Study of Compositional Processes in Oil Recovery from Andrew Field due to Submiscible Gas Injection

Abstract

Abstract This study is part of a development study to investigate the mid and late life options for the Andrew Field. The focus was placed on the benefit of Compositional Simulation compared to the Black Oil Simulation in predicting the recovery of condensate from gas cap and residual oil in water-invaded area of oil-leg. The impact of converting a former horizontal producer, which has been shut in for a period of time due to high water-cut, to gas injection was considered. One dimensional (1D) simulation models of the oil-leg were used to ascertain the Minimum Miscibility Pressure (MMP) with injected gas and the nature of displacement mechanism. Different grid block dimensions were used to consider the effect of numerical dispersion on MMP. A pressure of about 6000 psi, significantly higher than current reservoir pressure, would be needed to produce a fully miscible system. A Minimum Miscibility Enrichment (MME) analysis was carried out whereby a conceptual addition of 50:50 mixture of C3:C4 components in terms of molar proportion as NGL was added to the injected gas stream. NGL enrichment of 35% and above results in a miscible injectant at saturation pressure. Strongly vaporising gas drive process dominates in all cases and therefore continuous injection of lean gas will provide sub-miscible displacement of residual oil. 1D Compositional and Black Oil Simulation models of the gas cap region were used to measure the condensate recovery of the gas cap under different possible depletion scenarios. Condensate recovery due to primary depletion is 40% whilst different gas injection plans have resulted in higher condensate recovery, an average around 85%. Three dimensional (3D) Black Oil and Compositional sector models of a pattern element were used to investigate the benefit of converting a producer to a gas injection well, to mobilize the residual oil in the water-invaded area. Results indicate substantial residual oil vaporization and condensate recovery by Compositional model, which is not predicted by the Black Oil model. There is reservoir heterogeneity due to the different structural setting and faulting in upper and lower hydrocarbon bearing units. Injecting to upper unit rather than the lower unit maximizes condensate recovery. Residual oil recovery by injecting to the lower unit is higher than the upper part due to higher residual oil saturation to water sweep and larger gas flooded area. This provides an important justification for gas injection, particularly into the lower unit.