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Abstract
The purpose of the experiments described in this paper is to investigate the efficiency of oil recovery by CO2 and C1 in fractured carbonate rock after tertiary gravity drainage in reservoir condition. Samples with different type of wettability have been selected and tertiary gas injection experiments were performed on 20 cm long and low permeable outcrop core surrounded with a fracture established with a novel experimental set-up. A mixture of live oil (C1- C7) with a known composition has been selected and PVT properties of the sample such as density, bubble point pressure, interfacial tension between the phases were measured in reservoir condition prior to experiment. The matrix block was saturated using the live oil, while the fracture was filled with a sealing material to obtain a homogeneous saturation. The sealing material was then melted and removed by increasing the temperature which in turn creates the fracture surrounding the core. Water was injected into the fracture at pressure above the bubble point to measure oil recovery by imbibition. After reaching to final oil recovery equilibrium gas was injected to the fracture in two successive stages where interfacial tension was reduced from 0.37 mN/m to 0.15 mN/m in order to measure oil recovery by gravity drainage. Non-equilibrium gas (CO2 and C1) were then injected in the last part of the experiment to study the effect of composition on final oil recovery. Results from tertiary gas injection experiments show that additional oil recovery could be obtained by injection of non-equilibrium gas, where diffusion and gravity drainage were the main elements of the displacement mechanism. Results also reveal that CO2 injection is a very efficient recovery method while injection of C1 can also improve the oil recovery.