Reinjection of used geothermal liquid s into the producing reservoir has been proposed by several investigators as an effective way to a,void disposal problems and to reduce the possibility of land subsidence in the reservo i r area. However, after a length of time, the reinjectoon water will break through at the production well, which will no longer be able to maiintain its original temperature. Gringarten and Sautyl have developed a simple steady- state model to study such a system of many production and in,jection wells, capable of calculating the temperature decrease as functions of time. The present paper describes a possibility of retarding the breakthrough time and reducing the temperature drop by means of screening wells. The basic physical idea is as follows. Consider a doublet of one production and one injection well. to the production well. well will correspond to those close to the straightline leading from the injection to the production well. Now if we have an extra screening well on this line in between the two wells, which extracts fluid and puts it back into the injection well, then these earliest streamlines will be "picked up" by the screening well and prevented from arrivi g at the production well. I n this way, not only is the breakthrough time increased, but also the rate of temperature drop is reduced. Besides the temperature difference of the reinjected water and the reservoir, two dimensionless parameters enter into the problem, the rate of extraction of the screening well relative to that of the production well, y, and the position of the screening well, CY, defined as the distance of screening well from the production well divided by the separation of the doublet. It is easy to see that the

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