Abstract
In the stimulation of fractured geothermal reservoirs, injection wellhead pressure, flow rate and microearthquake (MEQ) data are crucial feedbacks recorded in order to characterize the evolution of subsurface fluid flow. However, one of the hurdles to successful EGS development and operation is the lack of reliable evaluation for the initial and evolving hydraulic properties of the fractured reservoir. Specific spatial conditions (e.g., location and direction) of fracture permeability in the field are vital in defining reservoir response during stimulation and then production. To constrain the evolving permeability, we propose a model that maps the in-situ permeability based onto the Oda crack tensor using the moment magnitude of individual MEQs, assuming that the induced seismicity is controlled by the Mohr-Coulomb shear criterion. The MEQ catalog of locations, fault plane solutions, and moment magnitudes are used to estimate fracture apertures of individual events/fractures that are a dynamic function of in-situ stress, fluid pressure, shear displacement and fracture size. The corresponding in-situ 2D permeability tensors are computed and mapped at various scales within the reservoir. Results suggest that the permeability magnitude largely depends on MEQ moment magnitude and fracture frictional properties while permeability direction is dominantly controlled by fracture orientation. However, uncertainty remains within the results, which need improvements in constraint from laboratory and in-situ fracture characterization, the quality of seismic monitoring and reliability of appropriate assumptions.
1. INTRODUCTION
Enhanced Geothermal Systems (EGS) are engineered reservoirs created to recover the geothermal resource from high temperature but low permeability rock formations. In general, the creation and operation of an EGS project comprises two phases. In the first phase, the reservoir is stimulated to generate sufficient permeability through hydroshearing of pre-existing fractures in the reservoir [1]. During stimulation, elevated fluid pressures induce microearthquakes (MEQs) by decreasing the effective normal stresses on the fracture planes. As microseismic events provide valuable feedback to the stimulation, microseismic monitoring is one of the most effective ways to characterize the underlying active processes and the evolution of permeability in the reservoir. This is accomplished through the use of moment tensors derived for the events [2]. Moment tensors, in turn, may be mathematically interpreted to estimate the geometry of the fracture zone, the orientation of fracture planes and the dynamics of fracture development. In the second phase, a production well is subsequently planned and optimally accommodated according to the monitored spatial distribution of MEQs in order to maximize thermal production. Although there is wide agreement that microseismicity may signal the enhancement of permeability in EGS reservoirs (Figure 1(a)) and it has become a method to optimally locate a production well, the quantification of the assumed enhancement and the physical connections between MEQ data and spatial permeability distribution and evolution are not well constrained.
