Reservoir stimulation and induced seismicity: Roles of fluid pressure and thermal transients on reactivated fractured networks

• We show that thermal effects may have important impacts on induced seismicity.
• We identify changes in b-value, timing and distribution of seismicity due to thermal and pressure effects.
• These behaviors are in turn controlled by fracture density, spacing, orientation and geometry.
• Permeability evolution within reservoir at different stress regime.

We utilize a continuum model of reservoir behavior subject to coupled THMC (thermal, hydraulic, mechanical and chemical) processes to explore the evolution of stimulation-induced seismicity and related permeability in EGS reservoirs. Our continuum model is capable of accommodating changes in effective stresses that result due to the evolving spatial variations in fluid pressure as well as thermal stress and chemical effects. Discrete penny-shaped fractures (∼10–1200 m) are seeded within the reservoir volume at both prescribed (large faults) and random (small fractures) orientations and with a Gaussian distribution of lengths and location. Failure is calculated from a continuum model using a Coulomb criterion for friction. Energy release magnitude is utilized to obtain the magnitude-moment relation for induced seismicity by location and with time. This model is applied to a single injector (stimulation) to the proposed Newberry EGS field (USA). Reservoir stimulation is assumed to be completed in four zones at depths of 2000, 2500, 2750 and 3000 m. The same network of large fractures (density of 0.003 m−1 and spacing 300 m) is applied in all zones and supplemented by more closely spaced fractures with densities from 0.26 m−1 (deepest zone) through 0.5 m−1 (shallow zone) to 0.9 m−1 (intermediate depth zone). We show that permeability enhancement is modulated by hydraulic, thermal, and chemical (THMC) processes and that permeability increases by an order of magnitude during stimulation at each depth. For the low density fracture networks, the increase in permeability reaches a smaller radius from the injection point and permeability evolution is slower with time compared to the behavior of the higher density fracture network. For seismic events that develop with the stimulation, event magnitude (Ms) varies from −2 to +1.9 and the largest event size (∼1.9) corresponds to the largest fractures (∼1200 m) within the reservoir. We illustrate that the model with the highest fracture density generates both the most and the largest seismic events (Ms = 1.9) within the 21 day stimulation. Rate of hydraulic and thermal transport has a considerable influence on the frequency, location and time of failure and ultimately event rate. Thus the event rate is highest when the fracture network has the largest density (0.9 m−1) and is located at depth where the initial stresses are also highest. Also apparent from these data is that the closely spaced fracture network with the higher stress regime (at the deeper level) has the largest b-value ∼0.74.