Role of Differential Stress in Induced Seismicity in Enhanced Geothermal System: a Review of Previous Hydraulic Stimulation Tests

Abstract

Massive hydraulic stimulation is the key operation to improve the permeability of deep underground heat exchanger in Enhanced Geothermal System (EGS) and numerous stimulation tests have been performed during the past four decades. The induced seismicity (IS) associated with stimulation operation and its impact is a controversial issue for EGS development. This paper attempts to study the role of differential stress in IS with a geomechanical perspective by investigating previous hydraulic stimulation tests in seven EGS projects where massive volume of fluid was injected into the long open well section with interval of tens to hundreds of meters in the granite formation. Specifically, we focus on the role of differential stress in the onset of seismicity which is thought as an indicator of activation of fracture shear slip and the induced large magnitude events (LMEs) with which the public are concerning. Based on the recognition that fracture slip is controlled by the ratio of shear to normal stress acting on its plane, slip tendency analysis is carried out to provide more insight. The analysis and discussion of previous stimulation tests suggests: 1) the injection pressure for activating shear slip and the associated onset of seismicity is mainly controlled by field differential stress and 2) the differential stress condition is one of the necessary factors to raise LMEs and the amount of maximum injection pressure achieved over that at onset of seismicity is another important factor to induce LMEs.