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

Two laboratory experiments were performed to investigate the molecular weight selectivity of crude oil hydrocarbons mobilized into CO2 at the reservoir temperature (42 degrees 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 C-21-C-36 increased dramatically from 10.34 to 21.13 MPa (from 1500 to 3500 psi), even though both pressures were above the MMP. In addition, when crude oil was first equilibrated at 20.68 MPa (3000 psi) and then lowered before sampling the "miscible" phase, substantial deposition of the higher molecular weight hydrocarbons occurred with the result that the hydrocarbon composition and concentrations were similar to those when the crude oil was exposed to only the lower CO2 pressures without first being exposed to any higher pressure. The sand flush experiments showed similar results to the "miscible" phase samplings in that both the total hydrocarbons and the fraction of C-21-C-36 recovered increased dramatically with pressure, regardless of whether the pressures were below or more than double the MMP. The crude oil samples remaining after CO2 exposures also showed large increases in molecular weight as well as higher viscosity and lower American Petroleum Institute (API) gravity than the original crude oil. As predicted by both lab experiments, produced crude oil samples collected from two wells before and after CO2 breakthrough showed significant bias against high-molecular-weight hydrocarbons, with the fraction of C-21-C-36 produced dropping by as much as 60-80% after CO2 breakthrough compared to crude oil samples collected from the same wells before CO2 breakthrough. These results verify that crude oil during a CO2 flood does not move as a homogeneous "miscible" phase and that hydrocarbon dissolution (vaporization into the "miscible" phase) of lower molecular weight hydrocarbons dominates oil recovery in both the field and lab experiments.