Numerical Analysis of Fracture Propagation Behavior in Hot Dry Rock During Radial Borehole Fracturing

Abstract

ABSTRACT Radial well fracturing is a promising stimulation method, which shows great potential in addressing the issues suffered in enhanced geothermal system (EGS), such as poor connectivity and difficulty in controlling the fracturing paths. To determine the applicability of radial borehole fracturing in HDR, a THM-FDEM coupling model for studying the fracture propagation behavior of radial borehole fracturing is established. Firstly, the reliability of the coupled THM model is validated by radial borehole fracturing experiments. Then, the effects of azimuth, number, length and spatial distribution of radial boreholes on fracture propagation behavior are analyzed. The results show that implementing radial borehole fracturing in HDR can overcome the adverse effects of in-situ stress, allowing fractures to expand in the desired direction. Increasing the number and azimuth of radial borehole while decreasing their length can increase the angle of fractures deviating from the direction of maximum horizontal principal stress. In the case of double-branch, a branch angle of 60° can result in stress interference between radial boreholes, inhibiting fracture propagation from the toe of the radial borehole. The research findings of this study are expected to provide valuable guidance for the field application of radial borehole fracturing in HDR geothermal reservoirs. INTRODUCTION Geothermal energy, as a clean and renewable energy resource, can promote the development of electricity generation, heating, tourism, and other industries (Mock et al., 1997; Fridleifsson et al., 2001; Armstrong et al., 2016). HDR geothermal formations have the characteristics of high temperature, dense lithology, low permeability, and poor crushability. The building of an enhanced geothermal system (EGS) by hydraulic fracturing is the major means of extracting deep-subsurface heat energy (Anderson et al., 2019; McClure et al., 2014). However, there are some technical issues in the practical application of EGS technology, such as poor connectivity between injection and production wells, and difficulty in controlling the fracturing paths.