Molecular Diffusion in Naturally Fractured Reservoirs: A Decisive Recovery Mechanism

Abstract

ABSTRACT The purpose of this paper is to demonstrate the importance of the Fick's molecular diffusion in fractured reservoirs and to present a numerical solution to predict the related mass transfer. As opposed to porous media where molecular diffusion is generally small, in naturally fractured reservoirs, molecular diffusion may be very important, as the dispersive flux through fractures rapidly increases the contact area for diffusion. Fick's molecular diffusion potential may even override viscous forces when hydrocarbon or inert gases are injected and the fracture spacing is small. The importance of Fick's diffusion is largely illustrated in the present paper showing results from analytical models applied to field cases. A method to predict molecular diffusion is presented and has been implemented in a very recent dual porosity, fully implicit, fully compositional, EOS based reservoir simulator. One advantage of the proposed algorithm, which is an extension of the Sigmund correlation, when compared to others, is that it requires only the component critical properties and other parameters used in any equation of state. Results from the above referred reservoir simulator are also presented to illustrate the effect of molecular diffusion while injecting separator gas or inert gas in a North Sea volatile oil reservoir. Solutions for molecular diffusion of gas in fractured undersaturated oil reservoirs, using single porosity simulators, are also described in the paper.