Gravity drainage is the dominant production mechanism in fractured reservoirs, where injection of hydrocarbon gases is believed to improve oil recovery. This paper presents the results of an experimental study of the effect of injecting CO2 into fractured media and its influence on the performance of gravity drainage and ultimate oil production. Both miscible (i.e. high pressure) and immiscible (i.e. low pressure) schemes for CO2 are investigated in this study. The results obtained from these experiments are used to compare the effect of miscible versus immiscible gases, i.e. CO2, on gravity drainage. For these experiments a special stain steel holder was designed allowing an open space around a core of 30 cm long and 5 cm in diameter, simulating a matrix with surrounding fractures. Carbon dioxide and normal decane were used as solvent and oil, respectively. While all experiments were conducted at constant temperature of 35 °C, six series of experiments were carried out at 250, 500, 750, 1000, 1250, and 1500 psi. A high pressure calibrated glass-gauge was connected to the bottom of the fractured media model allowing the produced oil be collected and measured continuously under the pressure and temperature conditions of the experiments. Analyzing the results show that injecting CO2 at higher pressures improves the recovery factor of gravity drainage mechanism in fractured media, significantly.
CO2 flooding has been considered an inefficient process for enhancing oil recovery from naturally fractured reservoir. This is mainly due early gas breakthrough through fractures and very low oil recovery. However, miscible CO2 injection can be used to improve oil recovery in such reservoirs. In fractured reservoirs, the main production mechanism is gravity drainage. The performance of this process in naturally fractured reservoirs is a function of a series of parameters such as fracture-matrix geometry, size, matrix-fracture flow interaction, and so on (1). In the case of application of miscible CO2 both viscosity and density of oil and injected CO2 play important roles. At highpressure conditions, the density of liquid CO2 is about the same and sometimes larger than the oil in-place (2). Therefore, since fractures are filled with dense CO2 during miscible CO2 injection, oil recovery may be reduced due to the presence of denser solvent around the matrix. However, significant reduction in interfacial tension and capillary pressure would lead to a better ultimate oil recovery (3). Several studies show that viscosity ratio, density differences, matrix permeability, and production rate have significant impact on oil recovery by miscible CO2 in fractured environments (4,5). Firoozabadi and Markeset used nC14 and nC10 to study miscible displacement in a fractured medium (6). Both of these solvents were liquid at room conditions, which leads to first contact miscible conditions between the solvent and oil in the matrix. In another study Jamshidnezhad et. al showed that displacement rate has a great impact on oil recovery from fractured reservoir under miscible conditions (7). It is clear that in a miscible displacement process in fractured reservoirs, oil is swept completely from fractures during the early stages of production.