Hydraulic fracturing in a stiff naturally fractured rock mass such as a shale gas reservoir or a geothermal site in an igneous rock involves a complex interaction between the stress and flow effects of the induced fracture process with a geometrically complex, strongly anisotropic, and undoubtedly heterogeneous medium. Deformation mechanisms include the opening of the primary hydraulic fracture, opening of near-by joints, and small-scale shearing of near-field and more distant joints that are not fully wedged open by the high hydraulic pressures. Aspects of initial state (joint system, stresses, mechanical properties…) are complex and must be better understood. The changes to the system during hydraulic fracturing include large scale stress and pressure changes, as well as irreversible and geometrically complex changes to the joint systems. Because of the different scales involved, large-scale modeling must involve some type of upscaling, but the best approach to this in different cases and for different processes remains obscure. The article attempts to give a physical portrait of the process to help guide model development, but also to help understand what happens in real cases. Some examples of simple 2-D modeling efforts in simulated jointed media are given to show how the physical understanding can, qualitatively, be supported and extended by careful analysis, albeit of a preliminary nature.

1. Introduction

Some applications of high pressure injection and hydraulic fracturing in naturally fractured rock (NFR) masses include jointed rock mass grouting to reduce permeability and perhaps increase rock mass strength; jointed igneous rock fracturing at depth to aid in the extraction of geothermal energy; deep slurried solid waste injection; and, development of oil and gas trapped in stiff jointed rock masses. The main driver of research at present is the rapid advance of technology in the hydrocarbon industry to exploit shale gas, shale oil and tight gas deposits. The following sections are general discussions, and detail is unreferenced, although many sources over the years could be detailed (Curtis 2002, Harris 2015).

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