Drilling horizontal wells with subsequent multi-stage hydraulic fracturing have become essential for economic production of oil and gas from unconventional resource plays. The selection of appropriate intervals for perforation clusters (in cased holes) is one of the keys for a successful fracturing job in horizontal wells. An inappropriate interval may lead to extremely high breakdown pressures, or even an inability to breakdown, which can prevent that part of the well from contributing to the production.
Wellbore staging where perforation intervals are selected is based on minimum horizontal stress along the lateral has proven to be effective and has increased production contribution of each stage. We found this approach can be effective only in normal stress regime. In other stress regimes, e.g., strike-slip and reverse stress regimes, breakdown pressures were discovered to be higher in intervals of lower horizontal stress, contrary to what we typically see in normal stress regime.
This study adds practical soundness to theoretical methodologies of fracture breakdown pressure prediction, and improves the completion design to meet the expected fracturing pressure in the tectonically stressed environment of unconventional gas wells in Saudi Arabia.
To design completions that can withstand breakdown pressures in various stress regimes, hoop stress and the resultant fracture breakdown pressure must be taken into account. This study reviews the process of openhole image log analysis, selection of the uniaxial compressive strength (UCS) correlation, estimation of maximum horizontal stress and obtaining hoop stress. It collects and analyzes fracture breakdown pressures observed in unconventional gas wells. Various geomechanical aspects that influence breakdown pressure are investigated and compared with the actual breakdown pressures observed.