FRACTURE MECHANICS ANALYSIS OF THE VALIDITY OF HYDRAULIC FRACTURING AS A TECHNIQUE OF IN-SITU STRESS DETERMINATION (ROCK)

Hydraulic fracturing is a widely used method for stress determination in deep boreholes.; Interpretation of the data obtained from this technique is based on the restrictive assumption that the principal stresses in the rock are oriented normal and parallel to the borehole axis. Although this may be true in some situations, it is likely that there are many geological situations in which this assumption is not met (e.g. folded and or faulted structures).; This thesis describes experimental studies using 5 inch cubes of quartz glass to investigate the mechanics of critical and early post-critical stages of hydraulic failure around a wellbore for the general case where principal stresses are not normal and parallel to the wellbore axis. Experimental results are analyzed using fracture mechanics principles.; It was found that hydraulic fractures tend to propagate along the wall of the borehole, parallel to the wellbore axis, even when the applied field stresses are not normal and parallel to the hole. This is due primarily to the following: (i) The critical flaw (which is small with respect to the wellbore packed-off region) once initiated, propagated unstably, faster than the rate of fluid flow into the fracture surfaces. Under this condition of a "non-pressurized crack" the crack propagates more readily along the hole than into the solid mass. (ii) The maximum tensile stress direction at the wellbore (wall) and in the immediate vicinity of it is close to that of the tangential tensile hoop stress. Axial fractures were obtained even in cases where the principal stresses were highly inclined (('(TURN))45(DEGREES)) to the axis of the wellbore.; Thus, the current technique cannot be used to indicate reliably the direction and magnitudes of principal stresses at depth, and must either be replaced or supplemented by other techniques in order to remedy this.; In its present form, the hydraulic fracturing technique does serve to provide the following important information: (i) orientation of secondary principal stresses in the plane perpendicular to the borehole axis, (ii) approximate magnitude of secondary principal stresses.; Alternative ways of improving the current technique are suggested.