An analysis of horizontal fracture initiation during hydrofrac stress measurements in granite at North Conway, New Hampshire

Abstract

S U M M A R YThe in-situ state of stress was investigated using the hydro-fracturing technique in a cored, borehole in granite at North Conway, New Hampshire, USA. A total of 21 successful tests were made in fracture-free intervals to a depth of 579m. Of the 18 tests in which oriented impressions were taken of induced fractures, 16 were found to be sub-horizontal. Laboratory and finite element simulations show that these fractures could not have been a result of stresses induced by the packer system. Although a conventional analysis in which the borehole wall is assumed to be a smooth continuum would not admit the possibility of horizontal fracturing, we present a simple analysis, based on the experimental result of Jaeger (1963) and the theoretical work of Haimson (1968), that shows that if the fluid intrudes micro-cracks in the borehole wall, horizontal fractures can be initiated where the least horizontal stress is only modestly greater than the vertical stress. For rocks of microdarcy permeability the wellbore pressure must be raised to breakdown levels in less than ten seconds if the effect is to be suppressed.The shut-in pressures provided an unusual opportunity to measure the vertical stresses. The results show that the vertical stress closely followed the calculated overburden to a depth of 400 m. Three measurements below 415 m, however, yielded vertical stresses which are distinctively sub-lithostatic.

Appalachian Stress Study: 1. A detailed description of in situ stress variations in Devonian shales of the Appalachian Plateau

“…It is of note that had infiltration been significant, we would expect to observe evidence of horizontal fracture initiation given the inferred stress regime above the H-sand [Evans et al, 1988 Figure 3) and the water level baled back to 600 m below surface, the level remained stable for 2 years until the wells were refilled with brine several months prior to stress testing. The Wilkins well was loaded with brine some 2 years prior to stress testing.…”

Appalachian Stress Study: 3. Regional scale stress variations and their relation to structure and contemporary tectonics

“…An R h value of unity will thus be inferred even though the true value is greater. This substantially increases the likelihood that conditions for horizontal fracture extension at the nucleation point will be met [Evans et al, 1988]. Fractures which initiate axially may rotate to the horizontal plane after propagating a short distance from the well bore [Warren and Smith, 1985].…”

“…The latter possibility is greatly enhanced where the fracture is formed through perforated casing, since then the hoop stress component induced at the borehole wall by "well bore" pressurization (which plays the dominant role in promoting axial fracture initiation in open holes) is greatly reduced. This substantially increases the likelihood that conditions for horizontal fracture extension at the nucleation point will be met [Evans et al, 1988].…”

On the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity

“…Further complications can arise where the minimum principal stress is close to aligned with the borehole axis, and the preferred orientation of fracture propagation is in the plane normal to the borehole axis. In this case, the fracture can rotate from axial to lie normal to the minimum principal stress after propagating a short distance outside the wellbore stress concentration (Warren and Smith, 1985;Evans and Engelder, 1989) or even initiate as a transverse fracture (Evans et al, 1988). Subsequently, constant rate injections are repeated for several cycles to reopen and further propagate the fracture, commonly with periods of venting in between.…”