Parametric Investigations of a Modified SWAG Injection Technique

Abstract

Abstract The natural energy of hydrocarbon reservoirs gets depleted as production continues. The restoration of reservoir energy involves the injection of gas and/or water to support reservoir pressure and to provide a sweeping mechanism. Due to the unfavorable gas-oil mobility ratio, gas injection alone often results in early breakthrough and thus poor sweep efficiency. Water and gas might be injected alternatively, as in water-alternating-gas (WAG) processes, or together, as in simultaneous water-alternating-gas (SWAG) processes, to improve the sweep efficiency. These processes improve the sweep efficiency by stabilizing the displacement front. In this paper, we study a new design of SWAG process, in which water is injected at the top of the reservoir and gas is injected at the bottom. The difference in water and gas densities provides a sweeping mechanism in which water tends to sweep hydrocarbons downward and the gas tend to sweep the hydrocarbons upward. It is expected that the two displacement mechanisms will work together to enhance the overall sweep efficiency and thus the oil recovery. In this paper, the drive mechanisms responsible for oil production under this injection technique are analyzed. The effects of several design parameters on these drive mechanisms are investigated for both homogeneous and heterogeneous reservoirs. Such parameters include; mobility ratio between water and oil phases, viscosity ratio between gas and oil phases, lateral aspect ratio, location of the water and the gas injection wells, and injection rates of water and gas. Introduction Water Alternating Gas (WAG) injection method was originally proposed to improve sweep efficiency of gas injection through better mobility control. In recent years, WAG injection process has gained an increasing interest as an improved oil recovery method especially with the aging of mature reservoirs. Higher Oil recovery factors are anticipated in WAG projects compared to waterflooding due to the combined improvements in the microscopic displacement efficiency associated with gas injection and the macroscopic sweep efficiency of water injection. Christensen et al.1 presented an extensive review of WAG applications on 59 field cases. The first reported WAG field pilot was in the North Pembina field, Alberta, Canada in 1957. The review shows that WAG has been applied successfully in most field trials. The majority of these projects have resulted in significant incremental oil recoveries ranging from 5 to 10%. Attanucci et al.2 investigated the effects of water and gas slug sizes and tapering sequence on the performance of WAG process. Their results indicated that implementing WAG process with reduced injection cycle lengths has the potential to increase the recovery efficiency and to improve oil lift efficiencies which will result in reducing operating costs.