Strain superposition and fault stability during sequential hydraulic fracturing
T. R. Harper, 2017. "Strain superposition and fault stability during sequential hydraulic fracturing", Geomechanical and Petrophysical Properties of Mudrocks, E. H. Rutter, J. Mecklenburgh, K. Taylor
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Numerical simulations of a naturally fractured shale reservoir are used to investigate the influence of sequential hydraulic fractures on the shear displacements and evolving stability of a fault. The effect of the heterogeneous elastic strains arising from natural fractures and faults in the reservoir on the displacements around hydraulic fractures is simulated. The displacements experienced by the hydraulic fractures depart strongly from the elliptical distribution characteristic of homogeneous elastic media, and are a result of the influence of natural fractures and faults by both superposition of elastic strains and inelastic behaviour. It is shown that interaction between the pre-existing reservoir strains and those induced by hydraulic fracturing influence shear displacements along the fault. Displacements on the fault respond to the hydrofracture-induced shear strains, tend to be restricted to certain patches of the fault and mainly tend to stabilize the fault. The results imply that the numerous closely spaced hydraulic fractures characteristic of current shale reservoir completion practice may, to some extent, increase the stability of potentially unstable faults before they are intersected by a propagating hydraulic fracture. The stability of the fault as the fractures sequentially approach the fault depends upon the direction of approach and the dimensions of the hydraulic fractures.
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A surge of interest in the geomechanical and petrophysical properties of mudrocks (shales) has taken place in recent years following the development of a shale gas industry in the United States and elsewhere, and with the prospect of similar developments in the UK. Also, these rocks are of particular importance in excavation and construction geotechnics and other rock engineering applications, such as underground natural gas storage, carbon dioxide disposal and radioactive waste storage. They may greatly influence the stability of natural and engineered slopes. Mudrocks, which make up almost three-quarters of all the sedimentary rocks on Earth, therefore impact on many areas of applied geoscience.
This volume focuses on the mechanical behaviour and various physical properties of mudrocks. The 15 chapters are grouped into three themes: (i) physical properties such as porosity, permeability, fluid flow through cracks, strength and geotechnical behaviour; (ii) mineralogy and microstructure, which control geomechanical behaviour; and (iii) fracture, both in laboratory studies and in the field.