Optimization Leads to Successful Deepwater Extended-Reach Drilling Offshore West Africa
Authors: Wilson Adam 
Published: March 2016

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE/IADC 163485, “Offshore West Africa Deepwater Extended-Reach Drilling: Drilling-Optimization Case History,” by Hernando Jerez, SPE, Rafael Dias, SPE, and Jim Tilley, SPE, Halliburton, prepared for the 2013 SPE/IADC Drilling Conference and Exhibition, Amsterdam, 5–7 March. The paper has not been peer reviewed.

A deepwater extended-reach well for a major operator in west Africa required a tangent section in a soft formation. The detailed drilling plan included the use of a tailored bottomhole assembly (BHA) with a point-the-bit rotary-steerable system (RSS) and an extended-gauge bit. The BHA had to overcome the tendency of the formation to drop inclination while creating a high-quality wellbore to provide sufficient hole-cleaning capacity in the deeper sections.

Introduction

West Africa has seen a significant shift in drilling activity, mainly in deep water. Many of the major international operators are carrying out important exploration-and-development activities there. Typical field-development plans in offshore projects call for step-out directional and extended-reach wells, to reach and develop large areas from a single drilling platform.

A solid drilling plan is required for success with an extended-reach well. The plan should be comprehensive but feasible for achieving the desired results. In this case study, the extended-reach trajectory called for the delivery of build rates in the 17½-in.-hole interval not previously reached in the shallow soft formations, building up to an 85° sail angle. For the 12¼-in. tangent interval, the BHA must hold the angle and step out the well along a faulted shale and later turn the well more than 60° in azimuth to intersect the reservoir properly.

A proprietary, state-of-the-art BHA planning tool was used to model and design the appropriate BHA for each interval. Different iterations were performed to account for potential hole enlargement, build-rate response, vibration mitigation, and hole cleaning. The proposed solution was to use a point-the-bit RSS with attributes to overcome hole enlargement and deliver the planned trajectory and hole quality.

The RSS was used to drill all of the directional intervals. All intervals were drilled casing to casing in one run, with one BHA per section.

Case Study—Main Drilling Challenges

The case-study well is an oil producer, planned to reach approximately 6800-m total depth. The main objective is to drain the reserves located in an unswept area of the reservoir that cannot be reached with conventional well profiles.

The plan called for building angle in the surface hole to 85° inclination and keeping the sail angle close to the top of the reservoir, then turning 60° in azimuth and dropping to an inclination of 70°.

The critical well challenges included the ability to obtain sufficient and smooth build rates in a soft formation and manage hookload and torque-and-drag loads critical to the successful stepout of the well. Another critical challenge was the narrow mud-weight window, which, on the basis of the calibrated hydraulic model, was required to control rate of penetration (ROP) to keep the equivalent circulating density (ECD) in the safe-operation window.