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Poroelastic effects in the determination of the maximum horizontal principal stress in hydraulic fracturing tests—A proposed breakdown equation employing a modified effective stress relation for tensile failure

1989; Elsevier BV; Volume: 26; Issue: 6 Linguagem: Inglês

10.1016/0148-9062(89)91427-7

1879-2073

D. R. Schmitt, Mark D. Zoback,

Drilling and Well Engineering

In a hydraulic fracturing stress measurement, the greatest horizontal principal compressive crustal stress SHmax must be calculated using a breakdown equation which relates the fracture initiation pressure to in situ stress, tensile strength, pore pressure, and, for permeable formations, the Biot poroelastic parameter α. Using laboratory-derived α from core, SHmax magnitudes have been calculated with breakdown equations for impermeable, permeable and nonporous materials published by Hubbert and Willis [1] (Trans AIME210, 153–163), Haimson and Fairhurst [2] (Soc. Petrol. Engrs J. Sept, 310–318) and Pine et al. [3] (Int. J. Rock Mech. Min. Sci. & Geomech. Abstr.20, 63–72), respectively. Not surprisingly, there are major differences between computed values for SHmax associated with the different breakdown equations, especially for low compressibility rocks at great depth. This is illustrated with data from two field experiments in crystalline rock. Another problem is that while the published breakdown equations accurately estimate breakdown pressures in many laboratory hydraulic fracturing experiments in relatively high α rocks, in some cases with low α rocks, breakdown pressures are not predicted by any of the breakdown formulae. To resolve these problems we propose a breakdown equation based on a modified effective stress failure relation in which the tensile strength is dependent on a non-Terzaghi effective stress law. This proposed breakdown equation needs experimental validation in the laboratory and the field.