Lab and Reservoir Study of Produced Hydrocarbon Molecular Weight Selectivity during CO2 Enhanced Oil Recovery

Abstract

Two laboratory experiments were performed to investigate the molecular weight selectivity of crude oil hydrocarbons mobilized into CO2 at the reservoir temperature (42 °C) and pressures ranging from below the minimum miscibility pressure (MMP) of 9.69 MPa (1406 psi) to pressures well above the MMP (8.27–27.58 MPa, 1200–4000 psi). The hydrocarbon composition at equilibrium in the CO2-rich upper “miscible” phase was measured, as was the ability of four pore volumes of CO2 to recover crude oil hydrocarbons from a sand bed. Both experiments showed significant selectivity against producing higher molecular weight hydrocarbons at lower pressures. In addition, the bias against higher molecular weight hydrocarbons continued even at pressures well above the 9.69 MPa MMP. For example, both the total hydrocarbon concentration in the CO2-rich “miscible” phase and the fraction of C21–C36 increased dramatically from 10.34 to 21.13 MPa (from 1500 to 3500 psi), even though both pressures were above the MMP. In addition, ...