| Abstract |
Proppants are widely used in the oil and gas industry in combination with hydraulic fracturing, but have been used only sparsely in geothermal reservoirs. They help maintain fracture apertures after injection has ceased and the well reheats, but mineral deposition or dissolution may significantly affect their usefulness. A series of static experiments has been conducted under simulated geothermal reservoir conditions in order to determine the effects of mineral deposition. The experiments utilized: crushed quartz monzonite; sintered bauxite proppant; quartz sand proppant; and geothermal water from the Raft River geothermal field in south-central Idaho. Mixtures of granular solids and liquid were placed within an autoclave and reacted at 225°C and 275°C for 38 days to 10 weeks. In reactions at 275°C secondary mineralization was observed coating and cementing together the granular solids, with accompanied dissolution of the quartz monzonite. These precipitated phases include opal-CT, zeolite(s) and a calcium silicate mineral. No secondary mineralization was observed at 225°C. Silica, calcium, potassium and aluminum concentrations in the reacted geothermal waters were found to increase significantly by the end of the experiment. The silica concentrations exceed quartz saturation, indicating disequilibrium and dissolution of plagioclase and biotite, and possibly the ampoule. Despite the state of disequilibria in runs at 225°C, the absence of new mineral products suggests that reaction kinetics are slow and that mineral deposition is unlikely to degrade porosity-permeability at least over time scales of a couple of months. This is not the case at the hotter condition of 275°C, where precipitation of silica and zeolite(s) reflects an adjustment towards fluid-mineral equilibria. |