Flow Rate Effect on Retrograde Condensate Dropout in Trinidad Gas Reservoirs

Abstract

The gas reserves off the East coast of Trinidad contain a significant proportion of gas condensate fluids. When these fluids are produced to surface, the components can be separated by fractionation into gas and liquid fractions at the surface facilities. The premium components, propane and butane, generate an additional source of income. However, if the pressure drops to below the dew point in the reservoir, some of the heavier hydrocarbon components will condense and be trapped in situ rather than be produced to surface, with a substantial loss in revenue due to non-production of these components. Reservoir engineers prevent this by maintenance of pressure by gas or water injection, but economic analysis usually shows that, if the oil/gas ratio is less than 50 barrels per million standard cubic feet (scf) of reservoir fluid, pressure depletion is the overall most economical route than any pressure maintenance scheme. This is because the cost of the extra equipment and operation expenses are not recovered by the sale of the produced heavier components. Some of these valuable premium components are then left in the reservoir. All of Trinidad's gas condensate reservoirs are currently produced by pressure depletion, as their oil/gas ratios are significantly less than 50 barrels per million standard cubic feet. However, the volume of condensate production at surface has shown unusual behavior as it is a reversible function of flow rate and not pressure direct. This article provides an insight into the PVT properties of condensate reservoirs relevant to the nature of the behavior of Trinidad's reserves, and considers how a producer may obtain the contracted rates of gas delivery of these high flow rate wells (>100 million scf/day), but still have maximum economic benefits from condensate recovery.