Typically, in the presence of water coning, a water-free production is economically unattainable since critical rates are too small. Downhole water sink (DWS) technology has been proved and demonstrated as capable of significantly increasing critical rates. In actual practices, however, DWS wells have produced oil above critical rates but with lower water cut that resulted in high oil rates and the need for water separation. In fact, water handling is an inherent downside of DWS since the downhole water drainage (for coning control) involves independent lifting of oil and large amount of water to the surface. In contrast, downhole water loop (DWL) technique would combine coning control with water-free production by recycling the drained water from the water sink (drainage) and source (injection) completions and keeping the water in-situ. Experiments show that the beneficial effect of water recycling on well performance is very strong – a moderate water recycling rate would increase critical oil rate by ten times comparing to the conventional well.
Although attractive, the DWL technique is constrained by well-reservoir system properties such as oil viscosity, relative oil permeability, bottom water thickness, well penetration and reservoir anisotropy. This study offers a simple analytical model describing the effects of the above parameters on the water-free oil production rate. The model has been derived using principles of the flow potential theory and anisotropy transformation supplemented with some empirical correlations. A comparison with reservoir simulator demonstrates time savings and acceptable accuracy of the analytical approach. A sensitivity study reveals factors most critical for using DWL in specific reservoirs: critical thickness of the bottom water, limiting value of the fluids mobility ratio and strong effect of permeability anisotropy in the reservoir and water layer. Also presented is a statistical assessment of reservoir systems potentially suitable for the DWL technique.