Field injectivity experiences with miscible recovery projects using alternate rich gas and water injection

Similar operating procedures were used in 4 miscible projects utilizing alternate injection cycles of rich gas and water to achieve an improved mobility ratio and, therefore, an improved sweep efficiency. In 3 of the projects, the water injection rates before and after rich gas injection were as anticipated. However, unanticipated reductions in water injection rates occurred in one project after the injection of rich gas. Laboratory tests suggest that this is caused by the presence of an unusually high trapped gas saturation which results in a decreased relative permeability to water in the invaded region of the reservoir. The trapped gas saturation could be higher than usual in the water bank as a result of a residual oil saturation after the passage of the rich gas bank. A general comparison of the 4 projects does not indicate why one project should behave so differently. It is concluded that controlled single-well injection rate tests, using the injection procedures proposed for a field, would be desirable in future operations until the cause and remedy for low water injection rates is developed. In this way, the maximum water injection rates may be determined prior to a full-scale expansion.