Comparative evaluation of immiscible, near miscible and miscible CO2 huff-n-puff to enhance oil recovery from a single matrix-fracture system (experimental and simulation studies)

In this paper, the performance and efficiency of improved oil recovery in fractured porous media utilizing the cyclic CO2 injection process (otherwise known as CO2 huff-n-puff) are examined for immiscible, near-miscible and miscible pressure conditions. The main approach involved the comparison of simulation results to experimental data obtained from a laboratory CO2 huff-n-puff model. Previous experimental studies performed showed promising results for CO2 huff-n-puff process in a fractured model saturated with light oil. In order to upscale the experimental results and perform further studies, fully compositional simulation software was utilized to simulate the cyclic CO2 injection experiments. Prior to this, the phase behavior of a mixture of CO2 and decane (nC(10)) was studied and minimum miscibility pressure (MMP) of CO2-nC(10) was found to be 1058 psi, which was in good agreement with experimental results obtained by rising bubble apparatus (RBA) and visual tests [9,10]. The results of six sets of huff-n-puff experiments performed under constant temperature of 35 degrees C and a wide range of pressures from 250 to 1500 psi were simulated. Simulation results were in good agreement with those obtained experimentally. Throughout these studies, it has been found that near-miscible CO2 huff-n-puff process can improve the recovery factor from the light oil saturated experimental model by a factor of two. It was also discovered that the injection of CO2 at pressures much higher than the MMP was not significantly beneficial to recovery factor and may require further economical analysis. Also, cyclic CO2 performance improved greatly when conditions transitioned from immiscible to near-miscible (i.e. operating and injection pressures were increased to 1000 psi) as indicated by a sharp increase in oil production. (C) 2011 Elsevier Ltd. All rights reserved.