Fracture and permeability evolution of geothermal reservoirs using elastic moduli and elastic wave velocity

Seismic based geophysical methods are seeing increased usage in evaluating geothermal resources in order to maximize resource potential. However, interpreting geophysical data (such as modulus and fracture density/alignment) generated from geothermal reservoirs remains difficult. This project focuses on producing new laboratory data of seismic attributes of fresh and hydrothermally altered rocks from a Philippine geothermal field (Southern Negros Geothermal Project - SNGP). Two types of rocks were obtained by sub-coring samples of low porosity (~1%) andesite and higher porosity (~10%) volcaniclastic samples from the SNGP. The formation of shear fractures that weakens the material decreases the attributes while increasing pressure conditions that strengthens the material via fracture closure increases the overall properties.

Samples were prepared with two offset drill holes to allow a natural fracture to permit fluid flow along the fracture. An embedded array of Acoustic Emission (AE) sensors allows elastic wave and induced microseismic data to be collected. Although not the focus of this study, monitoring of geothermal reservoirs with seismic sensors (AE in the laboratory) has the potential to resolve both the 4D source and character of the event (e.g. source mechanism, frequency vs magnitude). Larger scale implementation of this would therefore be of benefit in understanding associated geohazards in high temperature geothermal fields.