Stress Measurement Using Rock Core in an HDR Field

Abstract

If a geothermal reservoir consists of fractures, it will be significantly important to know the stress state of the area of interest of the geothermal field because the behavior of underground fractures is greatly controlled by the in-situ stress state and any fractures could contribute to production of geothermal fluid. Two core-based stress measurement methods (DSCA and ASR) were conducted in the Hijiori HDR test site in Japan to study applicability and reliability of these methods. DSCA is considered to show a good result in aspect of principal stress directions. The E-W direction is thought to be strongly compressed at the site from the DSCA result and other investigations. Principal stress ratios estimated from DSCA fall into a reasonable range of values though there are not enough reliable data for comparison. ASR is also considered to be applicable to an HDR field, but measured anelastic strain was very small. In this ASR study, the E-W direction is estimated to be strongly compressed. This direction of major horizontal stress is also consistent with various investigations, including DSCA, for the upper reservoir. At present, DSCA is considered to be worth studying further and the ASR method needs more improvement for use in geothermal fields.