Combination of a chemical blend with CO2 huff-n-puff for enhanced oil recovery in oil shales

The oil recovery from extremely low permeability shale oil reservoirs by depressurization is only about 7-10% of the original oil in place (OOIP). The objective of this work is to evaluate a hybrid process of a chemical blend (CB) with CO2 huff-n-puff to improve the oil recovery in shale cores. The chemical blend contained an anionic surfactant and a persulfate salt in 2000 ppm KCl. Outcrop Mancos shale core plugs were injected with a light oil and the resulting cores were used to evaluate the liquid recovery factor during huff-n-puff cycles. Shale cores saturated with oil were subjected to CO2 huff-n-puff cycles at three pressures at 75 degrees C. Oil recovery efficiency of the CO2 with the chemical blend was compared with those treated with just CO2 huff-n-puff. Fluid volumes inside cores before and after huff-n-puff cycles were estimated from NMR T2 relaxation time data. D2O was used in the chemical blend to suppress the NMR response from injected blend. The findings from this study show that CO2 huff-n-puff is an effective technique to enhance oil recovery from oil shales. CO2 huff-n-puff was able to recover most of the added oil and a part of the initial fluid. Increasing pressure had a negligible effect on the oil recovery as long as the pressure was above 2800 psi. Application of the chemical blend with CO2 significantly improved the fluid recovery factor compared to pure CO2 huff-n-puff. First injecting a gas and the pressurizing the wells with the CB can improve the oil recovery factor in shale wells.