Pressure-transient behavior during cold water injection into geothermal wells

During injection testing\ the pressures in geothermal wells used for reinjection sometimes initially increase but then decline as injection continues[ Injection tests carried out at the Yutsubo geothermal _eld in Kyushu\ Japanexhibit this peculiar behavior[ During injection testing of Yutsubo well YT!1the observed downhole pressures eventually began to decline despite sustained injection rates[ We have carried out numerical simulation studies using a radial ~ow model to examine this behavior[ Double porosity "MINC# models are adopted\ in which the fracture porosity increases as a result of both cooling and pressure build!up\ 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[ 0887 CNR[ Published by Elsevier Science Ltd[ All ri`hts reserved