Pressure-Transient Behavior During Cold Water Injection

Abstract

During injection testing, pressures in geothermal wells sometimes decline after an initial period of increase despite continued injection. The injection tests carried out at the Yutsubo geothermal field in Kyushu, Japan exhibit this peculiar behavior. During injection testing of Yatsubo well YT-2, observed downhole pressures eventually began to decline despite sustained injection rates. We have carried out numerical simulation studies using a radial flow model to examine this behavior. Double porosity ("MINC") models are adopted, in which the fracture porosity increases due both to cooling and to pressure buildup, and the permeability is very sensitive to porosity changes. This extreme sensitivity of fracture permeability to porosity appears to be necessary to reproduce the late-time pressure decline, and suggests that fractures were opened by injection-induced cooling near the well.