In oil-rim fields, a thin oil layer lies between an aquifer and a gas cap. Oil may be produced from such fields by use of horizontal wells. Production will lower the local gas/oil contact (GOC) near the well in a process called gas coning. After gas breakthrough, the gas/oil ratio (GOR) from the well may vary strongly with the production rate. The ability to predict this dependency is essential for production optimization for such fields.
We have developed a mathematical model that can predict gas-coning behavior and the resulting rate-dependent GOR for periods of several months or more. Despite a simple model structure and short computation times, the accuracy of the predictions is good. We combine a dynamic model that describes the essential reservoir behavior with a highly simplified description of the interaction between the well and the surrounding reservoir. The full model has three adjustable parameters that allow us to fit the behavior to individual wells, using historical oil-and gas-production rates. The model forms the basis of the GORM (gas/oil ratio model) computer program that, since early 2003, is in regular use for production planning and optimization at the Troll field. We also have tested the model on wells in other fields, with encouraging results.
Note that the model describes the behavior of individual wells only. The focus is short-and medium-time production optimization, not reservoir management.