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Vertical Versus Inclined: A High-Stress Environment Where Deviated Wells are Easier Than Vertical

Abstract

In most locations where Chevron is operating, vertical wells are easier and faster to drill than deviated wells. This is due to the relative magnitudes of the three principal stresses and the vertical stress being the greatest stress (normal faulting as with fields in Gulf of Mexico, North Sea, offshore West Africa). But Chevron also operates fields where high horizontal stresses are at play and the vertical stress is not the greatest stress acting on the wellbore. In one such field, vertical wells incurred significant geologic NPT due to wellbore instability with negative impact on Value Based Well Objectives. Detailed geomechanical modelling with significant emphasis given to high-quality borehole image data indicated high horizontal stresses; in particular, SHmax is greater than the vertical stress. Vertical wells drilled in this stress regime (strike-slip) are more difficult to drill than vertical wells drilled in normal faulting regimes. This is due to the two high horizontal stresses acting on the vertical wellbore wall. The geomechanical model and further afield wells indicated deviated wells, while not without their challenges, would be easier to drill with respect to less borehole instability and reduced losses. Well planning efforts for the first deviated well in the field focused heavily on the most challenging formations for stability and how to best reduce operational risks while encountering the reservoir targets at an optimal trajectory and achieving all the well objectives. The deviated well (~60 degrees) was in fact drilled with fewer wellbore stability issues and losses than earlier vertical wells. Drilling parameters and practices were optimized to get the best ROP, minimize bit trips and reduce the risk of cuttings bed build-up. The deviated well drilled the three critical hole sections ~30 days faster than the previous vertical well despite having ~660 meters / ~2150 feet more to drill.