Safe Operating Window: Wellbore Stability Is More Than Just Fluid Density

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 94732, "Integrated Management of the Safe Operating Window: Wellbore Stability Is More Than Just Fluid Density," by T. Hemphill, SPE, Halliburton Baroid, prepared for the 2005 SPE Latin American and Caribbean Petroleum Engineering Conference, Rio de Janeiro, 20-23 June. The drilling of challenging wells, especially those in deepwater, high-pressure/high-temperature, and extended-reach environments, often is characterized by a narrow safe-operations window between formation pore pressure and fracture gradient. Overstepping the window bounds can lead to unwanted incidents of lost well control and/or lost circulation. Maintenance of wellbore stability (WBS) while drilling is critical to the success of today’s challenging wells, yet applications of such modeling have largely centered only on predictions of surface drilling-fluid density requirements. An integrated approach demonstrated the use of incorporating drilling-fluid hydraulics and barite-sag potential in the wellbore-construction process to help avoid unwanted surprises that can lead to unstable wellbores while drilling in technically challenging environments. Introduction Recently, for an extended-reach-drilling (ERD) well in the North Sea, hole cleaning and hydraulics optimization were combined with wellbore-stability modeling to produce a more comprehensive analysis of a large-diameter interval to be drilled with water-based drilling fluid. In that study, a fully coupled wellbore-stability analysis was used to estimate the minimum drilling-fluid density to prevent shear collapse and the maximum density to prevent formation fracturing during initial stages of the drilling operation. Changes in key drilling parameters such as pump rate, drillstring rotation speed, and rate of penetration were made in a “what if” fashion to determine their relative effects on wellbore stability. Following the optimization process developed for drilling the well on paper, the large-diameter hole was drilled, and the casing was cemented successfully. In the subsequent 12¼-in. tangent interval, drilled with low-toxicity mineral-oil-based mud (LTMOBM), the hole was drilled without problems, but hole instability occurred once drilling operations ceased. The hydraulics used in the actual drilling process near the bottom of the 12¼-in. interval was entered into the wellbore-stability model to re-evaluate the effects of drilling operations on wellbore stability. In this second study, a chemoporoelastic model accounting for chemical interaction between the drilling fluid and the formation, as well as full coupling between the change in pore pressure and effective stresses, was used.