Analysis of Thermally Induced Permeability Enhancement in Geothermal Injection Wells

AEiSTRACT Reinjection of spent geothermal brine is a common means of disposing of geothermal effluents and maintaining reservoir pressures. Contrary to the predictions of twefluid models (tweviscosity) of nonisothermal injection, an increase of injectivity, with continued injection, is often observed. Injectivity enhancement and thermally-affected pressure transients are particularly apparent in short-term injection tests a t the Los Azufres Geothermal Field, Mexico. During an injection test, it is not uncommon to observe tha t after an initial pressure increase, the pressure decreases with time. As this typically occurs far below the pressure at which hydraulic fracturing is expected, some other mechanism for increasing the near-bore permeability must explain the observed behavior. This paper focuses on calculating the magnitude of the nearbore permeability changes observed in several nonisothermal injection tests conducted at the Los Azufres Geothermal Field. In order to evaluate the pressure transient da ta and calculate the magnitude of the thermally induced permeability changes, a new analytic solution for calculating pressure transients with time-varying sandface flowrates and temperatures has been developed. The effects of temperature-dependent fluid and rock properties, as well as a moving thermal front, are explicitly included in the calculations. Based on this new solution, a technique is developed for calculating the reservoir permeability, skin factor of the well, and near-bore permeability increases. The results of these calculations indicate that the permeability increases bv a factor of 5 in the near-bore Instituto De Investigaciones Electricas Departamento De Geotermia Cuernavaca, Morelos, Mexico