In an oil well a quantity of water is mixed with the oil which flows to surface tanks from underground formations. This water is separated from the oil and then injected back into the underground formations. A high percentage of water can make the production of oil uneconomical, due to the expense of circulating the water through a "water loop" which begins and ends in the underground formation.
The industry is currently experimenting with methods for reducing the amount of formation water in oil production. One method involves the creation of a "water sink" which alters the shape of the oil/water contact. Another method involves using biological or chemical agents as "blockers" to block off water channels in the reservoir.
An example of the "water sink" method is described in a paper by A. K. Wojtanowicz of Conoco Inc. and H. Xu of Louisiana State University in an article entitled "A New Method to Minimize Oilwell Production Watercut Using a Downhole Water Loop" published by the Petroleum Society of the Canadian Institute of Mining, Metallurgy and Petroleum as paper No. CIM 92-13. According to this method a pump is placed downhole and used to drain formation water from around the well creating the water sink. This reduces formation water produced into the well with the oil and, consequently, the water content in oil that flows to surface. The water which is pumped to create the water sink is preferably pumped a relatively short distance from one underground formation into another underground formation.
The "water sink" method proposed by Wojtanowicz and Xu relies upon having a highly porous and permeable reservoir with a single relatively stable oil/water interface. A very detailed understanding of the characteristics of the reservoir rock is required; information which is often not available. Even when the information is available, conditions favourable to the water sink method are often not present. Porosity and permeability of the rock vary considerably in some reservoirs causing a breakthrough of the water high in the producing zone. Other reservoirs have multiple oil/water contacts, making control of formation water through the water sink method impractical.
The "blocking" method, using biological or chemical agents to block off water channels in the reservoir, also has its drawbacks. It is difficult to control the blocking agents when they are injected. The treatments are expensive and often must be repeated in order to achieve the desired effect.