Nicholas C. Davatzes, Temple University (PI) - Borehole Geophysics, Geomechanics

Kurt Feigl, University of Wisconsin (PI) - InSAR, Geodetics

Herb Wang, University of Wisconsin (PI) - Poroelasticity

Robert Mellors, LLNL (PI) - Seismology, InSAR

Bill Foxall, LLNL (PI) - Reservoir Mechanics, Seismology, InSAR

Peter Drakos, ORMAT (Co-I) - Geothermal, Geologist

Stimulation and management of enhanced geothermal system (EGS) reservoirs involves manipulating the fluid pressure at depth to create new permeability. These pressure changes cause two distinct effects: (1) slip on fractures that can often be detected as seismic events and (2) expansion or contraction that can be seen as displacements at the ground surface. To date, managing EGS stimulation has relied on monitoring induced seismicity to map the extent of fluid pressure perturbations and to identify the volume of connected porosity that results from opening new fractures or dilation that occurs where fractures slip. These pressure changes occur throughout the lifespan of the reservoir and could reveal the changing geometry of the stimulated fracture network over time including the portion of the stimulated volume participating in subsequent exploitation and the development of short circuits. Thus, the primary objective of the proposed research is to develop a framework and automated tools to monitor stimulation and reservoir management that complements analysis of seismicity to better constrain the geometry of the stimulated volume.

The use of seismicity to map permeability creation presumes that the percolation of the stimulation fluid pressure initiates shear failure of fractures due to the reduction of normal stress resulting in slip and/or creates new opening-mode fractures, and that these failure events are sensible through microseismic monitoring thus revealing the extent of stimulation. However, the volume of increased fluid pressure must first expand to invade well-oriented, highly stressed fractures, which may be preceded by stress changes in the host rock. The expanding pressurized volume will also have a complex relationship to the history of pumping at the surface, the initial permeability structure, and the stresses measured at the well. Currently, no tool effectively provides direct monitoring of the progress of fluid pressure into the natural fracture network or surrounding formation. This problem is exacerbated by the simple fact that the displacement of fracture walls may be either aseismic or below the detection threshold of the local seismic network.

This lack of tools to map the pressurized volume leaves several key issues un-resolved including: (1) How does access to the stimulated volume evolve during and after stimulation? (2) What is the geometry of the pathways that connect pore space enhanced by stimulation? (3) How is induced seismicity related to injection/production and the volume experiencing pore pressure change? (4) What is the driving mechanism of induced seismicity? (5) Does the 3D stress and fracture model from well analysis used in designing the stimulation plan predict the growth of the reservoir?

In order to achieve this objective, we propose to use interferometric synthetic aperture radar (InSAR) to map the surface deformation resulting from fluid injection and extraction at the Brady’s Hot Springs EGS experiment and hence infer the resulting time history of strain and the shape of the stimulated volume at depth using poroelastic modeling and inversion. We will take advantage of new satellites that provide significantly improve spatial resolution over past InSAR studies to monitor deformation in during and for the following three years in order to map the stimulated volume and track its evolution. We will integrate this monitoring with prior deformation associated with production at Brady’s captured by SAR images since 1992 and pumping records from 1992 to the end of the proposed project. Similarly, we will use new techniques to better constrain the lowest detectable magnitude of local seismicity, their locations and focal mechanisms that in conjunction with the InSAR analysis will help elucidate the relationship of seismicity to stimulation.

The research will result in automated tools for determining the geometry of the enhanced reservoir from InSAR and the distribution of induced seismicity as well as provide a post assessment of the Mohr-Coulomb prediction scheme of induced seismicity.