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Plane strain fracture toughness (KIc) values are determined for the modified ring (MR) test through numerical simulation of crack growth to highlight the sensitivity of MR KIc values on applied displacement or force boundary conditions, slip conditions at the specimen-platen interface, and the Poisson ratio (v) of the test material. Numerical calculation of fracture toughness in the MR test is traditionally conducted assuming a uniform force along the specimen loading surfaces and no slip between the specimen and the loading platens. Under these conditions KIc increases by 30–40% as v decreases from 0.4 to 0.1. When slip is allowed at the specimen-platen interface under a uniform force, KIc values are independent of v, and for any given v, are 5–25% less than those determined under a no-slip boundary condition. Under a uniform displacement of the specimen loading surfaces, KIc is essentially independent of v, regardless of specimen-platen interaction. Moreover, although KIc values determined under unifor displacement and no-slip boundary conditions are always higher than those determined under uniform displacement and slip-allowed boundary conditions, the average difference in KIc for these two methods is less than 5% for the two specimen geometries examined. This suggests that under uniform displacement conditions, KIc is essentially independent of specimen-platen interaction. Because KIc values determined from MR testing are strongly dependent on the modeling procedure, future reports of KIc determined from this test should be accompanied by detailed reports of the modeling procedure. Until further testing reveals the most accurate simulation technique, we advocate use of a uniform displacement formulation for KIc determination from MR testing because results from this method are insensitive to most modeling parameters. Numerical results from models conducted under uniform force, no-slip boundary conditions should be viewed as an upper bound to KIc.

In situ and laboratory measurements of ultrasonic velocity have been compared to test for the effect of strain relaxation on rock properties. For granites, the larger changes in compressional-wave velocity (Vp) correlate with higher in situ stresses, the maximum Vp being parallel to the directory of maximum compressive stress. For sedimentary rocks, the relation between changes in Vp and in-situ stress is dependent on lithology. In sandstone, the direction associated with the maximum decrease in Vp accompanying strain relaxation is parallel to the maximum compressive stress. Limestone is virtually isotropic with little decrease in Vp on strain relaxation.

Since hydraulic fractures exhibit a strong tendency to propagate in a plane normal to the least principal stress, the preferred plane of propagation under thrust stress regime ( S h > S v ) conditions is horizontal. This can lead to complications in applying the hydrofracture stress measurement technique as horizontal fractures do not directly sample the horizontal stress field. Recent experiences in conducting measurements in thrust regimes have highlighted two problems that might be encountered. The first is the possibility of horizontal fracture initiation at the wellbore. The second relates to the seemingly common problem of deciding whether ISIPs which lie close to the estimated overburden reflect least horizontal stress levels (which happen to coincide with Sv) or vertical stress levels (resulting from fracture rotation to a horizontal plane in response to S h > S v ). If the latter is true, then the ISIPs represent only lower bounds to the true values of S h . We present two datasets which have bearing on the two problems noted above. In the first we review measurements conducted in a vertical borehole penetrating granite in which the vast majority of induced fractures were horizontal at the wellbore. Evidence suggests that these fractures were not a result of packer-induced stresses or incipient natural fractures but rather were a consequence of both high horizontal stress levels and fluid infiltration of the wellbore wall during the 15 sec of pump time required to attain breakdown. The absolute magnitudes of the horizontal stresses is not determined. However, through consideration of the elastic stress distribution about a vertical borehole it is possible, in principle, to estimate the horizontal stress difference S H −S h from the horizontal fracture initiation pressure. In the second dataset we present measurements conducted in three boreholes penetrating a sandstone/shale sequence in which the induced fractures were determined to be vertical at the wellbore. However, by modelling the anticipated effects of topography it is clear that the ISIPs above a certain stratigraphic horizon consistently reflect the vertical stress, and not S h . No evidence of dual shut-in pressures, which might provide a measure of the magnitude of the least horizontal stress in these beds, was observed. Neither did the form of the post-shut in pressure decline for these beds display any characteristics that might have served to distinguish them as horizontal fracture controlled. This example shows that the presence of a vertical fracture trace at the wellbore cannot be taken as proof that the ISIP reflects S h .

The permeability of whole and jointed Barre granite was measured at pressures up to 2 kbars. Jointed samples were actually split cylinders joined by surfaces with controlled surface roughness. Samples with induced tension fractures were also measured. The permeability of the whole rock ranged from about 10−6 to 10−7 darcies. The permeability of the jointed rock ranged from about 8 × 10−5 darcies at low pressure down to that of the whole rock at high pressures. Permeability was not a simple function of the difference between external confining pressure (Pc) and internal fluid pressure (Pf). Changes in permeability were found to be proportional to (b dPf − a dPc) where b/a

In situ measurements of both instantaneous and time-dependent relaxation of the Algerie granite at a quarry near East Otis, Massachusetts, indicate that the orientation of instantaneous maximum expansion is 30° different from the orientation of the time-dependent maximum expansion. Based on comparison of these measurements with the regional contemporary tectonic stress and with measurements of petrofabrics of the sampled unit, it appears that the instantaneous relaxation orientation (ENE) is controlled by contemporary tectonic stress, whereas the time-dependent relaxation orientation is controlled by healed microcracks in the granite. The curves for time-dependent relaxation best fit a power-law function of time with the exponent varying between 0.211 and 0.872.