Experimental observations of the mechanics of borehole failure in porous sandstone

To investigate borehole failure under hydrostatic stress in sandstone of varying porosity, experiments were conducted on three sandstone varieties with 2-5 mm bores. Tennessee, Darley Dale, and Penrith sandstone-types were selected to represent most of the spectrum of sandstone porosity variations (respectively, 0.07, 0.12 and 0.28). Confining pressures ranged up to 500 MPa, and pore fluid volumometry was used to detect bore failure. Failed samples were studied micro structurally using optical and scanning electron microscopy. Each rock type failed with the development of oriented breakout features normally seen to develop around boreholes loaded under non-hydrostatic stress, and in Tennessee sandstone at least, these developed in a consistent orientation with respect to an external reference frame (dip-direction of foreset beds). All of the rock types showed strength and/or elastic anisotropy, thus the formation of oriented breakouts under hydrostatic loading is attributed to the effects of anisotropy. Two modes of breakout development were observed. In Darley Dale and Penrith sandstones, a combination of intergranular shear and extensional fracture produced broad and shallow features with breakout width unaltered during growth. In Tennessee sandstone a purely extensional mode of intragranular fracture was observed, leading to deeper breakout features. The samples tested showed higher failure pressures for smaller bore sizes. When failure pressure is normalized with respect to grain crushing pressure P-i*, and bore diameter with the product of porosity with grain size, all data lie on a single master curve, with bore failure pressure approximately 0.15 P-i* in the regime of bore size-independent behaviour. (C) 2003 Elsevier Ltd. All rights reserved.