The influence of thermal- Hydraulic- mechanical- and chemical effects on the evolution of permeability, seismicity and heat production in geothermal reservoirs

We utilize a numerical model to examine thermal-hydrologic-mechanical- chemical processes leading to the evolution of induced seismicity in naturally fractured dual-porosity media. We use a continuum model to examine the thermohydro-mechanical behaviors due to variation in fluid pressure and thermal stress on different fracture networks within a prototypical enhanced geothermal system (EGS). Discrete penny-shaped fractures are seeded within the reservoir volume with random orientations and a Gaussian distribution of lengths. Failure is calculated from Coulomb strengths. Energy release magnitude is utilized to obtain the magnitude-moment relation for induced seismicity by location and with time. This model is applied to the potential Newberry EGS field (USA) by assuming fracture sizes of 10 to 1200 m. Models are classified by their conceptualization of the fractured reservoir geometry as both networks of discrete fractures and with equivalent fractured media as fill-in. This model is applied to a doublet injector-producer to explore the spatial and temporal triggering of seismicity for varied fracture network geometries at shallow (2000m) and deep (2750) depths. First we consider the identical network of large fractures (300 m fracture spacing) in both shallow and deep zones and infilled with smaller (10-200m) more closely spaced fractures with densities of 0.5 m^-1 in the shallow zone (B) and of 0.9 m^-1 in the deeper zone D. Then we apply a different network where the spacing of the large fractures are halved (∼150m) in both zones but with the small closely spaced fractures retained with densities of 0.5m^-1 in the shallow zone (B) and 0.9m^-1 in the deeper zone (D). We evaluate the magnitude of seismic events that vary from -2 to +1.9 with the largest event size (∼1.9) corresponding to the largest fracture size (∼1200m) within the reservoir. We illustrate that the model with the higher fracture density generates both the most and the largest seismic events (MS =1.9), thus the evolution of seismicity is quickest and migration of seismic events is fastest with radius from the injector compared with the case for more widely-spaced fractures. Rate of hydraulic and thermal transport has a considerable influence on the amount, location and time of failure and ultimately event rate. Thus the event rate is higher when the fracture network has the larger density (0.9m^-1) with closely-spaced fractures (150m) and is located at depth where the initial stresses are highest (zone D). Finally, we evaluate the thermal energy recovered during the production and the results show that the highest thermal energy is recovered from the deeper zone (D) with the more closely-spaced fractures (150m).