The Role of Diffusion for Nonequilibrium Gas Injection Into a Fractured Reservoir

Abstract

Summary The Schönkirchen Tief oil field is located in the Vienna basin in Austria. It is a pervasively fractured dolomite reservoir that has been produced for more than 50 years. The field is at the tail end of production, the wells are perforated close to the top of the reservoir, and water is injected downdip. Because of the location of the field close to one of the main gas pipelines in Austria, it is planned to convert the field into high-performance underground gas storage (UGS). The field is characterized by a highly permeable fracture system and a less-permeable matrix system. It is expected that some incremental oil can be recovered because of gas/oil gravity drainage from the matrix. In addition to gas/oil gravity drainage, diffusion will have an effect on the oil recovery. The injected gas is leaner than the equilibrium gas in the reservoir. Hence, gas components diffuse from the fracture system into the matrix and components of the oil diffuse toward the fracture system. This results in a modification of the properties of the oil affected by diffusion. This type of gas injection results in a zone of decreased oil viscosity for gases such as CO2 and CH4 at the interface of the gas and the oil in the matrix. This zone of lower oil viscosity increases the gas/oil gravity-drainage rates. The results show that the effect of diffusion can increase cumulative oil production up to 25% compared with a case neglecting the effect of diffusion. The effect of diffusion could be determined for various parameters such as permeability, porosity, fracture spacing, and matrix-block height. While for some of the parameters the effect of diffusion scales with the square root of time (e.g., permeability), for others an exponential relationship has been determined (fracture spacing). The results derived for the example reservoir can be used more generally to screen whether the effect of diffusion should be incorporated into reservoir studies concerning nonequilibrium-gas injection and to determine how large the error could be in the case where diffusion is neglected.