Résultat de la recherche
Influence of Stress Path on Stress Memory and Stress Fracturing in Brittle Rocks
A two-dimensional numerical model based on the Distinct Element Method, previously calibrated to the laboratory properties of undamaged and damaged Lac du Bonnet (LdB) granite, was used to investigate the influence of stress path on the development of stress memory and stress fracturing in brittle rocks. Various cycles of loading and unloading, similar to those imposed during Kaiser Effect tests, were first applied to undamaged numerical specimens of LdB granite. The results of Kaiser Effect test simulations were found to be consistent with those of published laboratory and numerical investigations. Further simulations were conducted to investigate the influence of stress path resulting from the excavation of a tunnel on the depth of stress fracturing around the excavation boundary. For this purpose, the stress paths at points on and near the tunnel wall, obtained from a continuum finite element model, were applied to the calibrated numerical specimen. It was found that the amount of damage in the numerical specimens decreases rapidly with increasing distance from the excavation wall. The findings of this research shed some light on the influence of stress path and grain-scale heterogeneity on stress memory in brittle rocks and stress fracturing around underground openings.
Hydraulic Fracturing Mine Back Trials-Design Rationale and Project Status
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, Australia 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. Keywords: stress management, stiffness modification, shale gas analogue, mine-back experiments, model calibration, hydraulic fracture, naturally fractured rocks