Waterflooding of Gas-Condensate Reservoirs

Abstract

Summary Gas-condensate reservoirs are usually produced by primary depletion. This technique is normally an efficient means of producing the gaseous hydrocarbon components but can be very inefficient in producing the more valuable liquid components that are left in the reservoir in a condensed liquid phase (oil). The recovery efficiency of the liquid components decreases with increasing richness of the gas condensate, making for a large improved oil recovery (IOR) target in some reservoirs. The usual approach to improving liquid recovery is to recycle produced gas through the reservoir. However, this technique may not be economically attractive when there is the possibility of immediate gas sales because of the discounting applied to the gas value when sales are delayed. An alternative means of improving liquid recovery is to keep the reservoir pressure above the dewpoint for a period by injecting water. Depending on reservoir characteristics, water injection may be continued throughout field life or the reservoir may be pressure depleted after a period of injection. Special relative permeability data, describing the mobilization of waterflood trapped gas by expansion, are necessary for the latter case. This paper reports a simulation study that quantifies the potential benefits of the waterflood technique by use of simple reservoir models. For a fluid with a condensate to gas ratio (CGR) of 180 STB/ MMscf, total hydrocarbon recovery was optimized by injecting 0.25 hydrocarbon PV of water before pressure depletion. This increased the recovery efficiency of both the liquid and gaseous components, raising the total hydrocarbon recovery by 10% of the hydrocarbon mass initially present in the reservoir. For a richer, near-critical fluid with a CGR of 300 STB/MMscf, continued water injection gave the optimum total recovery, which was 21% of initial mass higher than for primary depletion. This improvement was achieved by greatly increasing the liquids recovery at the expense of a smaller reduction in the gas recovery. The results of this paper suggest that waterflooding of gas-condensate reservoirs might be a valuable IOR technique.