The Physical Mechanisms of Geothermal Reservoir During Hydraulic Injection Through Microearthquake Tomography

Abstract

Understanding the physical mechanisms involved during hydraulic stimulation is a key parameter to estimate the fluid flow and permeability increase within the geothermal reservoir. An attempt to infer the mechanical behaviour of the XO geothermal reservoir during injection is performed in this study. Microseismic is a key method for monitoring the percolation of the fluid within reservoir during the injection activity. In this study, we aim to determine the 3D seismic velocity structure of the XO geothermal reservoir using microseismic tomography as well as analyse its mechanical changes due to hydraulic injection using the evolution of the microseismicity. We use the microseismic data from 13 and 16 stations deployed before and after the injection, respectively. A total of 2,827 microseismic events were recorded from 2010 to 2013, in which only 135 microearthquakes were recorded before injection and significantly increased to 2,692 events after hydraulic fracturing. To analyze the mechanical behavior of the reservoir, first the hypocenter location accuracy must improved by using a cross correlation master technique. Then hypocenter relocation as well as the velocity structure is inverted using double difference technique. We use tomographic double difference inversion to determine the structure of Vp, Vs and the ratio Vp/Vs. The results of the 3D velocity model together with the microseismic propagation are used to analyse the changes in mechanical behaviour that occur in the reservoir during and after the hydraulic injection.