INSTABILITIES DURING LIQUID MIGRATION INTO SUPERHEATED GEOTHERMAL-RESERVOIRS

We examine the stability of a vaporizing liquid front migrating throug h a permeable rock. We show that such liquid-vapor fronts may become u nstable if a sufficient fraction of the liquid vaporizes. This instabi lity is a result of the different speed of the fluid on each side of t he front. We also identify that short-wavelength perturbations are sta bilized by thermal conduction, while long-wavelength perturbations are stabilized as a result of the compressibility of the vapor. Furthermo re, under conditions typical of geothermal reservoirs, where the press ure is similar to 10 atm and the temperature is similar to 200 degrees -300 degrees C, we find that if the permeability of the system is smal ler than similar to 10(-15) m(2), then these two stabilizing mechanism s overlap and the system becomes stable to perturbations of any wavele ngth. We also examine the role of gravity in suppressing the instabili ty of an ascending front and promoting the instability of a descending front. We apply our results to the vapor-dominated geothermal reservo irs at Larderello, Italy, and the Geysers, California, to predict cond itions under which liquid fronts, advancing into superheated vapor, ar e stable.