Simulating Hydraulic Fracturing with Varied Well Placement for Preconditioning in Cave Mining
Authors: Fu W. , Damjanac B. , Ghazvinian E. , Fuenzalida M. 
Published: September 2023

ABSTRACT
Hydraulic fracturing is becoming increasingly used for orebody preconditioning in cave mining. This approach aims to enhance caveability, cave propagation and resource recovery, improve rock fragmentation, and mitigate the seismic hazard during caving operations. To date, there is still limited experience with hydraulic fracturing in caving applications. Additional challenges arise because in these applications hydraulic fracturing needs to be conducted in the vicinity of caves and excavations, where both magnitudes and orientations of the stresses are significantly altered compared to the far-field stresses. In this study, we present a workflow for hydraulic fracturing simulation and optimization in cave mining. Specifically, discrete element method (DEM) models are constructed to simulate hydraulic fracture propagation based on a lattice approach, in which the deformation and fracturing of the rock matrix are simulated by the lattice, and fluid flow is implemented within the fractures and fully coupled with the mechanical deformation. The stress field affected by mining operations, as predicted by calibrated continuum models explicitly representing the existence of the cave, is imported into the DEM model. Different hydraulic fracturing preconditioning scenarios corresponding to varied locations of boreholes are studied to investigate the influence of an existing close cave on hydraulic fracture trajectories, interactions and optimum spacing. This study highlights the importance of optimizing fracturing designs based on the location of the boreholes and the impact of the cave on the stress field.

INTRODUCTION
Hydraulic fracturing has been widely used as a reservoir stimulation technique, which involves the injection of high-pressure fluid into the rock formation to create fractures, allowing for increased permeability of the rock matrix as well as enhanced energy resource recovery (Economides & Nolte, 1989). The most prominent examples of hydraulic fracturing applications include enhancing hydrocarbon production in unconventional reservoirs and developing fracture networks in enhanced geothermal systems, in which interconnected pathways are created to facilitate the flow of oil, gas, or hot water towards the wellbore for extraction (Birkholzer et al., 2021; Moore et al., 2018).
In recent years, there is an increasing interest in the utilization of hydraulic fracturing for preconditioning in cave mining (He et al., 2016). Cave mining is a mining method that uses gravity to extract fragmented ore from underground mines. The method is primarily used for the extraction of ore bodies that are deep and generally low grade in nature, which makes conventional mining methods economically unviable. Cave mining operations can be challenging because the high strength of rock mass and high stress conditions at large depths can result in poor fragmentation of the ore and make it prone to seismic hazards. The use of hydraulic fracturing to precondition the strong orebody by fracturing has been found effective for improving caveability and reducing seismic hazards during cave propagation and breakthrough (Flores, 2019).