The ability to generate deep flow in massive crystalline rocks is governed by the interconnectivity of the fracture network and its permeability, which in turn is largely dependent on the in-situ stress field. The increase of stress with depth reduces fracture aperture, leading to a decrease in rock mass permeability. The frequency of natural fractures also decreases with depth, resulting in less connectivity. The permeability of crystalline rocks is typically reduced to about 10 −17 -10 −15 m 2 at targeted depths for Enhanced Geothermal Systems (EGS) applications, i.e., > 3 km. Therefore, fluid injection methods are required to hydraulically fracture the rock and increase its permeability.

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The ability to generate deep flow in massive crystalline rocks is governed by the interconnectivity of the fracture network and its permeability, which in turn is largely dependent on the in situ stress field. The increase of stress with depth reduces fracture aperture, leading to a decrease in rock mass permeability. The frequency of natural fractures also decreases with depth, resulting in less connectivity. The permeability of crystalline rocks is typically reduced to about 10 À17 -10 À15 m 2 at targeted depths for enhanced geothermal systems (EGS) applications, that is, >3 km. Therefore, fluid injection methods are required to hydraulically fracture the rock and increase its permeability. In the mining sector, fluid injection methods are being investigated to increase rock fragmentation and mitigate high-stress hazards due to operations moving to unprecedented depths. Here as well, detailed understanding of permeability and its enhancement is required. This paper reports findings from a series of hydromechanically coupled distinct-element models developed in support of a hydraulic fracture experiment testing hypotheses related to enhanced permeability, increased fragmentation, and modified stress fields. Two principal injection designs are tested as follows: injection of a high flow rate through a narrow-packed interval and injection of a low flow rate across a wider packed interval. Results show that the development of connected permeability is almost exclusively orthogonal to the minimum principal stress, leading to strongly anisotropic flow. This is because of the stress transfer associated with opening of tensile fractures, which increases the confining stress acting across neighboring natural fractures. This limits the hydraulic response of fractures and the capacity to create symmetric isotropic permeability relative to the injection wellbore. These findings suggest that the development of permeability at depth can be improved by targeting a set of fluid injections through smaller packed intervals instead of a single longer injection in open boreholes.

Last year, a joint Mining and Oil & Gas industry consortium was established in Canada to conduct hydraulic fracturing HF tests accompanied by a mine-back of fractured regions to assess HF models and microseismic monitoring data during controlled experiments. Details about the displacement field, fracture aperture and extent, and micro-seismic parameters could then be verified and used as calibration data for modeling of HF processes in igneous and dense sedimentary rocks.Various injection experiments are planned and they will include pre-fracturing rock mass characterisation using best available current techniques, dense arrays of multi-parameter wall and borehole-mounted instruments, and the treated volume will be mined through to assess fracturing effectiveness, existing fractures and new fracture interactions, and to determine if pathways can be identified for improving currently available numerical and fracture network modeling tools.In this paper we present the results of the experimental design and planning phase, outlining objectives and justifications for planned experimental layouts. Preliminary plans for a first mine-through trial at Newcrest Mining's Cadia East mine in New South Wales, "ustralia are described. The hypotheses advanced in this experimental design, supported by evidence from the literature, are that activation and development of a fracture network by hydraulic stimulation is possible if the injection procedure is designed such that injection pressures and rates are maintained within an optimal window, thereby producing conditions under which effective stress management for risk mitigation in deep mining can best be achieved. The evaluation of these hypotheses is the focus of the current high level experimental plan presented in the paper.

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