Passive Seismic Monitoring: Mapping Enhanced Fracture Permeability

Abstract

The economical performance of a geothermal reservoir strongly depends on how accurate the in situ hydraulic conductivity can be determined. Averaged reservoir conductivity and near-well hydraulic properties can be determined by well test analyses. However, direct monitoring of hydraulic parameters further away from wells remains a challenging task.

We present a new technique for spatially mapping regions of enhanced hydraulic conductivity in EGS (Enhanced Geothermal Systems) reservoirs. Our approach is based on the concept of self-propping of fractures that experienced shear-deformation during hydraulic stimulation. These deformation processes radiate seismic signals (“microearthquakes”) which carry information on the location and the amount of shear displacement that occurred in the reservoir. By mapping the (cumulative) shear deformation of stimulation induced seismicity, a detailed 3-D image of the stimulated reservoir is obtained.

We apply the approach of cumulative shear slip mapping (CSSM) to a field data example in the Cooper Basin EGS reservoir (Australia). During stimulation activities, about 45,300 microseismic events were induced from which approximately 32,200 could be located. Seismic activity aligns along a single, subhorizontal fault. Cumulative slip determined by mapping the slip contribution of individual events onto the fracture zone indicates that most fracture patches in the reservoir slipped repeatedly, accumulating up to several centimetres of shear displacement. Based on CSSM we have determined an optimum location for a new production well in the Cooper Basin reservoir. This well was successfully completed in 2008 and exhibits excellent connectivity to the reservoir.