Assuring Stability in Extended-Reach Wells: Analyses, Practices, and Mitigations
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 105405, "Assuring Stability in Extended-Reach Wells - Analyses, Practices, and Mitigations," by Stephen M. Willson and Stephen T. Edwards, BP plc; Anthony Crook and Adam Bere, Rockfield Software; Daniel Moos and Pavel Peska, GeoMechanics International; and Nigel Last, BP plc, prepared for the 2007 SPE/IADC Drilling Conference, Amsterdam, 20–22 February.
While the departure lengths of proposed extended-reach-drilling (ERD) wells are becoming more challenging, wellbore-stability assurance technologies (both in the preplanning and execution phases) are developing at an equal pace. Several new developments in theoretical understanding and predictive capability of rock failure surrounding wells drilled at a high angle to bedding are described that are required to solve problems encountered in these challenging environments. Rigsite processes for the integration of this new understanding with real-time diagnostic measurement and monitoring provide the means to deliver borehole-stability assurance for ERD wells drilled in the most challenging environments.

Introduction
Approximately 10 years ago, the ERD program in the Niakuk field, North Slope, Alaska, was suspended because of severe wellbore-instability problems in the 8.5-in. sections of successive ERD wells. Studies to investigate these problems highlighted the importance of integrating results of wellbore-stability prediction, drilling-fluid optimization, hydraulics and cuttings transport, operational practices, and use of a pressure-while-drilling tool.
There has been a steady progression in the vertical depth and horizontal-departure length of ERD wells. Wells with horizontal departures in excess of 40,000 ft at vertical depths of less than 10,000 ft are now considered as viable to access satellite reserves from existing facilities or, in the case of environmentally-sensitive arctic environments, to develop offshore fields from onshore locations. New understanding and predictive capability for assessing instability in wells drilled at high angles to bedding are presented.

Wellbore Instability in ERD Wells
Additional considerations are needed that are subtler compared with conventional high-angle wells, and extra assurance steps are necessary. Significant variations in seabed depth or ground elevation may occur along the ERD well path. Simple assessments of pore pressure, fracture gradient, and in-situ stresses will not apply in these cases. To predict pore pressures and fracture gradients accurately, the varying water depth must be taken into consideration. The lower fracture gradient (relative to that for a shallower-water vertical-well profile) existing over the long tangent section of extended-reach wells drilled from the escarpment to access deeper-water reserves is an important aspect in the well design.
The presence of the free surface of the escarpment can reduce the magnitude of the near-surface fracture gradient below that which is expected for intra-basin locations. Failure to recognize this effect in the well-planning stage could lead to circulation losses in the tophole sections of the well. Where such problems are of particular concern, 2D or 3D finite-element predictions of stress states may be warranted.