INFILTRATION EFFECTS IN THE TENSILE RUPTURE OF THIN-WALLED CYLINDERS OF GLASS AND GRANITE - IMPLICATIONS FOR THE HYDRAULIC FRACTURING BREAKDOWN EQUATION

To study the effects of fluid infiltration on fracture initiation in low porosity rock, hollow cylinders of Westerly granite and glass were brought to rupture by pressurization of the internal cavity in a manner analogous to a hydraulic fracturing field test. The samples were subject to hydrostatic confining pressures as high as 60 MPa. The dependence of the internal rupture pressures of the glass tubes on confining pressure were found to be consistent with linear elastic theory which includes a pre -existing inner wall flaw. The granite hollow cylinders were internally pressurized at rates of approx. 2.0, 0.6 and 0. 05 MPa/sec (referred to hereafter as fast, normal and slow). For a given pressurization rate fracture, initiation pressures as a function of confining pressure were extremely well fit by a line with slope of unity and pressurization rate dependent intercepts from 10.9, 14.7 and 17.5 MPa for the slow, normal and fast tests, respectively. Observed failure levels for the slow tests are in good agreement with predictions based on experimentally -derived strength and elastic moduli bounds for a saturated porous media. However, failure pressures in the fast tests are too high; that is, the difference between the fast and slow tests is larger than can be rationalized by the theoretical predictions. One possible explanation is that the pore pressures cannot be recharged during the short time of the fast tests. This is also implied by the results of similar tests in which fluid infiltration into the rock is not allowed during inner cavity pressurization. Reduced pore pressures are a possible result of the highly non-linear elastic behaviour (including dilatancy hardening) in the granite due to its low aperture microcrack porosity. The evidence we have found for diminished pore pressure effects on fracture initiation provides support for the empirically-determined hydraulic fracturing breakdown equation in low porosity crystalline rock that omits pore pressure.