Thermal effect on the evolution of hydraulic fracture conductivity: An experimental study of enhanced geothermal system

Abstract

This paper aimed to investigate the thermal effect on the evolution of hydraulic fracture conductivity during heat extraction. Meanwhile, the degradation mechanisms of fracture conductivity were analyzed in terms of the mechanical damage of fracture surfaces. Experimental results of micro-indentation tests on the studies granite show the existence of a threshold value of temperature. The Young's modulus and hardness of fracture surface significantly decreased when the thermal treatment levels were higher than 300 °C. Similarly, the degradation of fracture conductivity was limited (<10%) before 300 °C and then significantly deteriorated at 400 °C and 500 °C. Moreover, cyclic thermal treatment tended to aggregate the mechanical damages of fracture surfaces further. At the thermal treatment level of 300 °C, obvious thermal softening of fracture surfaces and degradation of fracture conductivity were observed only when the number of cycles was larger than 10, while both of these two phenomena became more severe with the increasing of the number of cycles at the thermal treatment level of 400 °C. The evolution of fracture conductivity can be accounted for by the temperature-related integrity of fracture surfaces and proppant pack. It was indicated that the thermal softening of fracture surface tended to enhance the proppant embedding, which might be beneficial for the alleviation of proppant crushing.