Investigation Of Gas Cycling Schemes For The Westerose D-3 Pool
Authors: Bachman R.C. , Papst W. , Legere R.F. 
Published: March 2010

Abstract
Due to the considerable value of liquids within the gas cap, an engineering study was undertaken on the Westerose D-3 pool to determine the potential for gas cycling with either methane or nitrogen. This paper describes the laboratory and reservoir engineering work used in the evaluation process.
Laboratory experiments concentrated on determining the effect of the two possible injection gases on the current gas-cap gas. Following a multi-component mathematical characterization of the reservoir fluids and a field history match via a numerical reservoir simulator, forecasts were generated for various cycling cases. As a result of the higher density of nitrogen compared to the gas-cap gas or methane, significant coning of nitrogen into the oil leg producers was observed. This led to a recommendation for a methane gas cycling scheme which is currently being implemented.

Introduction
An engineering study was undertaken in the Westerose D-3 pool to determine the potential for gas-cap cycling with either methane or nitrogen. Methane and nitrogen were chosen as cycling gases due to their availability and low cost compared to other alternatives.
Considerable effort was made to establish the composition of the current gas-cap gas. Laboratory work was undertaken examining the effects of pressure depletion and contact by the injection fluid on the current gas-cap gas. Numerical work was then performed to match the PVT studies using a multicomponent representation of the fluids.
After establishing an appropriate reservoir description by history matching past pool performance with a numerical reservoir simulator, a series of forecasts examining the various production options were evaluated. As a result of this study, a methane gas cycling scheme was recommended to partners.

Gas Cycling Overview
A cycling scheme in the Westerose D-3 pool was studied because of the value of natural gas liquids in the gas-cap gas. A cycling project accelerates the production of natural gas liquids, enhancing the present value when compared to blowdown at the completion of oil production. There is also potential for revaporizing residual liquids in the gas cap and gas swept oil zone.
Other cycling schemes such as that at the neighbouring Bonnie Glen pool were reviewed. This pool has been undergoing a twostage gas cycling scheme in the gas cap above a diminishing oil zone(1,2l. The lower liquid content of the gas-cap gas and the lower residual oil saturation in the Westerose D-3 pool prohibit a similar production strategy.
Technical and economic issues were examined to determine the feasibility of cycling. The quantification of the natural gas liquids carried in the gas-cap gas was of primary importance. It was also necessary to determine the retrograde behaviour of the gas-cap gas as a result of pressure depletion during blowdown. Another important issue was to examine the retrograde effects of the gas-cap gas contacted by the dry gases methane or nitrogen. Other issues included defining the capabilities of the dry gases to vaporize residual liquid and determining the merits of each of the cycling gases on the displacement process in the gas cap.