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

Abstract

Summary Numerical studies were performed to investigate the effects of localized feed zones on the pressure transients in two-phase reservoirs. Gravity effects were shown to affect the pressure transients significantly because of the large difference in the densities 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 as a result of gravity drainage of cooler liquid water. The vapor zone acts as a constant-pressure boundary and helps to stabilize the pressure decline in the system. The pressure transients at observation wells were shown to depend greatly on the location (depth) of the major feed zone; 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 caused by production at a deep feed zone.