontal boreholes monitored stimulation events and flow tests. The other two boreholes were used to perform and carefully measure water injection and production. More than a dozen stimulations and nearly one year of flow tests in the testbed were performed. Detailed observations of processes occurring during stimulation and dynamic flow tests were collected and analyzed. Flow tests using ambient-temperature and chilled water were performed with intermittent tracer tests to examine system behavior. We achieved adaptive control of the tests using close monitoring of rapidly disseminated data and near-real-time simulation. Numerical simulation was critical in answering key experimental design questions, forecasting fracture behavior, and analyzing results. We were successful in performing many simulations in near-real-time in conjunction with the field experiments, with more detailed simulations performed later. The primary objective of Experiment 2 is to examine hydraulic shearing of natural fractures at a depth of 1.25 km in amphibolite at SURF. The stresses, rock type, and fracture conditions are different than in Experiment 1. The testbed consists of 9 boreholes, in addition to two earlier-drilled characterization boreholes. One borehole is used for injection, two fans of 2 monitoring wells have several geophysical measurement tools grouted in, and four open boreholes surrounding the injection hole are adaptively used for production and monitoring. We have encountered approximately five fracture set orientations in the testbed, and designed our testbed accordingly to maximize the potential for shear stimulation. Three stimulations have been performed to date from the injection borehole, each intersecting at least one production borehole. Different methods have been used for each stimulation, including a ramped flow, a high flow rate, and oscillating pressure.

Enhanced Geothermal Systems could provide a substantial contribution to the global energy demand if their implementation could overcome inherent challenges. Examples are insufficient created permeability, early thermal breakthrough, and unacceptable induced seismicity. Here we report on the seismic response of a mesoscale hydraulic fracturing experiment performed at 1.5-km depth at the Sanford Underground Research Facility. We have measured the seismic activity by utilizing a 100-kHz, continuous seismic monitoring system deployed in six 60-m length monitoring boreholes surrounding the experimental domain in 3-D. The achieved location uncertainty was on the order of 1 m and limited by the signal-to-noise ratio of detected events. These uncertainties were corroborated by detections of fracture intersections at the monitoring boreholes. Three intervals of the dedicated injection borehole were hydraulically stimulated by water injection at pressures up to 33 MPa and flow rates up to 5 L/min. We located 1,933 seismic events during several injection periods. The recorded seismicity delineates a complex fracture network comprised of multistrand hydraulic fractures and shear-reactivated, preexisting planes of weakness that grew unilaterally from the point of initiation. We find that heterogeneity of stress dictates the seismic outcome of hydraulic stimulations, even when relying on theoretically well-behaved hydraulic fractures. Once hydraulic fractures intersected boreholes, the boreholes acted as a pressure relief and fracture propagation ceased. In order to create an efficient subsurface heat exchanger, production boreholes should not be drilled before the end of hydraulic stimulations.

We investigate the influence of foliation orientation and fine-scale folding on the static and dynamic elastic properties and unconfined strength of the Poorman schist. Measurements from triaxial and uniaxial laboratory experiments reveal a significant amount of variability in the static and dynamic Young’s modulus depending on the sample orientation relative to the foliation plane. Dynamic P-wave modulus and S-wave modulus are stiffer in the direction parallel to the foliation plane as expected for transversely isotropic mediums with average Thomsen parameters values 0.133 and 0.119 for epsilon and gamma, respectively. Static Young’s modulus varies significantly between 21 and 117 GPa, and a peculiar trend is observed where some foliated sample groups show an anomalous decrease in the static Young’s modulus when the symmetry axis (x3-axis) is oriented obliquely to the direction of loading. Utilizing stress and strain relationships for transversely isotropic medium, we derive the analytical expression for Young’s modulus as a function of the elastic moduli E1, E3, ν31, and G13 and sample orientation to fit the static Young’s modulus measurements. Regression of the equation to the Young’s modulus data reveals that the decrease in static Young’s modulus at oblique symmetry axis orientations is directly influenced by a low shear modulus, G13, which we attribute to shear sliding along foliation planes during static deformation that occurs as soon as the foliation is subject to shear stress. We argue that such difference between dynamic and static anisotropy is a characteristic of near-zero porosity anisotropic rocks. The uniaxial compressive strength also shows significant variability ranging from 21.9 to 194.6 MPa across the five sample locations and is the lowest when the symmetry axis is oriented 45° or 60° from the direction of loading, also a result of shear sliding along foliation planes during static deformation.

The phonon equilibrium distribution function can be obtained by combining the mechanically derived solid elastic energy density with a phonon description. Thermal resistance like scattering changes the energy transfer rate due to a raise in film thickness past the Casimir limit. The procedure for the derivation of the distribution function is discussed.

A bare, four copper layer printed wiring board with simple trace patterns was built for modeling and experimental validation purposes. In-plane elastic properties of the core materials in the board were measured as a function of temperature. Thermoelastic lamination theory was utilized to predict the warpage of the board when subjected to an infrared reflow process, with emphasis on studying the influence of thermal gradients through the board, its support conditions and CTE differential on the warpage process. Board layers with traces were approximated with quasi-homogeneous effective properties obtained using micromechanics theory. An experimental system that employs the shadow moire technique in a simulated infrared reflow environment was used to evaluate the warpage for comparison to modeled results.