Summary
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.
Introduction
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.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
16 September 2006
- Meeting paper published:
3 October 2006
- Revised manuscript received:
30 April 2008
- Manuscript approved:
5 May 2008
- Version of record:
25 October 2008
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