Miscibility effects on performance of cyclic CO ₂ injection in hysteretic tight oil reservoirs

Dependency of relative permeability on saturation path during cyclic CO ₂ injection (CCI) in various operational constraints affects the oil recovery in different ways. A compositional reservoir sector model is built based on the available production data of hydraulically fractured horizontal well in Bakken formation. The work discusses the simulation results of the CCI and investigates the contributions of non-wetting phase's relative permeability hysteresis in oil production below and above the minimum miscibility pressure (MMP). A CMG-GEM model is built based on the Bakken geological settings, well production and live oil PVT data. Relative permeability hysteresis model is incorporated within the simulator using the Killough's method. A cyclic CO ₂ injection (CCI) EOR scheme is designed and implemented in the numerical model. Effects of structural trapping and hysteresis-induced CO ₂ /gas retardation on oil recovery are studied during CCI in which a strong flow reversals may occur. The results of simulation revealed that in non-hysteretic model, performing cyclic CO ₂ injection at immiscible (2000psi) and miscible (5000psi) conditions increases the recovery up to 12.8% and 22.64% respectively. Recovered oil after inclusion of relative permeability hysteresis demonstrate major corresponding effects of gas retardation, CO ₂ trapping and improved water permeability. The results show mole fraction of CO ₂ invading the reservoir remains constant at miscible condition and is not affected by hysteresis. Yet in hysteretic model, the oil recovery factor is slightly declined as the relative permeability to water is improved. The immiscible-hysteretic model incorporates high residual gas/ CO ₂ gas saturation at the end of each production (imbibition) cycle which increases gradually with historical gas saturation. CO ₂ mole fractions in both gas and oil phases are intensely decreased due to hysteresis following by decline in CO ₂ injectivity. Residual CO ₂ trapped during early cycles, limits the CO ₂ extent in reservoir and makes the recovery less efficient. In addition to residual saturation, oil composition varies due to different rates of vaporization and diffusion by CO ₂ as a result of its uneven distribution in reservoir. We accurately evaluated the efficiency of cyclic CO ₂ injection in to Bakken tight oil reservoir by incorporating gas-trapping mechanisms in the model. Shortcomings of uncertainties associated with the previous simplified non-hysteretic reservoir models is reduced. Various operational conditions are tested. Our results draw a distinction amongst underlying mechanisms of recovery induced by hysteresis at different miscibility conditions.