Shinsuke Nakao

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 field in Kyushu, Japan, exhibit this peculiar behavior. During injection testing of Yutsubo well YT-2, the 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 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.

Geomechanical Stability Analysis of Reservoir Rock Related to CO2 Injection in Japan

ing “effective stress”, which is acting on the reservoir and cap rock system. The change in effective stress causes the geomechanical response of deep strata, e.g. deformation of rock, induced seismicity. We evaluated the geomechanical stability of reservoir and caprock at Tomakomai candidate site in order to evaluate the effect of planned CO2 injection on geomechanical stability of the reservoir based on “slip tendency”. The slip tendency around injection point where maximum pore pressure built-up was estimated about 0.495, which means geomechanical instability will unlikely occur by planned CO2 injection.