Oil production from thin oil zones sandwiched between a gas cap and an active aquifer is often limited to uneconomic rates due to water and gas coning. Horizontal wells seem to provide a promising solution to the coning problem. In order to avoid premature gas breakthrough, the horizontal wells should normally be positioned as far away from the GOC as possible. This assumption has been examined further; wells completed below the WOC have been compared to wells in the oil zone. This strategy relies upon oil coning into the well completions through the water zone; the so called "inverse coning" process.

In this study, production from the 12 m thick oil zone in the Troll West Gas Province has been simulated with a 500 m horizontal well completed in the water zone below the WOC. The effects of various rock and fluid parameters have been evaluated, and a correlation for time to gas breakthrough has been developed based upon the simulation results.

The calculated gain in oil production with the well below the WOC ranged from 15% to over 50% in this study, depending on the reservoir and operating conditions. Based upon cumulative oil production considerations, an optimal completion depth of 3-4 m below the WOC was determined. Cone behaviour was shown to be dominated by permeability level, fluid mobilities, completion depth and liquid production rate. The primary disadvantage with these wells is the sustained increased water cut.

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