Pressure Transient during Cold Water Injection into a Geothermal Reservoir

Influence of Borehole Capacity on Pressure Buildup

Abstract

Numerical simulations of pressure transients during nonisothermal injection tests in geothermal reservoirs were carried out. The downhole pressure response to a cold water injection test (into a hot aquifer) was shown to consist of three portions when pressure disturbance was plotted as function of the logarithm of time. The first is a straight-line portion of which the slope is dictated by the viscosity of hot water. This initial period lasts until approximately one wellbore volume of water has been injected. Next, when cold water begins to enter the formation, pressure begins to increase very rapidly. The slope of the [P-log (t)] curve becomes very steep. Finally, at later times, another asymptotic straight-line region is approached. In the porousmedium case, or the fracturedmedium case when its fracture spacing is not too large, the slope of this late-time asymptotic region is governed by the viscosity of cold water. For larger fracture separations, the slope of this late-time region is governed by the average of injected and reservoir fluid viscosities. An approximate analytical solution for this problem (“borehole capacity effects”) was also developed for the porous-medim and the fractured-medium of large fracture spacing cases.