Some Problems in Estimating Horizontal Stress Magnitudes in "Thrust" Regimes

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 (Sh > Sv) 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 So) or vertical stress levels (resulting from fracture rotation to a horizontal plane in response to Sh > Sv). I f the latter is true, then the ISIPs represent only lower bounds to the true values of Sh. 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 , Sh 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 Sh. 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 Sh.