Scaled model studies and numerical simulation of inert gas injection with horizontal wells

Gas injection is one of the oldest enhanced oil recovery techniques used for recovering light oils. Recently, there has been renewed interest of gas injection in both light and heavy oil reservoirs. It has been pointed out that in order to apply gas injection, gravity stabilization is required to avoid early gas breakthrough. In absence of natural dip, such gravity stabilization may be achieved through horizontal wells placed on top of the reservoir while producing through another horizontal well placed at the bottom of the reservoir. The possibility of such recovery technique is investigated in this paper. A series of partially scaled 2-dimensional model studies indicated that close to 70% oil in place may be recovered by using gas injection from the top of a heavy oil reservoir of oil viscosities of 1000 and 4000 mPa.s. Due to scaling requirements, the porous medium was replaced by glass beads of much higher absolute permeabilities. Two additional runs were also conducted with 750 mPa.s and 7500 mPa.s oils using high-pressure geometrically scaled physical models. These results indicated that it is much harder to maintain a stable gas front in a geometrically scaled high pressure 3-dimensional model than in a partially scaled 2-dimensional model. In order to maintain a stable gas front, one has to operate the injection/production wells at a very low rate. While this process is found to be rather slow, oil recovery if a stable gas front is maintained. If the frontal stability is lost, gas injection does not increase oil production beyond that of a primary depletion case. Several tests with variable gas composition were conducted. The presence of CO₂ as opposed to nitrogen improved recovery to some extent. Finally, numerical simulation of field cases is presented. The danger of invoking an unstable displacement front is demonstrated through numerical simulation.