A Model for Gas Coning and Rate-Dependent Gas/Oil Ratio in an Oil-Rim Resevoir
- Are Mjaavatten (StatoilHydro ASA) | Robert Aasheim (Norsk Hydro) | Steinar Saelid (Prediktor AS) | Oddvar Groenning (Prediktor AS)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- October 2008
- Document Type
- Journal Paper
- 842 - 847
- 2008. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 5.5 Reservoir Simulation, 5.6.4 Drillstem/Well Testing, 5.5.8 History Matching, 4.3.4 Scale, 5.6.9 Production Forecasting, 1.6.6 Directional Drilling, 5.1 Reservoir Characterisation
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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.
The Troll field is located in the North Sea 80 km off the west coast of Norway. It covers an area of 700 km2. It contains a thin oil layer between a large gas cap and an aquifer. The field consists of three provinces, as shown in Fig. 1. In the Troll east province, the oil layer is very thin, so this province has no oil producers. Gas production from Troll east started in 1996. In the Troll west oil and gas provinces, the oil layer is between 12 and 24 m thick. The oil here is produced by use of long horizontal wells. Oil production started in 1995 (Haug 1992; Henriksen and Storegjerde 1997; Hauge and Horn 2005).
The Troll oil subsea system is one of the world's largest subsea developments, with more than 100 wells. Water depths vary from 315 to 340 m.
After gas from the gas cap breaks through into a well, the GOR will be strongly rate-dependent, with GOR increasing with the production rate. The gas-processing capacity on the platform limits the total allowable gas production from the oil wells. To maximize oil production in this situation, it is essential to know how the GOR from individual wells will change with time and in response to changes in production rate from that well. To our knowledge, no models were available that could predict the rate dependency of the GOR successfully. We, therefore, started a research and development activity that resulted in the model described in this paper.
|File Size||2 MB||Number of Pages||6|
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