Numerical study on a multifracture enhanced geothermal system considering matrix permeability enhancement induced by thermal unloading

Abstract

We explore an enhanced geothermal system (EGS) reservoir with horizontal wells connecting multiple hydraulic fractures. In particular we follow the implications of complex thermal–hydraulic–mechanical on the thermal unloading of rock matrix on the permeability evolution. The accuracy and reliability of the proposed model is validated/verified against an existing analytical solution and the Fenton Hill demonstration project. Then, the effect of the fracture number on the heat extraction process is discussed. Results show that matrix permeability enhancement induced by the thermal unloading should be considered in the coupling model. Compared with the results from the traditional model, the maximum temperature difference at the production well can reach 20 K. More fractures in the EGS are advantageous for expanding the cooling range along the production well and have a great impact on the temperature. Generally, an EGS with more fractures is prone to obtain a higher production temperature. In the EGS, fracture number plays an important role in considering both the heat transportation rate and the cooling range in a geothermal exploration process. Per the consideration of geothermal exploitation in the matrix and fracture, there is only a slight difference in the heat extraction ratio for the four numerical cases.