Evolution of Induced Seismicity Due to Interactions between Thermal, Hydraulic, Mechanical and Chemical Processes in EGS Reservoirs

摘要

We explore the complex interaction of coupled thermal, hydraulic, mechanical and chemical (THMC) processes that influence the evolution of EGS reservoirs in general, and in particular with reference to strong, low-permeability reservoirs with or without relic fracturing. We define and describe dominant behaviors that evolve with the evolution of the reservoir: from short-term stimulation through mid-term production and culminating in long-term decline. The injection of fluid under pressure in a rock mass may change the effective stress at early times and result in micro seismicity induced by shear events on reactivated fractures. Changes in thermal stress and chemical changes in the mid- to long-term injection period may also generate seismic activity at later times. In most geothermal reservoirs the induced seismicity results from fluid injection and migrates within the reservoir with time as driven by the various interactions of thermal, hydraulic, mechanical and chemical processes. These processes migrate through the reservoir as fronts at a variety of different length-scales and timescales. We use a continuum model of reservoir evolution subject to coupled THMC processes to explore the evolution of stimulation- and production-induced seismicity in a prototypical EGS reservoir. The model which is discussed here is capable of accommodating changes in stress that result from change in fluid pressure as well as thermal stress and chemical effects. This model is applied to both a single injector and doublet geometry to explore the spatial and temporal migration for triggering of seismicity as stimulation evolves into production. We use varied fracture network geometries in our models to examine various stimulation and production scenarios. The individual models are realized by different fracture density, fracture distribution (~lm to 100m) and spacing between fractures (~lm to 10m). The approach is successfully calibrated against short-term observations in the Cooper Basin (Australia) and applied to explore the expected evolution of moment magnitude and the triggering of seismicity. Modeled b-values (~0.68 to 0.72) at different locations and times are in good agreement with observations (~0.7 to 0.8). For longer injection periods, predicted changes in energy release generate moment magnitudes which vary from -2 to 2 for small to large fractures. Tracking of the hydrodynamic and thermal fronts illustrates a transition in the triggering of seismicity with time. At early time (days to months) - higher flow rates driven by the fluid pressure result in, larger magnitude events. For later time (>1year) thermal drawdown and potentially chemical influences principally trigger the seismicity but result in a reduction in both the number of events and their magnitude. As a result of this decrease in the number of events (both large and small) both b- and a-values decrease with time.