A General Solution of Heat Transfer Model in Heterogeneous Enhanced Geothermal Systems with Temperature-dependent Variables

Abstract

Geothermal resource from Enhanced geothermal system (EGS) is considered as promising clean energy to meet increasing global energy demand with minimized environmental footprint. Heat extractions in EGS have been extensively studied both analytically and numerically with a general assumption of a homogeneous reservoir, however, EGS is always with strong heterogeneity in the real-world setting, due to temperature-dependent rock properties and non-uniform stimulated fractures. Heat transfer in such heterogeneous fractured porous media largely depends on the contact interfacial features and thermal properties of rock and fluid as a function of space and temperature. Therefore, a good understanding of the effect of reservoir heterogeneity on heat extraction is considerably important to properly evaluate, design and implement EGS development projects. This paper aims to investigate the temperature distribution in a heterogeneous geothermal reservoir and characterize the impact of heterogeneity on thermal extraction. In this study, pseudo-temperature, pseudo-time, and pseudo-distance are introduced to reduce the radial flowing diffusivity equation with varying coefficients to that with constant coefficients in front of derivatives. Effect of reservoir heterogeneity is represented and characterized by the function of the heterogeneity index, which includes all heterogeneity related properties. And the heterogeneity index is then applied to the generalized diffusivity equation with new transformed variables for the analytical solution in Laplace domain. This work could contribute to EGS operators to perform better project evaluation, production forecast, and subsequent decision-making