A simple segregated flow model was developed to describe an alternate water and gas (WAG) injection process in a linear reservoir system. The model was developed base on the geometries of segregated fluid interfaces and on a volume balance of the injectants and the initial reservoir fluids. Results obtained from the model showed good agreement with published experimental and field data.
Assumptions in the model development includes immiscible water and gas phases, incompressible fluid flow, no trapped oil volumes and a uniform initial water saturation. Segregated flow interfaces depict the injected gas channeling to the top and the injected water channeling to the bottom of the oil zone. It assumes a pistonlike displacement and a fully segregated flow process. process. The model tracks the progress of the respective interfaces and estimates the cumulative injection required to gas and water breakthrough. The model considers the fillup of initial free gas volume, The model can be easily extended to a five spot pattern by utilizing a streamtube approach, or by using an independently determined areal sweep efficiency.
A comparison of published experimental production data for different gas/water injection ratios with that predicted from the model showed excellent agreement. The model was satisfactorily used to predict the production performance of a field WAG process.
Because of dwindling oil reserves and the high cost of finding new oil, petroleum engineers have been actively looking for more efficient and cost effective ways to improve oil recovery from existing reservoirs. Water injection was introduced as one such way to increase oil production. Although waterflooding is able to produce incremental oil over primary recovery, many early waterfloods have already passed or are quickly approaching their economic limits. Miscible gas injection has been developed as an enhanced oil recovery method to recover additional oil over that recoverable by waterflooding.
Theoretically, miscible gases have the ability to completely strip and extract the oil from the reservoir. However, miscible gases are generally lighter than in-situ oils and thereby tend to segregate to the top of the production interval. Adverse mobility ratios and reservoir heterogeneity can aggravate the poor vertical sweep efficiency. In order to alleviate the severity of gravity differences and improve the vertical sweep efficiency, the industry has developed a WAG process in which gas and water are injected concurrently or as alternating slugs.
The WAG process is quickly gaining acceptance in the petroleum industry. There are several commercially successful WAG petroleum industry. There are several commercially successful WAG floods in existence today with many more planned for the future. Undoubtedly, the attractiveness of the WAG process will also increase in the petroleum industry.