THE eFFECTS OF ADSORPTION ON INJECTION INTO GEOTHERMAL RESERVOIRS

The effects of an adsorbing phase on the injection of liquid and eventual production of vapor from a lowporosity, vapor dominated geothermal reservoir was studied. The magnitude of delay caused by adsorption, diffusion partitioning, preferential partitioning, and permeability variation were compared. Results were then compared to measured tracer production data at the Geysers to determine the most likely delay mechanism for injected tracer at the Geysers. A one-dimensional numerical model describing vapor flow in a porous medium in the presence of a sorbing phase was used to investigate the delay of injected tracer caused solely by the sorbing phase. An analytical model was used to describe delay effects due to diffusion partitioning of tracer from the vapor phase into the liquid phase. Properties of steam and tracer used in Geysers tracer studies were compared to determine the effects of preferential partitioning. Finally, a streamline model of a tracer study was used to determine the magnitude of permeability delays possible using permeability values measured at the Geysers. It was concluded that adsorption alone has very little effect bn the delay of injected tracer indicating that little recharge of the adsorbed mass occurs for a typical injection program at the Geysers. Diffusion partitioning was shown to have a larger effect on tracer delay than adsorption while preferential partition was shown to have no effect. Permeability variation was shown to have the largest effect on tracer delay. Tracer delay was shown to be approximated closely by known permeability variations even when adsorption and diffusion effects are ignored. indicate that at high pressures, the low porosity portions of the Geysers reservoir may be almost completely saturafed by an adsorbed phase (Shang, et. al., 1993). These same measurements have also provided increasingly accurate descriptions of adsorption isotherms in the Geysers allowing increasingly accurate estimates of the effects of an adsorbed phase on reservoir performance for a wide range of pressures. Due to pressure decline in the Geysers, injection programs have been undertaken in a number of areas in the reservoir. The purpose of these injection programs is to increase reservoir pressure, replace produced liquid mass, and iultimately increase production rate and the productive life of the reservoir. In order to assess the effectiveness of injection programs it is necessary to closely monitor the propagation of injected liquid in the reservoir. Monitoring of injected fluid may be accomplished either by monitoring production and pressure response in the reservoir or b y introducing tracer along with the injected liquid and studying the production of the tracer. Production of the tracer provides information on the propagation of injected liquid in the reservoir. In the Geysers, several tracer tests have been carried out and tracer production has been used to infer characteristics of the fluid propagation through the reservoir. In general, production of tracer at the Geysers has been characterized by long production times indicating some delay mechanism which significantly spreads the tracer concentration in the reservoir. It is the purpose of this paper to investigate the possible tracer delay mechanisms at the Geysers and to determine the size of delay of each. Specifically, the effects of adsorption on tracer delay are investigated to determine the influence of adsorption on injection programs, at the Geysers.