THE INJECTION OF WATER INTO AND EXTRACTION OF VAPOUR FROM BOUNDED GEOTHERMAL RESERVOIRS

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 approx-imately 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 extrac-tion. 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 ex-traction of vapour from the reservoir increase with time. However, the rate at which liquid is pumped into the reservoir inay initially decrease but subse-quently 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 in-crease with time. In this case, the pressure a t the injection pump may increase but subsequently de-creases with time, again if a sufficient fraction of the liquid vaporisrs