Effects of permeability heterogeneity on heat extraction performance in geothermal reservoirs with carbon dioxide working fluid

Permeability is an important formation parameter, highly affecting the fluid flow and hence, the heat extraction performance of geothermal reservoirs. In realistic circumstances, this parameter is inherently heterogeneous, and therefore assuming homogenous permeability within the modeling process leads to erroneous outcomes. In this study, thermo-hydraulic-mechanical numerical simulation is performed to quantify the effects of permeability heterogeneity in terms of variance and correlation length on the evolution of heat mining functionality indicators including production average temperature, output thermal power, flow impedance, and heat extraction ratio using a typical horizontal doublet geothermal model equipped with transient rock and fluid properties as well as supercritical carbon dioxide fluid under two production modes of constant pressure and mass flowrate well boundary conditions. To consider uncertainty effects in permeability variance, 20 realizations for each permeability variance of 0.25, 0.5, and 1 are taken into account. The results indicated that when permeability is assumed constant within the modeling process, the output thermal power and flow impedance are underestimated while average production temperature and heat extraction ratio are overestimated under constant mass flowrate well boundary condition. This is while under constant pressure well boundary condition, heat extraction ratio, production temperature (at the initial heat production period), and output thermal power is overestimated as opposed to underestimation of flow impedance. Under constant pressure well boundary condition, it is observed that output thermal power is larger and production temperature, heat extraction, and flow impedance become smaller for larger values of correlation length. With regard to uncertainty in permeability variance, the emerged findings demonstrated that under constant mass flowrate condition, production temperature and heat extraction ratio are more likely reduced while flow impedance and output thermal power are more likely increased with more permeability heterogeneity. However, under constant pressure boundary condition, production temperature is more likely increased and output thermal power is more likely reduced as the degree of permeability variance is increased. The comparison between thermo-hydraulic and thermo-hydraulic-mechanical coupling indicates that mechanical effects should be considered when permeability heterogeneity prevails though the levels of discrepancy between these two coupling methods differ for different heat production performance indicators and under different well production strategies.