Abstract
Abstract
Implementation of EOR processes in naturally fractured reservoirs is a challenging task due to presence of extreme heterogeneity. Among potential EOR techniques for implementing in these environments application of CO2 for displacement purposes and/or pressure maintenance might be a viable option. This paper presents the results experimental and simulation studies of the effect of injecting CO2 into fractured media and its influence on oil recovery. Both miscible (i.e. high pressure) and immiscible (i.e. low pressure) schemes for CO2 are investigated in a core of 100 md permeability. The results obtained from these experiments are used to compare the effect of miscible versus immiscible conditions. For these experiments a special stainless steel holder was designed that allows 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. The core was saturated with oil with and without presence of residual water saturation. 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.
Introduction
Gas flooding, e.g. CO2 flooding, is considered an inefficient process for enhancing oil recovery from naturally fractured reservoir. This is mainly due early gas breakthrough through fractures that lead to low oil recovery. However, miscible CO2 injection, under controlled operating conditions, could be used to improve oil recovery in such reservoirs. In naturally fractured reservoirs, where the main production mechanism is gravity drainage, the performance of CO2 injection is affected by a series of parameters such as fracture-matrix geometry, size, matrix-fracture flow interaction, and so on (Li et. Al, 2000). Even in the case of injecting CO2 in fractured reservoirs under miscible conditions, the relative values of viscosity and density of oil and injected CO2 determine the extent of gravity override and early breakthrough of CO2, as well as oil flow from matrix to fractures. At very high-pressure conditions, the density of CO2 is close to and sometimes larger than oil viscosity (Darvish et. Al, 2006). 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, which hinders drainage of oil into fractures. However, it is expected that significant reduction in interfacial tension and capillary pressure would lead to improved ultimate oil recovery (Uleberg, 2002). Several studies show that viscosity ratio, density differences, matrix permeability, and production rate have significant impact on oil recovery for solvent flooding in fractured environments (Slobob, 1964 and Thompson, 1969). Firoozabadi and Markeset used nC14 and nC10 to study miscible displacement in a fractured medium (Firoozabadi, 1994). Both of these solvents were liquid at room conditions, which leads to first contact misciblity 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 flooding conditions (Jamshidzadeh et. al, 2004). It is clear that in a miscible displacement process in fractured reservoirs, oil is swept completely from fractures during the early stages of production. This leads solvent surrounding the matrix blocks, which are still saturated with oil. From then on, oil displacement from matrix into fracture would strongly depend on diffusion, advection and gravity. Therefore, one can conclude that low displacement flow rate will results in a better diffusion and advection of solvent in the oil in matrix, reducing the interfacial tension and increasing the rate of drainage from matrix into surrounding fractures.
In addition to the above parameters, temperature has significant impact on oil recovery under miscible and immiscible conditions (Holm, 1986). At low temperatures, when pressure drops below minimum miscibility pressure (MMP), oil recovery decreases sharply. However, at higher temperatures change in oil recovery is more gradual (Stalkup, 1978).
This paper presents the findings of a series of experimental investigations on the performance of oil drainage from matrix into a surrounding fracture filled with CO2 under immiscible and miscible conditions. These experiments have been conducted under gravity drainage conditions, and not flooding situation. Additionally, these experiments were simulated by utilizing a commercial simulator, CMG-GEMTM. The findings of this study can be used for designing successful CO2 EOR processes for naturally fractured reservoirs, as well as utilizing the underground storage of CO2 in fractured oil reservoirs.