The 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 579 m. 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. Two vertical fractures indicated a N60d̀E orientation for the maximum horizontal stress.
|Original language||English (US)|
|Number of pages||14|
|State||Published - May 1988|
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology