Seismic based geophysical methods are seeing increased usage in evaluating geothermal resources in order to maximize resource potential. However, interpreting geophysical data (such as velocities and dynamic modulus and fracture density/alignment) generated from geothermal reservoirs remains difficult. Here we present the results of a new laboratory study measuring seismic attributes of fresh and hydrothermally altered rocks from a Philippine geothermal field (Southern Negros Geothermal Project - SNGP) during triaxial deformation. Two types of rocks were obtained by sub-coring samples of low porosity (~1%) andesite and higher porosity (~10%) volcaniclastic samples from the SNGP. Samples were prepared with two offset drill holes to allow a natural fracture to permit fluid flow along the fracture. An embedded array of Acoustic Emission (AE) sensors allows elastic wave and induced microseismic data to be collected. We measure a significant reduction in elastic wave velocities and moduli, with the exception of Poisson's ratio, after shear fracture development. An initially pre-fractured permeability of approximately 10−17 m2 is measured. We find that the permeability decreases from 2.0 × 10−14 m2 to lower than 7.4 × 10−15 m2 as the confining pressure is increased from 5 MPa to 30 MPa. A concomitant increase in P and S-wave velocities, dynamic bulk and Young's moduli are also measured. Finally, we simulate a geothermal ‘venting’ situation by intentionally releasing the high pore fluid (water) pressure from 10 to 50 MPa to ambient pressure, generating a swarm of AE that increases in duration with higher pore pressure. We postulate that this is due to fluid phase change (liquid to gas) and movement along the natural fracture plane and damage zone.

A number of key processes, both natural and anthropogenic, involve the fracture of rocks subjected to tensile stress, including vein growth and mineralization, and the extraction of hydrocarbons through hydraulic fracturing. In each case, the fundamental material property of mode‐I fracture toughness must be overcome in order for a tensile fracture to propagate. While measuring this parameter is relatively straightforward at ambient pressure, estimating fracture toughness of rocks at depth, where they experience confining pressure, is technically challenging. Here we report a new analysis that combines results from thick‐walled cylinder burst tests with quantitative acoustic emission to estimate the mode‐I fracture toughness (KIc) of Nash Point Shale at confining pressure simulating in situ conditions to approximately 1‐km depth. In the most favorable orientation, the pressure required to fracture the rock shell (injection pressure, Pinj) increases from 6.1 MPa at 2.2‐MPa confining pressure (Pc), to 34 MPa at 20‐MPa confining pressure. When fractures are forced to cross the shale bedding, the required injection pressures are 30.3 MPa (at Pc = 4.5 MPa) and 58 MPa (Pc = 20 MPa), respectively. Applying the model of Abou‐Sayed et al. (1978) to estimate the initial flaw size, we calculate that this pressure increase equates to an increase in KIc from 0.36 to 4.05 MPa·m1/2 as differential fluid pressure (Pinj − Pc) increases from 3.2 to 22.0 MPa. We conclude that the increasing pressure due to depth in the Earth will have a significant influence on fracture toughness, which is also a function of the inherent anisotropy.

Seismic geophysical methods are not commonly applied in geothermal energy exploration and development compared with oil and gas. However, the use of such methods and models has the potential to greatly improve the use of routinely measured surface networks in geothermal areas in terms of seismic attribute changes in sace and time, with the aim of improving our understanding of deep geothermal systems and processes. To date, linking rock physics properties to such geophysical data has been rare. This would be especially beneficial in terms of evaluating enhanced permeability and fracture density to parameters such as elastic wave velocity and elastic models since the effects of geothermal processes to field seismicity and seismic attributes remain a challenge. Here we present the results of a new laboratory study that examines the seismic attributes of fresh and hydrothermally altered rocks from a Philippine geothermal field (SNGF) while undergoing shear fracture development and permeability evolution via tri-axial deformation experiments. Cylindrical samples of 100 by 40 mm cylindrical were prepared and two notches cut to stimulate shear fracture formation in a known 30-degree plane. This fracture was accessed by pore fluid via two pre-cut miniature boreholes drilled at both ends of the samples along the length and offset with each other. Using these, the formation and evolution of a natural fracture and damage zone and resulting permeability can also be examined. This setup allows the experiment to gather a range of micro-seismic data and elastic wave velocities in different ray paths and key rock physics attributes (e.g. static and dynamic moduli) whilst the fracture permeability develops. Initial results reveal a significant reduction in all seismic attributes (P/S-wave velocities and elastic moduli) after fracture development except for Poisson's ratio which shows the opposite. This opposite signature is again observed whilst increasing the confining pressure after failure and positively affecting most seismic attributes while negative on Poisson's ratio. Further, reduction in fracture permeability coincides with increasing P and S wave velocities, dynamic bulk modulus, Lame's first coefficient and Young's modulus. Pilot high temperature experiments focusing on Acoustic Emission show significant occurrence of events during and after pore fluid decompression that results from rapid fluid movement, and phase changes from liquid to gas, in the damage zone. The resulting data trends are correlated to each other to establish relationships between properties. With this, we present a link between temperature and fracture permeability and how this is manifested in terms of seismic data. The results of the study will be important in interpreting surface seismic models of volcanic geothermal fields in the Philippines and how it relates to crucial reservoir parameters.

The mechanical dynamics of volcanic systems can be better understood with detailed knowledge on strength of a volcanic edifice and subsurface. Previous work highlighting this on Mt. Etna has suggested that its carbonate basement could be a significant zone of widespread planar weakness. Here, we report new deformation experiments to better quantify such effects. We measure and compare key deformation parameters using Etna basalt, which is representative of upper edifice lava flows, and Comiso limestone, which is representative of the carbonate basement, under upper crustal conditions. These data are then used to derive empirical constitutive equations describing changes in rocks strength with pressure, temperature, and strain rate. At a constant strain rate of 10‐5 s ‐1 and an applied confining pressure of 50 MPa, the brittle‐to‐ductile transitions were observed at 975 °C (Etna basalt) and 350 °C (Comiso limestone). For the basaltic edifice of Mt. Etna, the strength is described with a Mohr‐Coulomb failure criterion with μ ~ 0.704, C = 20 MPa. For the carbonate basement, strength is best described by a power law‐type flow in two regimes: a low‐T regime with stress exponent n ~ 5.4 and an activation energy Q ~ 170.6 kJ/mol and a high‐T regime with n ~ 2.4 and Q ~ 293.4 kJ/mol. We show that extrapolation of these data to Etna's basement predicts a brittle‐to‐ductile transition that corresponds well with the generally observed trends of the seismogenic zone underneath Mt. Etna. This in turn may be useful for future numerical simulations of volcano‐tectonic deformation of Mt. Etna, and other volcanoes with limestone basements.

Unconventional hydrocarbon resources found across the world are driving a renewed interest in mudrock hydraulic fracturing methods. However, given the difficulty in safely measuring the various controlling factors in a natural environment, considerable challenges remain in understanding the fracture process. To investigate, we report a new laboratory study that simulates hydraulic fracturing using a conventional triaxial apparatus. We show that fracture orientation is primarily controlled by external stress conditions and the inherent rock anisotropy and fabric are critical in governing fracture initiation, propagation, and geometry. We use anisotropic Nash Point Shale (NPS) from the early Jurassic with high elastic P wave anisotropy (56%) and mechanical tensile anisotropy (60%), and highly anisotropic (cemented) Crab Orchard Sandstone with P wave/tensile anisotropies of 12% and 14%, respectively. Initiation of tensile fracture requires 36 MPa for NPS at 1-km simulated depth and 32 MPa for Crab Orchard Sandstone, in both cases with cross-bedding favorable orientated. When unfavorably orientated, this increases to 58 MPa for NPS at 800-m simulated depth, far higher as fractures must now traverse cross-bedding. We record a swarm of acoustic emission activity, which exhibits spectral power peaks at 600 and 100 kHz suggesting primary fracture and fluid-rock resonance, respectively. The onset of the acoustic emission data precedes the dynamic instability of the fracture by 0.02 s, which scales to ~20 s for ~100-m size fractures. We conclude that a monitoring system could become not only a forecasting tool but also a means to control the fracking process to prevent avoidable seismic events.