Gas Injection IOR for Widely Varying Initial Compositions

Abstract

Abstract The paper presents the effect of initial reservoir fluid composition on oil recovery for reservoirs under gas injection. To analyze the effect of initial fluid composition, a series of fluid systems were selected based on isothermal gradient calculation from a North Sea field. The systems ranged from low-GOR oils to high-GOR gas condensates, with a continuous transition from gas to oil through a critical mixture. In this compositional reservoir simulation study, a 3D dipping reservoir with dip angle of 3.8 degree was used. The reservoir layer permeabilities were varied based on Dykstra-Parsons model. Different average reservoir permeability was used to quantify the effect of gravity. For oil reservoirs, the oil recovery of oil increases with increasing initial solution gas-oil ratio. The oil recovery increases gradually for low-GOR to moderate-GOR oil reservoirs. For moderate-GOR to near-critical oil, the oil recovery increases rapidly. The increase in oil recovery as the reservoir oil becomes more volatile is due to decrease in oil viscosity and the vaporization effects of the injection gas. For gas reservoirs, the condensate recovery increases rapidly from a near-critical gas towards near 100% condensate recovery for high-GOR systems. The oil recovery also depends on the permeability distribution. The oil recovery is higher in the case of high permeability at the bottom than high permeability at the top due to gravity segregation effect. The effect of gravity segregation on oil recovery is more pronounced in high permeability reservoirs. For lean gas condensate the oil recovery is almost independent on the permeability distribution. The paper shows the variation of oil recovery with initial reservoir fluid composition for different permeability distributions, gas injection period, critical saturation, vertical to horizontal permeability ratio, level of pressure maintenance, and injection gas composition.