Oil Recovery from Matrix during CO2-Foam Flooding of Fractured Carbonate Oil Reservoirs

Abstract

Abstract The recovery of oil during CO2 flooding of fractured carbonate oil reservoirs strongly depends on the transport of CO2 from the fracture to the matrix. Molecular diffusion of CO2 from the fracture to the oil in the matrix will therefore affect the rate of oil production from the matrix to the fracture. CO2-foaming agents have been shown to increase the macroscopic sweep efficiency in CO2 floods. However, whether the foaming agent either increases or decreases the rate of oil production from matrix to fracture needs to be understood. The effects of a CO2-foaming agent on the rate of oil production during CO2 flooding of fractured carbonate oil reservoirs at reservoir conditions have been studied in a fractured model. The fracture model was created by transferring the waterflooded core plugs into stainless steel cells with an annulus space around the core plugs. The fractures were flooded by injecting CO2 gas, co-injecting CO2 gas and aqueous foaming agent solution, or injecting aqueous foaming agent solution outside the core plugs. CO2 bulk diffusion coefficients for synthetic seawater, stock tank oil and an aqueous solution of a CO2-foaming agent were determined using the pressure decay method. Co-injecting CO2 gas and aqueous foaming agent solution was found to give slightly higher oil recovery than CO2 injection. Bulk CO2 diffusion coefficients measured in synthetic seawater and aqueous solution of foaming agent show that the tested foaming agent does not any significant effect on the transport of CO2 at the experimental conditions used. For the CO2- synthetic seawater and CO2-stock tank oil systems the bulk diffusion coefficients were estimated to be of the same order of magnitude as published in literature at reservoir conditions. Introduction In view of the declining oil reserves from the existing oil fields, there is an increasing need for the oil and gas industry to improve the recovery efficiency (Chakravarthy et al. 2006). Residual oil left after waterflooding from fractured and nonfractured reservoirs can be a target for improved oil recovery methods. The mobility and recovery of such oil can be improved by lowering their viscosity, which could be accomplished by injecting gases (hydrocarbon, nitrogen and carbon dioxide) or light hydrocarbons (Jamialahmadi et al. 2006). CO2 flooding is a well recognized and tested Enhanced Oil Recovery (EOR) method. Although the microscopic sweep efficiency of CO2 flooding is very high, the volumetric sweep is a cause for concern (Bernard et al. 1980). CO2 dissolves easily into oil, it reduces oil viscosity, swells the oil and extracts the light components in oil at sufficiently high pressures. However, the local efficiency with which CO2 gas displaces oil depends strongly on the phase behaviour of the mixtures of the gas and oil (Jessen et al. 2005).