Coupled hydro-thermo-mechanical modeling of hydraulic fracturing in quasi-brittle rocks using DEM

摘要

This paper presents a novel contribution towards modeling of fully coupled hydro-thermo-mechanical quasi-brittle rock behavior. Specifically, the new developments towards modeling of coupled processes in quasi-brittle rocks are used for better understanding fracture initiation and propagation for Enhanced Geothermal Reservoirs (EGS). Permeability of quasi-brittle hot dry rock is enhanced with hydraulic fracturing technique, where new fractures are formed from the previously drilled deep borehole. Technical challenges during hydraulic fracturing of EGS are very high rock temperatures that exceed 200° C and high in-situ stresses at depths of 3-5 km. The approach used in this study is the Bonded Particle Model (BPM) that was previously developed within the Discrete Element Method (DEM) framework. The main advantage of bonded DEM models is the ability for explicit fracture propagation and stress-strain behavior modeling. PBM is extended to account for conductive-convective heat transport and to enable hydro-thermal fluid-solid coupling. Fracturing of hot dry rocks is studied and the impact of temperature difference between rock and fracturing fluid is investigated. The results presented in this paper show thermal damage and thermal micro-cracking at the borehole perimeter and along the new fracture surfaces. Damage mechanisms were studied and new micromechanical understanding of temperature difference effect on hydraulic fracturing between rocks and fracturing fluid is given in the paper.