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.References and illustrations at end of paper
Inspired by the success o f the first long-term test with a horizontal well on Troll1, a second long-term test was carried out in 1991. This 4 months long-term production test with an 800 m horizontal well was performed in a 13 m oil column sandwiched between a large gas cap and an active aquifer in the Troll West Gas Province. The horizontal section target depth was 1 m above the WOC, but approximately 25% o f the well trajectory was actually the WOC.The observed production data from the test were history matched with a numerical simulator. The effects o f various geological features on water cut and GOR development were studied, and several simulation models matching the production history were built. These models were used t o evaluate the effects on oil recovery o f various design parameters such as; length o f the horizontal section, completion depth, liner diameter and liner roughness.The production history indicated that inverse coning took place in the parts o f the well situated below the initial WOC. The history matching confirmed the results from the first long-term test, but also demonstrated the strong sensitivity o f history matching and long-term production predictions t o small changes in geological interpretations and t o frictional pressure loss in the horizontal section.Large upside potential was found in the Troll West Gas Province for longer horizontal wells, deeper completions and super-critical production.References and illustrations a t end o f paper 909
This paper presents the results of two long-tenn horizontal well tests in the Troll field. Planning and evaluation of the tests and simulation of and consequences on field development are summarized. The two tests demonstrated significant production potential from 12-to 22-m oil columns and verified the pretest assumption that one horizontal well could replace four vertical wells.
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