Minimum miscibility pressure calculation for oil shale and tight reservoirs with large gas-oil capillary pressure

Shale and tight rocks are associated with tiny pore throats, on the order of nanometers, and subsequently large capillary pressure. The calculation of the minimum miscibility pressure (MMP) in nanopore space is complex because the phase compositions from flash calculations are affected by capillary pressure. This paper examines the effect of capillary pressure on the calculation of MMP using cubic equation-of-state (EOS) and three techniques: the method of characteristics (MOC), multiple mixing cells, and slim tube simulation. Ternary mixtures of hydrocarbons and real reservoir fluids are considered. Using MOC, capillary pressure changes both liquid and vapor compositions and alters the tie lines. The reason for the change in the MMP is illustrated graphically with ternary and quaternary diagrams. The modified slim tube simulation tool is also used to estimate MMPs of CO₂ with Bakken and Eagle Ford oil. We use an upgraded flash calculation in the slim tube procedure to estimate MMPs with large capillary pressures for real reservoir fluids. The results show that high capillary pressure changes liquid and vapor phase compositions and this change tends to either decrease or increase the CO₂ MMP depending on the original oil composition. The importance of MMP for gas injection in shales as well as the effect of large gas-oil capillary pressure on the characteristics of immiscible floods in shales is discussed.