The effects of multiple large scale asperities on deformation and permeability of a single fracture

Dillabough, Graham Douglas (1996) The effects of multiple large scale asperities on deformation and permeability of a single fracture. Masters thesis, Memorial University of Newfoundland.

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Abstract

The testing of physical analogues has been shown to be important in the development of theoretical models describing the relationships between fluid flow in fracture systems, and the state of stress and deformation within the host rock of the fracture. The flow and transport characteristics related to fracturing are often enhanced by the effects of the state of stress in the rock walls, which may open up secondary pathways and cause chemical alterations in the wall rocks. -- The objective of this study was to describe the influence of two, parallel, large scale fracture roughness asperities on: 1) the fracture stiffness, displacement and hydraulic transmissivity, 2) the local stress field in the adjacent wall rock, 3) the mode and patterns of secondary fracturing, and 4) the effects on host rock permeability all as a function of changes in both normal and shear stress. The overall study approach included a series of experiments on an instrumented high strength concrete model of a fracture plane in a stiff biaxial loading frame, and comparison of the experimental data with numerical model simulations. -- The concrete model (200 x 200 x 300 millimetres) was cast with 92 strain gauges mounted on concrete coupons embedded along the centre of the model, and 17 manometer ports intersecting the fracture plane to measure the fluid pressures across the length of the model. The fracture plane was formed by pressing an aluminium plate matching the form of the asperities into the wet concrete. This plate was separated from the wet concrete by a piece of geotextile. When the concrete was set, the second half of the model was cast on the geotextile (which was left in place). This geotextile provided the form of a uniform small scale roughness on the fracture surface after the blocks were separated. Four 6.4 millimetre diameter wells were drilled into the top of the model. These wells were used to estimate the matrix permeability changes of the concrete of the model as it was subjected to changing load conditions. -- Results of the study confirmed that the large scale asperities had a significant influence on the local strain field, as seen in the data recovered from the 78 strain gauges that survived the casting and pre-test processes. Three normal and four shear loading cycles showed repeatable internal strain patterns that reflected the effects of the applied boundary conditions. Fracturing in the model was assumed to be caused by tensile stress concentrations at the base of the asperities, and at the ends of the sample. -- Displacement data indicate permanent normal closure of the fracture was greatest for the first normal loading cycle, and decreased over the following two cycles. During the final four cycles, the permanent fracture closure became essentially identical at about 20 per cent of the original closure. This suggests that fracture seating occurred during the first normal loading cycle, which is consistent with other experimental work. -- Fracture transmissivity was seen to decrease logarithmically with the cube root of the normal stress applied to the sample, and decreased further as the sample was sheared, causing closure of the aperture at the large scale asperities. -- The four wells drilled into the top of the model were pressurised with 345 kPa (50 psi) water, and shut in. The pressure decline of each well was monitored sequentially throughout the shear cycles and these data were used to estimate the relative permeability of the concrete. All four wells showed similar decline curves over the periods that each well was monitored. There was no indication that the permeability of the concrete was enhanced by the shear stress applied except by the propagation of secondary fracturing through the wells as the concrete failed. -- The post-test finite element analysis confirmed the assumption that induced tensile fracturing was the primary mode of failure in this experiment. Under normal load, finite element analysis showed that the large scale asperities caused local changes in the orientation of the stress field. Under combined normal and shear load, there were significant tensile stress concentrations associated with the restraining and releasing bends of the fracture, which provided the focus points for the onset of failure of the blocks.

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