Numerical Simulation of Hydraulic Fracturing Considering Grain Boundary Weakness and Fluid-Rock Heat Transfer
Authors: Ikeda S. , Ogata S. , Yasuhara H. , Kishida K. 
Published: August 2024

ABSTRACT:
When simulating the hydraulic fracturing process in hot dry rock, particularly in estimating the fracture length from the wellbore and the breakdown pressure, it is crucial to account for the mineral grain distribution within the rock, as well as the heat transfer between the fracturing fluid and the rock. Current numerical modeling approaches that consider the heterogeneity of actual rock are mostly based on the Discrete Element Method (DEM) or the Combined Finite-Element and Discrete-Element Method (FDEM). In this study, we propose a method to integrate mineral distribution, particularly focusing on the weakness of grain boundaries, into Finite Element Method (FEM) analysis. This method is grounded in continuum damage theory, incorporating failure assessments on each finite element while considering weak planes: mineral grain boundaries. Unlike methods that create a mesh considering the actual grain distribution, our approach involves assigning material properties to each analytical element, which are created independently of the mineral distribution, and takes weak planes into account during failure assessments. Using this method, we conducted a parametric study to investigate how thermal stress affects the fracture propagation and the shape of the fracture propagation path.

1. INTRODUCTION
Hydraulic fracturing originally served as a technique for measuring crustal stress. It has attracted attention for its application in developing shale oil, shale gas, and geothermal resources. Generally, simulations of hydraulic fracturing predominantly considered mechanical-hydrological coupling. However, an actual reservoir is mostly composed of hot dry rock (HDR), thus typical hydraulic fracturing involves injecting cooler fluid into HDR, inducing thermal stress in the rock. For simulating hydraulic fracturing in energy generation, consideration of thermal and geochemical reactions has become increasingly important in addition to the traditional mechanical-hydrological aspects. Consequently, an analysis of hydraulic fracturing coupling thermo-hydro-mechano-damage (THMD) interaction is desirable to explore the mechanism of fracture propagation.
Research has addressed hydraulic fracturing considering thermal stress. Zhang et al. (2021) conducted a hydraulic fracturing experiment at a laboratory scale under triaxial compression conditions, performed a 3D numerical simulation of hydraulic fracturing under thermal stress conditions, and validated the model with experiments. Additionally, they have conducted a numerical simulation using different types of fluids, water, and supercritical CO2, and studied how fluid properties affect the morphology of fractures. Xue et al. (2023) also conducted a hydraulic fracturing experiment as well as a heating-cooling test of granite to study the morphology of fracture propagation under thermal stress conditions. They developed a numerical model based on Finite Element Method (FEM), verified its reliability with the experiments, and discussed the effect of modeling parameters and thermal stress in the process of hydraulic fracturing. Some other studies numerically investigate the hydraulic fracturing process; however, most models use probability distribution to define mechanical properties (e.g., Zhang et al. (2019), Tang (1997)). Yoshioka et al. (2022) revealed that mineral grain distribution, particularly in the wellbore vicinity, impacts fracture propagation path and tortuosity. Therefore, conducting hydraulic fracturing simulations that consider the mineral and weak plane distributions of actual rock is crucial.