Experimental Investigation of Thermal Effect on Fracability Index of Geothermal Reservoirs

The thermal effect is not considered in the current fracability index model. In this paper, we investigated experimentally the formation capacity of fracture networks in hot dry rocks at different temperatures, by using rock mechanics triaxial testing system and acoustic emission monitoring. A new brittleness index with consideration of thermal effect is proposed on the basis of crack volumetric strain calculated at different stress levels, which captures the following two characteristics: (1) thermal micro-crack density in the crack closure stage and (2) crack propagation, nucleation and coalescence by external stress. Then, a comprehensive fracability index model is established, which integrates the thermal effect and fracture toughness at different temperatures and confining pressures. The fracability index calculated by the new model is consistent with acoustic emission characteristic, microstructure analysis andb-value analysis, which verify that the proposed model is reliable. The results show that both temperature and confining pressure have a significant effect of formation capacity of fracture networks in hot dry rocks. This study provides new insight into improving the thermal efficiency in geothermal reservoirs.

Automated summary

This study proposed a new brittleness index model considering thermal effect based on rock mechanics testing and acoustic emission monitoring, and established a comprehensive fracability index model. The results verified the reliability of the model and showed that both temperature and confining pressure significantly affect the formation capacity of fracture networks in hot dry rocks at different temperatures and confining pressures.