Depletion oil recovery for systems with widely varying initial composition

Abstract The principal depletion drive mechanism is the expansion of oil and gas initially in the reservoir-neglecting water influx. The main factors in depletion drive reservoir performance are total cumulative compressibility, determined mostly by initial composition (gas-oil ratio), saturation pressure, PVT properties, and relative permeability. In this paper, we systematically study the effect of initial composition on oil recovery, all other parameters held constant. We also evaluate other aspects of reservoir performance, but the main emphasis is surface oil recovery including condensate. To analyze the effect of initial composition, a series of fluid systems was selected by a recombination of separator samples at varying gas-oil ratios. The systems ranged from low-GOR oils to high-GOR gas condensates, with a continuous transition from gas to oil through a critical mixture. Black oil and compositional material balance calculations, 2D fine-grid, and 3D coarse-grid models have been used to investigate the effect of initial fluid composition on reservoir depletion performance. Systematic variation of relative permeabilities was also used to map the range of fluid systems, which were most sensitive to relative permeability. For reservoir oils, the depletion recovery of surface oil initially increases with increasing initial gas-oil ratio. Oil recovery reaches a maximum for moderate-GOR oil reservoirs, followed by decreasing oil recoveries with increasing initial solution GOR. A minimum oil recovery is reached at a near-critical oil. For gas reservoirs, depletion drive condensate recovery increases monotonically from a near-critical gas towards near 100% condensate recovery for veryhigh GOR systems. STO recovery from oil reservoirs depends increasingly on gas-oil relative permeabilities as initial solution GOR increases, up to a point. At higher initial solution GORs, oil recovery becomes less dependent on relative permeability and, as the fluid system transitions to a gas at the critical point, relative permeability dependence rapidly diminishes. Condensate recovery from www.elsevier.com/locate/petrol gas condensate systems is more or less independent of gas-oil relative permeabilities, with only slight dependence for nearcritical gases.