Precipitate formation in a porous rock through evaporation of saline water

Abstract

We examine the motion of a high-pressure aqueous solution, through a low-permeability fracture, towards a low-pressure well. As the liquid decompresses in the fractures it expands, and for sufficiently high initial temperature the liquid reaches the boiling point. A vaporization front then develops, so that vapour issues from the well. As the fluid evaporates near the well, the salt concentration of the residual fluid increases. If the salt concentration increases beyond the saturation limit, then the evaporation leads to precipitation of salt in the fracture. We find a new family of self-similar solutions to describe the boiling and precipitation in a single idealized fracture, which at long times remains approximately isothermal owing to the cross-fracture heat transfer. The solutions describe the mass of salt that precipitates as a function of the initial salt concentration, the reservoir temperature and pressure, and the well pressure. In fact, this family of self-similar solutions is multi-valued: we identify a liquid-advection-dominated regime, in which the boiling front advances slowly and the fracture porosity decreases significantly, and a boiling-dominated regime, in which the boiling front advances more rapidly, and less precipitate forms at each point in the fracture. As the pressure difference between the well and the far field reservoir increases, these solutions converge, and eventually coincide. Beyond this critical point, there is no similarity solution, since the advective flux of salt from the far-field would produce more precipitate than can be taken up in the fracture adjacent to the boiling front. Instead, the rock will become fully sealed through precipitation, thereby suppressing flow into the well. We extend the model to show that an analogous result also occurs within an extensive porous layer. However in that case, the system is not isothermal; instead, the heat flux is supplied in the direction of flow, while the cross-flow heat flux is small. We discuss the relevance of the work to the natural venting of steam in high-temperature geothermal systems.