http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Optimizing injected solvent fraction in stratified reservoirs
Waterflooding has become standard practice for extending the productive life of many solution gas drive reservoirs, but has the disadvantage of leaving a substantial residual oil volume in the reservoir. Solvent flooding has been offered as a method whereby oil may be completely displaced from the reservoir, leaving no residual volume. Field results have demonstrated that solvent floods suffer from early solvent breakthrough and considerable oil by-passing owing to high solvent mobility. The injection of both water and solvent has been demonstrated to offer advantages. Water partially mitigates both the adverse mobility and high cost of solvent floods, while solvent mobilizes oil which would be left in the reservoir by water alone. The process is equally applicable to reservoirs currently at residual oil saturation (tertiary floods) and to reservoirs at maximum oil saturation (secondary floods). In stratified reservoirs high permeability layers may be preferentially swept by solvent floods, while low permeability layers may be scarcely swept at all. Presence or absence of transverse communication between layers can modify overall sweep efficiency. This work is a study of water-solvent injection in stratified reservoirs based on computer simulation results. Fractional oil recovery as a function of injected solvent fraction, permeability contrast between layers, initial oil saturation, and presence or absence of transverse communication between strata has been determined. Results are presented as a series of optimization curves. Permeability contrast between layers is shown to be the dominant control on fractional oil recovery. Transverse communicating reservoirs are shown to require a higher solvent-water ratio in order to attain recoveries comparable to transverse noncommunicating reservoirs. In actual field projects, water and solvent are injected alternately as discrete slugs. This process is known as "WAG" for "water-alternating-gas". In the simulations used in this study, continuous water-solvent injection at a fixed fraction rather than true WAG was employed. It is demonstrated that the two methods give equivalent results. In summary, this work is the first comprehensive study of the behavior of stratified reservoirs undergoing water-solvent injection.