Review of WAG field experience

In recent years there has been an increasing interest in water-alternating- gas (WAG) processes, both miscible and immiscible. WAG injection is an oil recovery method initially aimed to improve sweep efficiency during gas inje ction. In some recent applications produced hydrocarbon gas has been reinje cted in water-injection wells with the aim of improving oil recovery and pr essure maintenance. Oil recovery by WAG injection has been attributed to co ntact of unswept zones, especially recovery of attic or cellar oil by explo iting the segregation of gas to the top or the accumulating of water toward the bottom. Because the residual oil after gasflooding is normally lower t han the residual oil after waterflooding, and three-phase zones may obtain lower remaining oil saturation, WAG injection has the potential for increas ed microscopic displacement efficiency. Thus, WAG injection can lead to imp roved oil recovery by combining better mobility control and contacting unsw ept zones, and by leading to improved microscopic displacement. This study is a review of the WAG field experience as it is found in the li terature today,(1-108) from the first reported WAG injection in 1957 in Can ada to the new experience from the North Sea. About 60 fields have been rev iewed. Both onshore and offshore projects have been included, as well as WA G injections with hydrocarbon or nonhydrocarbon gases. Well spacing is very different from onshore projects, where fine patterns often are applied, to offshore projects, where well spacing is in the order of 1000 m. For the fields reviewed, a common trend for the successful injections is an increased oil recovery in the range of 5 to 10% of the oil initially in pl ace (OIIP). Very few field trials have been reported as unsuccessful, but o perational problems are often noted. Though the injectivity and production problems are generally not detrimental for the WAG process, special attenti on has been given to breakthrough of injected phases (water or gas). Improv ed oil recovery by WAG injection is discussed as influenced by rock type, i njection strategy, miscible/immiscible gas, and well spacing.