CO2-GOGD Subsurface Analysis in Fractured Carbonate in the Sultanate of Oman

Abstract

Abstract The North Oman "A" Field is a giant fractured carbonate reservoir, that has been in production for more than 50 years. In recent decades, the predominant production scheme has been via Gas Oil Gravity Drainage (GOGD). In this scheme, hydrocarbon gas is injected into the high permeability fractures to allow for oil production from the low permeability matrix. In this paper we will study the option of replacing the hydrocarbon injection gas by sub-critical CO2 gas. This would have the advantage of storing CO2 in the matrix while making more hydrocarbon gas available for sales. For the CO2 injection to be successful, the CO2 that is injected into the high permeability fractures needs to move into the low permeability matrix rock, even though there is no forcing viscous drive mechanism. The mechanisms by which the CO2 can go into the matrix are: Gas-Oil gravity drainage (CO2 gas in fractures is pulled into matrix to replace produced oil) Gas-Gas gravity drainage (CO2 gas in fracture has higher density than hydrocarbon gas in the matrix) Diffusion (Differences in concentration differences lead to CO2 moving into matrix). In this paper we will present both analytical models and numerical modeling that show the relative contribution of each of these mechanisms. In term of relative contribution, both analytical and numerical modeling shows that Gas-Gas gravity drainage is the most effective method from transferring denser CO2 gas to the matrix, followed by gas-oil gravity drainage. Diffusion only provides a small contribution. In absolute terms, modeling results indicate that significant volumes of CO2 and commercially significant volumes of hydrocarbon gas can be recovered from the reservoir. Overall, our assessment is that from a subsurface perspective, switching injection gas in GOGD from hydrocarbon gas to CO2 is attractive from both a commercial and an environmental perspective. The novel contribution in this paper is that we present a subsurface analysis of a switch from hydrocarbon gas to CO2 in GOGD developments. This analysis is relevant because it provides a way to store CO2 while also providing extra sales gas.