Breakdown pressures due to infiltration and exclusion in finite length boreholes

The theory of effective stress suggests that the breakdown pressure of a borehole should be a function of ambient stress and strength of the rock, alone. However, our experiments on finite boreholes indicate that the breakdown pressure is a strong function of fracturing fluid type/state as well. We explain reasons for this behavior including the roles of different fluid types and state in controlling the breakdown process. We propose that the fluid interfacial tension controls whether fluid invades pore space at the borehole wall and this in turn changes the local stress regime hence breakdown pressure. Interfacial tension is modulated by fluid state, as sub- or supercritical, and thus gas type and state influence the breakdown pressure. We develop expressions for the breakdown pressure in circular section boreholes of both infinite and finite length to validate our hypothesis. Importantly, the analysis accommodates the influence of fluid infiltration or exclusion into the borehole wall. For the development of a radial hydraulic fracture (longitudinal failure) the solutions are those of Detournay and Cheng (1992) and show a higher breakdown pressure for impermeable (Hubbert and Willis, 1957) relative to permeable (Haimson and Fairhurst, 1967). A similar difference in breakdown pressure exists for failure on a transverse fracture that is perpendicular to the borehole axis, in this case modulated by a parameterη = ν / (1-ν)α, where ν is Poisson ratio and α the Biot coefficient. A numerical solution is obtained for this finite length borehole to define tensile stresses at the borehole wall due to a decomposed loading from an external confining stress and interior pressurization. Numerical results agree with the breakdown pressure records recovered for experiments for pressurization by CO₂ and argon: