When liquid is injected into a geothermal reservoir, a fraction of the liquid may vaporise if the reservoir is sufficiently hot. The vapour forms at an approximately p1ana.r liquid-vapour interface and diffuses towards the far boundary of the reservoir. If vapour is extracted froin the far boundary, then once the new vapour has diffused across the reservoir, the rate of production of vapour at the liquid-vapour interface approximately balances the rate of extraction. We find that if the pressure a.t the injection pump and extraction well is fixed, then the fraction of the liquid which vaporises a.nd the rate of extraction of vapour from the reservoir increase with time. However, the rate at which liquid is pumped into the reservoir inay initially decrease but subsequently increases with time, if a sufficient fraction of the liquid va,porises. If the inass flux of liquid injected into the reservoir is fixed, then aga.in both the fraction of the liquid which vaporises and the inass flux of vapour which may be extracted increase with time. In this case, the pressure at the injection pump may increase but subsequently decreases with time, again if a sufficient fraction of the liquid vaporisrs.

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