Summary
The wellbore pressure to initiate fluid loss during casing-shoe tests, lost-circulation events, etc. is a primary well-engineering constraint and is also frequently used to evaluate assumed wellbore- and field-scale stress distributions. Current modeling approaches commonly fit empirical equations or limited failure criteria to field data to estimate likely “fracture pressure”; a smooth bounding envelope on observed regional loss pressures is often assumed to reflect the background stress. Departures from predicted pressures are often attributed to complexities in local stresses or rock properties. This work describes and evaluates a model that combines reopening of existing fractures and initiation of new fractures, incorporating pure tensile modes as well as multiple near-wellbore shear modes. Compared with typical models, this mixed-mode model better matches a global data set of leakoff pressures; it also appears to extrapolate well, allowing better predictions when there are limited local data. In addition, field data cluster tightly around predicted initiation or reopening pressures, giving a different perspective on variations in test results and potentially allowing improved evaluations of underlying stress models.
