Correlation Between Joint Roughness Anisotropy on Deep Cores and Seismic Propagation Direction in Pohang EGS Site, Korea

Abstract

South Korea launched its first enhanced geothermal project in 2010 aiming to install MW class binary power plant. Until now, two wells have been drilled and a first hydraulic fracturing stimulation was conducted for 23 days in early 2016. The first well, PX-1, has a measured depth of 4,127 m, while the second well (PX-2), has reached a profundity of 4,348 m. During the completion of PX-2, rock core samples were recovered at a depth of 4,219 m, and they comprise a length of 3.6 m with a diameter of 10 cm with open and closed joints crossing them. Using the X-ray computed tomography, the specimens were scanned to analyze internal features as well as to obtain 3D representations in order to compute physical properties. For this study, nine joint surfaces were extracted, with an approximate angle of 30 degrees in relation to the drilling direction. Among other parameters, the Joint Roughness Coefficient (JRC) was measured at different directions. Nearly all surfaces displayed roughness directionality indicating clear anisotropy in the joint roughness. The average orientation of the maximum JRC values is 161.7 degrees measured anticlockwise from the horizontal, and view from the footwall plane. Furthermore, the JRC directionality was compared with the seismicity propagation direction in the hydraulic stimulation of Pohang PX-2 well and the estimated shearing direction based on the back-estimation of in-situ stress orientation. The estimated shearing direction showed good correlations with the minimum JRC orientation and the seismicity propagation direction, which implies that the analysis with joint roughness directionality can serve as an indirect method to estimate the shearing directions and investigate the orientation of paleo-stress.