Identification of pitfalls in PVT gas condensate modeling using modified black-oil formulations

A black-oil (BO) PVT model is a fluid characterization formulation that represents multi-component reservoir hydrocarbons as a binary mixture (i.e., two pseudo-components: “surface gas” and “stock tank oil”). The BO PVT model is widely used in the petroleum industry because it is relatively simple compared to fully compositional modeling in which all or most components are independently accounted for. Since computational complexity increases nearly exponentially with number of components used in the characterization, there always remains a strong incentive to embracing the simplified black oil (binary) characterization as long as the fluid phase behavior allows it. When representing a complex system with this simplified model, a number of limitations arising from its simplicity may exist. In this study, these limitations are highlighted by performing phase behavior simulations for a gas condensate fluid. Rigorous calculations of standard (BO) PVT properties (B_o, B_g, R_s, and R_v) of a the gas condensate reservoir of choice are performed through a series of flash calculations at the prescribed reservoir fluid depletion path. The study demonstrates that the BO PVT model violates the species material balance principle as reservoir pressure depletes while conserving overall mass. This violation can lead to significant errors when coupling the BO PVT model with tank material balance-based techniques. The simulation test case indicates that these models will significantly and consistently underestimate oil formation volume factor (B_o) and solution gas oil ratio (R_s) due to the shortcomings of the BO PVT model.