Permeability Change of Rock Fractures Estimated from the Scale of Microearthquakes in Geothermal Reservoir

Abstract

Despite its importance, the relation between microearthquakes (MEQs) and changes in fracture permeability during hydraulic stimulation of a geothermal reservoir has rarely been explored, and it is still not well understood. To investigate this relation, we first formulate a plausible scale dependence in fracture permeability for joints and faults. By combining this formulation with the concept of the seismic moment, we derive quantitative relations between the moment magnitude (Mw) of MEQs and the fracture permeability change in the directions orthogonal to (kfault,Û/kjoint) and parallel to (k fault,a/kjoint) the shear displacement, in the form wMjofaultkk46.0int,104.116~=Û and wMjofaultkk46.0int//,101.13~=. Despite the simplicity of the derivation, these relations have the potential to explain the results of field experiments on hydraulic stimulation, such as the enhanced geothermal systems at Soultz-sous-Foret and Basel. Additionally, we briefly introduce our recent project on exploring the link between MEQs and permeability change on the basis of rock deformation experiment under HP-HT conditions.