Improved fluid characterization and phase behavior approaches for gas flooding and application on Tahe light crude oil system

Abstract

N2 or N2/CO2 mixture is considered as injection gas to enhance oil recovery from Tahe oil field. The reservoir pressure and temperature are around 66 MPa and 414 K. The target light crude oil is produced from S117 well of the Tahe oil field. The two main objectives of this study are: 1) to find an effective fluid characterization procedure to calculate the MMP for the N2/CO2–S117 oil system accurately. 2) to identify the effect of injection gas composition (i.e. N2/CO2 mixture) on the gas drive mechanisms for the S117 light crude oil. The PVT and multiple-contact experimental data were measured and these data were used to characterize and validate different fluid characterization methods, including the ‘conventional methods’ and the direct PnA method. We proposed an integrated fluid characterization procedure to accurately calculate MMP. The novelty of the proposed fluid characterization procedure can be summarized as: 1) the procedure regains excellent match in both phase envelopes and critical points before and after lumping; 2) an improved direct PnA regression procedure with only two perturbation parameters was used to calculate pseudo-component properties. The comparison of calculated MMP to the experimental slim-tube data taken from literature shows that the integrated procedure using the direct PnA characterization has the best performance on MMP calculation. For the CO2/N2–S117 system, the vaporizing-gas drive mechanism controls the miscibility provided the CO2 mole fraction in the injection gas is less than 70%. The calculated MMP is independent of the CO2 mole fraction within the range approximately from 0 to 70% in the injection gas. At a higher CO2 mole fraction (ranging from 71% to 100%), the complex condensing/vaporizing (C/V)-gas drive mechanism dominates the miscibility and thus the MMP of the fluid system decreases dramatically as CO2 mole fraction increases.