Oil bypassing is a significant problem in reservoirs with water drive. The bypassing mechanism is commonly attributed to distortion of frontal displacement due to the effects of viscous fingering and permeability variations. However, considerable bypassing may also result from ineffective well completion design causing water saturation buildup at the well, low productivity of oil, and -in the result- well marginalization with unrecovered oil left behind. In this study, numerical reservoir simulation is used to investigate the well completion's length and placement that would optimize oil recovery in bottom and edge-water systems. The effect of a single completion's length (penetration)is evaluated for different reservoir conditions. Then, single completions are compared to dual well completions (Downhole Water Sink, DWS) for both newlycompleted and marginal (or watered-out) wells. For the bottom- water systems, the results show that the best completion strategy for both the new and the marginal wells is the use of DWS. The system would prevent or reduce the growth of water saturation around the well, thus increasing oil relative permeability. In the edge-water systems, on the other hand, the best completion strategy for both new and watered out wells is the use of either DWS or fully penetrating wells. The main advantage, however, of DWS over fully penetrating wells is the independent control of each completion. Total recovery is similar to that with partially penetrating wells, but it takes less than a third of the time (70% shorter).
Oil bypassing due to water invasion can be a significant problem in bottom and edge-water systems(1),(2). The main mechanisms of water invasion in bottom and edge-water drive reservoirs are coning and underruning, respectively(1),(2) Many different methods are used to reduce water invasion. Typically, they are classified as mechanical and chemical methods, as shown in Figure 1. By some authors, the mechanical methods involve the use of packers, bridge plugs, well abandonment, infill drilling, pattern flow control and horizontal wells(3). Among the most commonly used chemical methods are cement, sand, calcium carbonate, gels, resins, foams, emulsions, particulates, precipitates, microorganisms and polymers(3). Thus, the effective strategy for selecting a proper method should involve a careful diagnosis of the excessive water problem. Seright et al.(3) proposed a methodology to attack the various types of water problems by categorizing them from the least to most difficult. Water coning and underruning were considered the most difficult with no easy, low-cost solution. The authors maintained that gel treatments will almost never work for coning and underruning problems. Their observation was based on extensive reservoir and completion engineering studies and analyses of many field applications.
Gel treatments target the reduction of water inflow to wells implicitly assuming that oil inflow would increase. In case of water coning and underruning the flow of water is an integral part of flow deliverability and, as such, cannot be stopped or reduced without reducing the oil flow. Thus, strategies studied in this paper address the increased/accelerated oil recovery, instead of reduced water production.
