Hydraulic Fracturing of Deep Wells
Authors: Haimson Bezalel C. , Edl John N. 
Published: December 2007

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Abstract
Laboratory tests were conducted in simulated deep open-hole wells (0–30,000 ft.) for the purpose of studying their mechanical stability and their suitability for hydraulic fracturing. Two conditions of instability exists., The first occurs when the pressure in the well is zero or low as compared with the assumed hydrostatic horizontal in-situ stress. Experiments in Berea Sandstone concur that shear failure causing well collapse can indeed take place, although the minimum critical horizontal stress value expected to induce crushing appears to be conservative. The second condition of instability occurs when the well fluid pressure exceeds that of the hydrostatic horizontal in-situ stress. This case could occur due to pressure buildups in drilling and is also the basis of the hydraulic fracturing method. Three failure mechanisms are considered:
rock follows the Mohr criterion of shear failure rock is elastic and fails in tension when one of the principal stresses reaches a tensile value higher than the tensile strength same as (b), only rock is poroelastic.
Failure mechanism (a) poroelastic. Failure mechanism (a) becomes a distinct possibility in deep wells due to the high stress differentials that can develop at the well. Previous experimental results in an impermeable sandstone and at relatively low pressures validates failure mechanism (b). However, present tests in a sandstone and a marble of very low permeability yield results that depart considerably from (b) and appear to obey the relationship predicted by failure mechanism (c). The results are significant in that it appears that deep openhole wells do not behave as simple elastic materials and fail at pressures considerably lower than the expected values. The type of failure remains tensile rupture even at the highest simulated field stresses.

Introduction
Wells deeper than ever before are drilled today for oil. The successful use of hydraulic fracturing as a stress measuring method at 6000 ft. below surface, 1 could result in stress measurements in boreholes of even greater depth. Subsurface disposal wells already reach 10,000 ft. or more. It is now suggested that geothermal energy from hot rock could be successfully extracted, through wells from depths reaching 25,000 ft. The fact is, however, that most of the field experiences as well as the theoretical and experimental research have so far been associated with shallower wells (0–10,000 ft.).