Rock damage evolution in the production process of the enhanced geothermal systems considering thermal-hydrological-mechanical and damage (THM-D)

Abstract

Hot dry rocks (HDRs) geothermal relies mainly on enhanced geothermal systems (EGS) for production. During production, the evolution of rock stress distribution will cause damage to the rock, especially near fractures. The application of damage mechanics in the geothermal field has mostly focused on the process of hydraulic fracturing, with limited reports on damage research related to long-term water injection development. According to previous experimental studies, the effect of rock damage is significant and cannot be ignored. Therefore, based on a thermal-hydrological-mechanical and damage (THM-D) coupling model, it studies the rock damage evolution features and its role in changes in rock physical properties. It also analyzed the effects of the initial temperature, injection flow, fracture curvature/number, and stress difference on damage evolution. Results indicate that the damage area and degree continue to increase with time, but the growth rate becomes slower. There are two methods of damage evolution: the new damage caused by the expansion of the low-temperature region is throughout the process, and the original damage is aggravated in the later stages. Damage will cause changes such as reduced Young's modulus (negative feedback) and increased permeability (positive feedback) of the rock matrix, which in turn affect the damage evolution. More, damage increases with the initial rock temperature, injection mass flow, and stress difference increase, and the impact of the fracture curvature is small. The damage of complex fracture reservoirs is more pronounced than that of single fracture reservoirs. This study provides a tool for analyzing the impact of damage on the production performance of HDRs. The investigation of relevant patterns is expected to offer guidance for production strategies.