Diffusion model for predicting reservoir gas losses

Abstract

A numerical one-dimensional model has been developed to describe the influence of the evolution of geological parameters on the diffusive losses of gas from a reservoir. A theoretical model of these phenomena is presented based on the equation of mass transfer through porous media and on the thermodynamics of the gas-water system. The numerical model aims to predict gas losses over geological time and the distribution of free gas within the caprock and the overlying sediment. The equilibrium concentration of gas is calculated by a modified Henry's law for the water phase and the Lee-Kesler method for the gas phase. The diffusion coefficients are functions of temperature and pressure. The equations are solved by a control volume method and the numerical integration scheme is discussed. The model can handle any kind of boundary conditions as well as gas phase migration pathways through the caprock. The model is applied to a typical North Sea field caprock. The distribution of methane is calculated in different geological hypothesis.