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, experiments on finite-length boreholes indicate that the breakdown pressure is a strong function of fracturing fluid composition and state as well. The reasons for this behavior are explored, including the roles of different fluid types and state in controlling the breakdown process. The interfacial tension of the fracturing fluid is shown to control 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 super-critical, and thus gas type and state influence the breakdown pressure. Expressions are developed for the breakdown pressure in circular section boreholes of both infinite and finite length and applied to rationalize otherwise enigmatic experimental observations. 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 show a higher breakdown pressure for impermeable relative to a permeable borehole. 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 parameter η, which is a function of Poisson ratio and the Biot coefficient. These solutions are used to rationalize observations for mixed-mode fractures that develop in laboratory experiments containing finite-length boreholes. Predictions agree with the breakdown pressure records recovered for experiments for pressurization by CO2 and Ar – higher interfacial tension for subcritical fluids requires higher critical pressures to invade into the matrix, while supercritical fluid with negligible interfacial tension has less resistance to infiltrate into the matrix and to prompt failure. This new discovery defines mechanisms of failure that although incompletely understood, provisionally link lower breakdown stresses with mechanisms that promote fracture complexity with the potential for improved hydrocarbon recovery.