Improving Collision Avoidance and Target Accuracy in an Offshore Brownfield Redevelopment Combining Gyros and In-Field Referencing
Authors: Costeno Hugo , Djohor Mohamed , Raziyev Mukhamed , Sessarego Horacio L , Subroto Bramanta , Ismail Azah , Lee Swee Lyn , Mond Mondali 
Published: May 2016

Abstract
In 2012, a redevelopment infill drilling campaign took place in a brown field, offshore Malaysia. Accurate wellbore positioning was critical to place a well within a path that navigates 2200 ft through an antithetic faults panel, separated approximately 900 ft, and with a 120 ft general throw; and to intersect the five target reservoirs of the well. The complex well path also faced the challenges of infill drilling in a brown field, such as collision avoidance in a crowded field, the location of the target reservoirs relative to the available drilling slots; as well as trajectory restrictions due to completions design.
This paper presents the well design solutions that include a rigorous anti-collision analysis and a comprehensive survey programme. The survey programme consists of gyro while drilling (GWD) on the upper section of the well until the path is clear of magnetic interference from neighboring wells and the application of in-field referencing (IFR) correction for conventional measurement while drilling (MWD) magnetic survey based on accelerometers and magnetometers.
IFR improves survey accuracy with a multi-station analysis that corrects the survey error with the localised crustal effects in the magnetic field of the earth. It was used for well positioning in between faults and for target intersection. It enabled drilling the well with higher confidence while intersecting the target reservoirs. A reduction of 60% on survey uncertainty was observed. The more accurate wellbore survey also optimises collision analysis for future well plans and it gives more reliable control points to update the subsurface static model. The benefits were obtained without compromising the drilling performance given that no extra operational time is required for the survey correction.

Introduction
Context
The well is located in a brown field, offshore Malaysia. Three infill drilling campaigns have been executed since a decade ago to increase oil recovery. To maximise the use of existing assets and to optimise cost, infill drilling has been performed from existing drilling facilities by either accommodating new drilling slots or side-tracking idle wells. The well was planned to target several reservoirs located in crestal position in the southern block of a rollover anticline formed by growth faulting.
In the study area, faults show a general east-west trending direction. The reservoirs are entirely related to sealing against faults while within reservoirs the seals are marine flooding surfaces.
Well design considerations
The well design process consisted of identifying the available surface location -a well with high water cut, idle since 2007- and selecting the subsurface targets. Once drilling targets were identified and preliminary well trajectories were built, an integrated team approach was used to optimise the well paths, usually requiring a compromise between the desired and the practical approach.