Parametric Investigations of a Modified SWAG Injection Technique

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.

A review of gas enhanced oil recovery schemes used in the North Sea

“…This led to a change from WAG to SWAG in 1999. The SWAG process reduces the gas mobility and can improve injectivity, especially when the gas and water are co-injected using a single well (Al-Ghanim et al 2009;Heidari et al 2013;Algharaib et al 2007). The SWAG mechanisms of oil recovery, which are miscibility, IFT reduction, gravity drainage, and mobility control, are like those of WAG (Heidari et al 2013;Masalmeh et al 2010;Tunio et al 2011).…”

“…The phases could, however, be injected separately to improve injectivity. The gas is injected near the bottom of the formation and water near the top of the formation, using different horizontal injectors (Algharaib et al 2007). This is a more recent approach and has been used in the North Sea Siri field.…”

Understanding aqueous foam with novel CO2-soluble surfactants for controlling CO2 vertical sweep in sandstone reservoirs

“…Stone (2004a, b) proposed injection of water in an interval above the gas to increase reservoir sweep, which is called ''modified SWAG'' by Algharaib et al (2007). The main goal of separate injection is to reduce the effect of gravity segregation commonly encountered in gas-liquid flow in reservoirs with high vertical communication (Rossen et al 2006;Liu et al 2011).…”

Evaluation of the use of Water Alternated Gas Injection for Enhanced Oil Recovery

“…Several studies have been presented towards optimizing the performance of the WAG technique since the advent of EOR such as the use of evolutionary algorithms (Ferreira et al, 2018). These methods rely on the use of commercial reservoir simulators to investigate critical parameters associated with the WAG method of EOR (Algharaib et al, 2007;Shi et al, 2008;Mousavi Mirkalaei et al, 2011;Jiang et al, 2012). For the efficient design of the scheme, adequate evaluation of the effect of WAG ratio, water and gas injection rate, water and gas cycle period, field pressure on production rates and oil recovery factor is vital.…”