Numerical parametric investigation of thermal extraction from the enhanced geothermal system based on the thermal-hydraulic-chemical coupling model

Abstract

There are chemical dissolution and precipitation in geothermal formations with the long-term exploitation of hot dry rocks, which would induce the remarkable alteration of fracture transmissibility and evolution of thermal performance. The objective of this study is to identify the influence of various parameters incorporating chemical reactions to provide suggestions for the operation of the enhanced geothermal system (EGS). A 3D thermal-hydraulic-chemical coupling model is established for a fractured EGS considering silica-water reaction kinetics at fracture surface. Parametric study and sensitivity analysis are innovatively conducted to investigate the effects of key operation and formation parameters, including injection concentration (cin), injection temperature (Tin), injection rate (Qin) and initial formation silica reactant content (Ci), on thermal extraction performance. Results indicate that undersaturated injection causes chemical dissolution while oversaturated injection leads to silica precipitation. Higher cin relieves the thermal breakout (10 °C) but enhances the injection-production pressure difference (13.5 MPa). The selection of cin is an efficient way to adjust geothermal production, especially for oversaturated injection. The influence degrees of cin on production temperature (Tout) and pressure difference (Δp) under oversaturated injection are 8 times and 2 times larger than those under undersaturated injection. Tin (<80 °C) exerts little influence on the chemical reaction. The remarkable water storage phenomenon will be induced with the increase of Qin when precipitation occurs. Higher Ci results in lower Tout and Δp. The sensitivity analysis provides the rank of major parameters to thermal performance: Qin, Tin, cin and Ci. The standard sensitivities of Qin to Tout, Δp and net thermal power N are 0.78, 0.63, and 0.88 respectively. As for the contribution ratio, cin contributes the most (89%) to the variation of fracture aperture while Tin (<80 °C) contributes the least (nearly 0%). The research will provide a significant reference for the selection of parameters during the development of geothermal energy.