Some reservoirs have communication with a water zone (aquifer) underneath. When reservoir pressure drops due to production, the compressed water in an aquifer expands into a reservoir and it helps pressure maintenance. This mechanism is called “water drive”.
Water drive mechanism will be effective if an aquifer contacting reservoir is very large because water compressibility is very low. For example, an anticline structure with extensive water zone (aquifer) will have the most advantage from the use of a water drive mechanism. Conversely, stratigraphic reservoirs or highly-faulted reservoirs will have limited aquifer volume so water drive is insignificant.
Typically, characteristics of reservoirs which are influenced by water drive mechanism are a small pressure decline and a fairly constant producing GOR over a period of time. Small gas production comes from solution gas oil ratio (Rs) and producing gas oil ratio (Rp) is equal to solution gas oil ratio (Rs).
Water from an aquifer below a reservoir pushes oil towards producing wells and eventually producing wells will have higher water. When the percentage of water production is so high that production becomes not economic, this is called “water out.” Wells located at a low structure part will be watered out before wells are at a high structure. Watered-out wells are good candidates to convert to water injection wells for water flood operation.
Water drive mechanism is a very good drive and reservoirs can produce oil over 50% recover factors in many cases.