Success of SAG Foam Processes in Heterogeneous Reservoirs

Abstract

Abstract In principle, miscible gas injection can displace nearly all of the oil from the portions of a reservoir swept by gas. However, reservoir heterogeneity, low gas density and high gas mobility reduce sweep efficiency and drastically decrease recovery. Fortunately, the use of foam can reduce gas mobility and the effect of heterogeneity and therefore increase sweep efficiency. Based on the work of Shan and Rossen, an optimal design strategy is proposed here for surfactant-alternating-gas (SAG) foam processes and tested in a homogeneous reservoir and in two layered rectangular reservoirs. Fractional-flow theory, Stone's and Jenkins' model for gravity segregation, and an idealized model developed by Shan and Rossen are used design the process and interpret the results. In the homogeneous reservoir, fixed-pressure SAG processes result in better sweep efficiency than fixed-rate SAG processes. A one-cycle SAG process, where one big slug of surfactant is followed by one big slug of gas, gives better sweep than multiple-cycle SAG processes; sweep efficiency is higher and the process proceeds faster. Adsorption can be dealt with by adjusting the size of the first surfactant slug. The optimal SAG process gives much better sweep efficiency than a WAG process without foam. Simulations reveal factors not accounted for in Shan and Rossen's idealized model for a single-cycle SAG process; nonetheless the model accurately predicts the rate of advance of the foam front at the top of the reservoir and qualitatively predicts the shape of the foam front. A ten-cycle SAG process performs somewhat better than predicted by Stone's model for continuous injection. In heterogeneous reservoirs, a maximum-fixed-pressure SAG process also gives best results. The surfactant slug sized for a homogeneous reservoir is much too large in this case, however, and much of the surfactant wasted. Again, a one-cycle process SAG performs better than multiple, smaller cycles, but sweep is not as good as in a homogeneous reservoir. A high-permeability layer at the top of the reservoir leads to better sweep efficiency at gas breakthrough than when the reservoir has a low-permeability layer at the top. A low-permeability layer at the top allows early gas breakthrough once gas in high-permeability layers below it reaches the edge of the surfactant bank above. Compared to WAG processes, the optimal SAG process shows much better sweep efficiency.