A Model for Gas Coning and Rate-Dependent Gas/Oil Ratio in an Oil-Rim Reservoir (Russian)
- Are Mjaavatten (Norsk Hydro ASA) | Robert Aasheim (Norsk Hydro ASA) | Steinar Saelid (Prediktor AS) | Oddvar Gronning (Prediktor AS)
- Document ID
- Society of Petroleum Engineers
- SPE Russian Oil and Gas Technical Conference and Exhibition, 3-6 October, Moscow, Russia
- Publication Date
- Document Type
- Conference Paper
- 2006. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 1.6.6 Directional Drilling, 5.6.4 Drillstem/Well Testing, 5.6.9 Production Forecasting
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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 using horizontal wells. Production will lower the local gas/oil contact near the well in a process called gas coning (or more accurately, cresting). 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 optimisation for such fields.
We have developed a mathematical model that can predict gas coning behaviour and the resulting rate dependent GOR with a surprisingly high degree of accuracy over periods of several months or more. We combine a dynamic model that describes the essential reservoir behaviour 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 behaviour 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 optimisation at the Troll field. We have also tested the model on wells in other fields, with encouraging results.
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 Figure 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 using long horizontal wells. Oil production started in 19951-3.
The Troll Oil subsea system is one of the world's largest subsea developments, with more than one hundred 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 maximum 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 successfully predict the rate-dependency of the GOR. We therefore started a research and development activity that resulted in the model described in this paper.
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