Fracture Gradient from Leadoff Test: Pitfalls of Using LOT Data from Non-vertical Wells

Abstract

Abstract Leak off tests (LOTs) are a routine in the drilling industry, typically conducted after drilling a few feet below a new casing shoe. The leak off pressure (LOP) recorded during a LOT primarily establishes the upper mud weight (MW) limit for the next hole section. Additionally, it can help to determine the cement integrity and the minimum total principal stress, which is close to the minimum horizontal stress in normal or strike slip faulting regime. The maximum operating mud pressure is usually maintained slightly lower than the LOP to permit safe well-control operations. Field-wide LOP data generally show some spread at a given depth. The lower bound trend of LOP data is often used as the base case for fracture pressure prediction and for planning of future wells. For the field under investigation, values of LOP obtained at the 9 5/8" casing shoe showed a scatter of 1200 psi (3 ppg mud weight equivalent). The average depth of the casing shoe is ∼7900 ft and the LOTs are spread over a depth interval of only 260 ft. Taking the lower bound of a wide spread in LOT data for fracture gradient estimation may lead to an overly conservative well design, because the difference between the pore fluid pressure and the LOT-based fracture pressure is (too) small. This so-called "small drilling window" can constrain casing design and mud weight. For example, when exploring deeper HPHT prospects, an excessively low (so conservative) fracture pressure estimate may increase the number of casing strings needed to reach the target. This can significantly change the well and drilling economics, and put the exploration/drilling plan at risk. The study investigated the scatter seen in the LOPs to understand the cause of scatter in terms of rock mechanics, well trajectory and in-situ stresses. The redistribution of stresses around the borehole is known to depend on the orientation of the borehole with respect to in-situ stresses. This affects the measured LOP. Using rock mechanics models and field data, the study confirmed that LOP measured in inclined wells may need an adjustment (numerical correction) prior to use for predicting fracture pressures for future wells with a different trajectory (inclination and/or azimuth). The study also provides an understanding of the cause of losses reported in some wells in the field.