Special Considerations in Predicting Reservoir Performance of Highly Volatile Type Oil Reservoirs

Abstract

Introduction In estimating production gas/oil ratios and oil recoveries from reservoirscontaining highly volatile oils it is highly important to include condensatethat may be recovered from the gas produced from the reservoir. The volume ofthe hydrocarbon liquid condensed from the production of the solution gas thathas been liberated in some reservoirs may equal or even exceed the volume ofrecoverable stock-tank oil as estimated by present methods. The usual procedurein estimating future reservoir performance includes use of relativepermeability data of the reservoir rock to gas and oil and includesmaterial-balance calculations utilizing data on the volume of the reservoirfluids, oil and gas production and reservoir pressure decline, and data on thephysical properties of the initial reservoir fluids. The inaccuracy in theprocedure results from the false assumption that all of the free or liberatedgas that enters the well bore of reservoirs containing highly volatile oilsremains in the gaseous phase as it is produced. This paper presents a method for estimating future reservoir performance andoil recoveries based on special laboratory analyses of reservoir-oil samplesand recognition of the additional volume of hydrocarbon liquid that will berecovered at various stages of depletion from a solution-gas-drive reservoir.The method also includes calculations of the volume and composition of thehydrocarbon liquids that can be recovered by processing the produced gas in anatural-gasoline plant. The development of the method resulted from a study oflaboratory analyses of reservoir samples and of field data obtained from tworeservoirs containing oil of the highly volatile type. Relatively high pressures and temperatures prevail in oil reservoirsdiscovered at the greater depths of present-day drilling. The contained fluidsmay have high saturation pressures and large volumes of gas in solution. Theoils and gases under these conditions may be almost identical in compositionsand densities. Also, engineers have noted that, for a number of oil reservoirsand especially the deeper reservoirs, the API gravity of the stock-tank oilincreases as the production gas/oil ratios increase. They attributed thisincrease in API gravity of the stock-tank oil to the formation of condensate asthe liberated gas in the reservoir is being produced. T.P. 3017