Vapour extraction from a water-saturated geothermal reservoir

Abstract

We examine the decompression and associated flow of water through a high-pressure permeable rock towards a well of low pressure. Using a series of new analytical similarity solutions, and some asymptotic simplifications, we explore the controls on the vaporization as a function of (i) the pressure jump from the far field to the well; (ii) the initial superheat of the reservoir and (iii) the permeability of the system. We find that for a sufficiently large pressure decrease, the liquid becomes superheated and boils, leading to venting of vapour into the well. For a low-permeability system, the flux of liquid from the far field is rate limiting and a sharp boiling interface develops ahead of the well. However, in higher-permeability rock, the flux of water from the far field increases and there is insufficient heat conducted from the far field in order to boil this water. The boiling front is then located at the well, while the fluid ahead of the boiling front may become superheated leading to formation of a two-phase zone which spreads outwards from the well.