Analysis of Injection-Induced Micro-Earthquakes in a Geothermal Steam Reservoir, The Geysers Geothermal Field, California

ABSTRACT: In this study we analyze relative contributions to the cause and mechanism of injection-induced micro-earthquakes (MEQs) at The Geysers geothermal field, California. We estimated the potential for inducing seismicity by coupled thermal-hydrological-mechanical analysis of the geothermal steam production and cold water injection to calculate changes in stress (in time and space) and investigated if those changes could induce a rock mechanical failure and associated MEQs. An important aspect of the analysis is the concept of a rock mass that is critically stressed for shear failure. This means that shear stress in the region is near the rock-mass frictional strength, and therefore very small perturbations of the stress field can trigger an MEQ. Our analysis shows that the most important cause for injection-induced MEQs at The Geysers is cooling and associated thermal-elastic shrinkage of the rock around the injected fluid that changes the stress state in such a way that mechanical failure and seismicity can be induced. Specifically, the cooling shrinkage results in unloading and associated loss of shear strength in critically shear-stressed fractures, which are then reactivated. Thus, our analysis shows that cooling-induced shear slip along fractures is the dominant mechanism of injection-induced MEQs at The Geysers.

1. INTRODUCTION

The Geysers is the site of the largest geothermal electricity generating operation in the world and is also one of the most seismically active regions in northern California [1]. It is a vapor dominated geothermal reservoir system, which is hydraulically confined by low permeability rock units. As a result of high rate of steam withdrawal, the reservoir pressure declined until the mid 1990s, when increasing water injection rates resulted in a stabilization of the steam reservoir pressure. If The Geysers were produced without simultaneously injecting water, reservoir pressures and flow rates from production wells would decline fairly rapidly to uneconomical levels. However, the water injection has also resulted in an increased level of seismicity at The Geysers, which has raised concerns regarding the social, environmental, and economic impacts on the local communities [1]. For public acceptance, a good understanding of the causes and mechanisms of induced seisimicity is important and may pave the way for finding ways to minimize the level of seismicity while optimizing energy production. Over the past 25 years, a number of studies have been made to investigate the correlation between operational data and seismicity at The Geysers [1?9]. Perhaps the most comprehensive study in recent years was made by Mossop [8], who studied the correlation of induced seismicity and operational data from 1976 to 1998. Mossop [8] found three types of induced seismicity of high significance: i) Shallow, production-induced seismicity that has a long time lag, on the order of 1 year; ii) deep, injection-induced seismicity with short time lag, < 2 months; and iii) deep, production-induced seismicity with short time lag, < 2 months that appeared to diminish in the late 1980s. Injection-induced seismicity is typically clustered around injection wells, extending downward in plume-like forms Fig. 1 [9].