Effect of operating pressure, matrix permeability and connate water saturation on performance of CO2 huff-and-puff process in matrix-fracture experimental model

The main objective of study is to examine the performance and efficiency of CO2 huff-and-puff process for improving oil recovery and subsequent storage of CO2 in light-oil fractured porous media through designing and conducting targeted experiments.The experimental set-up consisted of a high-pressure stainless steel cell made specially to hold a cylindrical core with spacing around it, simulating a matrix and its surrounding fracture environment. The matrix was saturated with normal decane, which was used as oil during the experiments. A total of six separate sets of huff-and-puff experiments, using CO2 as solvent, were conducted under operating pressures of 250, 500, 750, 1000, 1250, and 1500 psi. The temperature was kept constant (35 °C) during all tests. Each set of the huff-and-puff experiments was conducted by injecting CO2 in the fracture system surrounding the core (injection step). Then, the system was shut-in for a period of 24 h to allow CO2 to diffuse from fracture into the oil in matrix (soaking period step). At the end of the soaking period, the pressure was released and the oil production was measured (production step). The above cycle was repeated until no more oil was produced.The results obtained show that when CO2 was injected at 1500 psi through a huff-and-puff process, it recovers more than 95% of oil from a fracture-matrix experimental model saturated with normal decane. Also indicates that at such a pressure, 45% of initial oil-in-place can be recovered during the first cycle. However, when CO2 was injected at pressures below CO2–nC10 minimum miscibility pressure (MMP), the maximum recovery factor achieved was less than 50%. Similarly, recovery factor of the first cycles performed at pressures below MMP were much lower (less than 30%) compare to those conducted at miscible condition. This indicates that miscible huff-and-puff process is a viable option for fractured porous media.As part of this study, effects of matrix permeability, and connate water saturation on the performance of huff-and-puff process in fracture-matrix core set-up at both immiscible and miscible conditions were studied. Results indicate that presence of connate water saturation is beneficial to immiscible CO2 huff-and-puff process while it has almost negligible effect on the performance of this process when performed at miscible conditions. In presence of connate water saturation more than 70% of oil-in-place was recovered at immiscible condition compare to a maximum of 45% recovered when matrix was 100% saturated with normal decane. Results of the tests performed in a core with permeability about 10 times higher than the original core shows similar production trends. However, recovery factor in the high permeable core was slightly less than twice of that in low permeable core at immiscible condition. According to this study, effect of core permeability was less pronounced when CO2 was injected at miscible conditions.