Breakdown pressure prediction with weight function method and experimental verification

Accurate estimation of fracture gradient is critical for both drilling and hydraulic fracturing. Fracture gradient by traditional methods is greatly overestimated due to the ignorance of preexisting fractures, nonlinear near wellbore stress concentration acting on fracture surfaces, or non-uniform fluid pressure distribution inside fractures. To overcome the limitations of current methodologies, a weight function method is firstly introduced to calculate breakdown pressure where there are a pair of symmetrical radial fractures collinear with borehole axis emanating from the borehole. The results are compared against both measured breakdown pressure in both field and laboratory and results of classical fracture models. Parametric studies are conducted to investigate the influence of pre-existing crack length, orientation, in-situ stress contrast, fracture toughness, and internal pressure distribution on breakdown pressure. It indicates that breakdown pressure (1) does not always decrease with increasing dimensionless crack length at different stress contrast, (2) for the fracture with non-uniform pressure is higher than the one with constant internal pressure, and (3) increases with increasing absolute value of inclination angle(1) and fracture toughness. Comparison of breakdown pressure by traditional and weight function methods demonstrates that breakdown pressure by traditional methods is overestimated. The proven weight function method simplifies the prediction of breakdown pressure, and shows a great potential in improving the accuracy of breakdown pressure prediction because it captures more physical factors during rock fracturing process.