“Improved near-well approximation for prediction of the gas/oil production ratio from oil-rim reservoirs”

 Publikasjon: “Improved near-well approximation for prediction of the gas/oil production ratio from oil-rim reservoirs”

In the Troll field in the North Sea a thin oil layer is located between a large layer of gas and a layer of water. The reservoir is situated in relatively homogeneous grit. The western part of the field contains an oil layer between 12 and 24 meters thick.  At this point oil is produced in long, horizontal wells.

When the oil in the grit reservoir flows against the wellbore, the production will lower the local gas-oil-contact (GOC) near the well in a process called gas coning. See figure 1. After this, the produced gas/oil ratio (GOR) from the well will vary strongly with the production rate. The ability to predict this dependency is essential for production optimization.

Statoil has developed a GOR model (called GORM) to calculate how the GOR will develop in the future. GORM utilizes a one dimensional model to describe the oil flow towards the well. This is a well-known simplification, which on several occasions provides a good description of the rate. Historical oil and gas production rates are applied to fit three adjustable model parameters.

GORM has been extensively tested. The model has relatively short computational time and performs remarkably well for medium term production optimization (weeks, months). The predictions during the first days after a large change in the production can, however, be poor.

The model equations have been analysed and the behaviour close to the wellbore has been compared to more accurate finite element simulations. The analysis revealed that the approximation in GORM have suitable medium-term properties, while one has to expect significant deviations close to the well, and accompanying weak predictions shortly after a significant change of the production rate.

Based on the result of the mathematical analysis, an improved model was formulated. The local flow of oil is described as a superposition of three flows:

- Radial flow towards the wellbore and towards a mirror reflection of the well

        - Flow to correct boundary conditions due to the radial flow

3.       - Flow caused by height variations of GOC

The new model handles the proved weaknesses in the near-well area: 2D flow of oil towards the well, gas breakthrough due to viscous gas fingering, as well as horizontal/vertical anisotropy. There are no significant differences compared to present implementation outside the near-well area.

Preliminary testing and mathematical analysis indicate that the new model shows remarkably improved qualities compared to existing GORM in the near-well area.  It should therefore work considerably better for short-term predictions (hours, days). The new model should have equally good medium and long term capabilities, and considerably improved short term and near-well behaviour, compared to the present implementation.

This article has been published in Computational Geosciences. Download it free from:   http://www.springerlink.com/content/q4v4v70h88hr35t4/