In bottom water drive oil reservoirs, the use of dual-completed wells with water drainage-dubbed: Downhole Water Sink (DWS) technology - has proved to control water coning, increase oil production rate and total recovery factor. Also, in field trials, the technology demonstrated sustainable production of oil-free drainage water. The ability to produce oil-free water makes DWS extremely attractive in offshore operations as no water processing is needed prior to overboard discharge. In land operations, oil-free water may be injected into the same well below the drainage completion or lifted to the surface and disposed into injection wells without processing.

Field tests and our studies have also showed that sustainable drainage of oil-free water becomes somewhat difficult as the two completions (top and bottom) may receive co-mingled inflows of the two fluids. The phenomenon indicates the existence of transition zone (with mobile oil and water) much larger than that explained by the capillary pressure effect. This paper presents the results of a study - using numerical simulator and a pie-shaped physical model-aimed at understanding the change of transition zone around producing wells with and without DWS.

The results show that, in conventional wells with water coning, the transition zone is small and constant away from the well but enlarges towards the wellbore. This transition zone enlargement effect occurs in conventional wells due to diffusion resulting from pressure distribution around the well. For DWS wells, however, the effect is not only more pronounced but it also alters the IPW plots – a basic tool for design. There is a commingled inflow envelope in addition to the envelope of segregated inflow.

Based on the above understanding, the paper shows how to design operational limits for DWS wells to maximize oil production from the top completion and maintain oil-free water drainage from the bottom completion.

You can access this article if you purchase or spend a download.