Effects of Steam/Liquid Counterflow on Pressure-Transient Data From Two-Phase Geothermal Reservoirs

Numerical studies are performed to investigate the effects localized feedzones on the pressure transients in two-phase reservoirs. It is shown that gravity effects can significantly affect the pressure transients, because of the large difference in the density of liquid water and vapor. Production from such systems enhances steam/liquid water counterflow and expands the vapor-dominated zone at the top of the reservoir. Subcooled liquid regions develop in the center of the reservoir due to gravity drainage of cooler liquid water. The vapor zone will act as a constant pressure boundary and help stabilize the decline in the system. It is shown that the pressure transients at observation wells depend greatly on the location (depth) of the major feedzone; if this is not accounted for, large errors in deduced reservoir properties will result. At shallow observation points pressures may actually increase as a result of enhanced steam upflow due to production at a deep feedzone. 12 refs., 17 figs.