Hydraulic fracturing operations in mining: conceptual approach and DFN modeling example

Abstract

Most of the hydraulic fracturing experiments by the mining industry in hard rocks were conducted to precondition the rockmass with the aim of improving caveability and fragmentation for block caving mining operations through the creation of hydraulic fractures (HF). Based on an extensive literature survey and models, it is suggested that successful preconditioning could be obtained through hydraulic treatment of the rockmass. This paper discusses the interaction between hydraulic fluid injection and the pre-existing discrete fracture network (DFN) in a rockmass subject to in-situ stresses. Three-dimensional numerical studies have been used in an initial attempt towards understanding how the rockmass and the pre-existing natural fractures response to fluid injection is affected by some of the DFN characteristics and borehole length. Results indicate that DFN characteristics control fluid percolation in low-permeability formations and influence stimulated rock volume. When injection pressures are lower than pressures required for hydraulic fracturing, borehole length does not influence significantly fracture surface area stimulated by slip. It is shown that representing the fractures explicitly in the numerical models and adopting a fully coupled hydromechanical modelling approach provide promising capabilities in the prediction of rockmass responses to fluid injection.

On the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity

“…Similarly, laboratory investigations by Doe and Boyce (1989) showed that the stress orientation defines hydraulic fracture propagation only for a stress field anisotropy ratio σ 1 /σ 3 > 1.5. At near isotropic stress conditions the fractures branch more strongly and without a preferred propagation direction, a phenomenom often referred to as high fracture complexity (e.g., Katsaga et al, 2015). During large-scale stimulations, there is a tendency for seismic clouds to develop perpendicular to the minimum principal stress direction σ 3 (Häring et al, 2008;Evans et al, 2005), particularly for HF operations (e.g., Rutledge et al, 2004), although for HS stimulations in crystalline rocks there are many examples in which the seismicity cloud is oblique to the σ 3 direction (e.g., Block et al, 2015;Murphy and Fehler, 1986;Pine and Batchelor, 1984), presumably reflecting the complex interplay between stress and the pre-existing fracture population that is suitably oriented for slip reactivation.…”

Numerical Investigation of Hydraulic Fracture Propagation in Naturally Fractured Reservoirs Based on Lattice Spring Model

“…Treatment with a sufficiently high-viscosity fluid prevents the fluid from leaking into the surrounding rock, which results in a significant stress concentration. As the injection rate increases, the leakage into the surrounding rock becomes less important [43]. Moreover, poroelastic stress changes can locally modify the given tectonic stress regime [44,45].…”

Analysis of seismic tomography data for definition of hydraulic fracturing parameters in underground coal mines

“…When carrying out geophysical study, a set of seismic equipment (Figure 1) was used to record data, consisting of: autonomous seismic recorders "R-1", providing seismic data in digital format (1) and geophones (2), determining the number of channels in the performed research. A percussive source of mechanical vibrations is used to generate the signal.…”

“…The use of hydraulic fracturing in coal mines can meet different requirements. Hydraulic action is distinguished in order to ensure the intensification of methane recovery from coal seams, and to eliminate the negative effect of a difficult-to-collapse roof [1][2][3][4]. For each of the listed objectives fracturing processes are characteristic and, as a consequence, changes in the geomechanical composition of rocks in the area of hydraulic impact are observed.…”