In attempting to drill a deviated 12-1/4″ 37° tanget section with WBM through the Laffan and Nahr Umr shale intervals, very little margin for error was faced in a well where failure was not an option. While problems in the Nahr Umr were to be expected, the instability of the Fiqa and Laffan shale layers became the catalyst for excessive non-productive time. Two lost holes preceded the successful completion of the section after taking the last resort step of a re-drill with Synthetic Oil Based Mud (SOBM).
Various literature is available on wellbore stability problems in the Nahr Umr Formation on a regional and field specific scale. Behaviour of the shale and how best to control it is also known to vary on a field by field basis. Failure mechanisms are largely related to the mineralogical composition of the shale itself in combination with the localized geomechanical forces exerted on the specific wellbore. This paper captures the specific experience encountered in the operator's first well in the country and provides insight into the severity of the risk of wellbore instability.
Numerous operational issues were encountered due to shale instability induced through drilling and tripping related mechanical forces. Before committing to the changeover to SOBM, exhaustive attempts were made to find an acceptable and workable compromise to the challenges faced while drilling the section with WBM. This included implementation of alternative methods of well construction including the use of innovative tools and hole enlargement to increase annular clearances with the BHA. Often it was observed that mitigation of one problem led to the onset of another problem.
The experience is used to present an overview of the limitations of the contingency options available and the impact of unplanned changes implemented during the well construction phase.
In addition to the engineering conclusions presented, the case study provides insight into the complexities of a start-up drilling operation. Where an entire field appraisal campaign depends on the successful accomplishment of primary objectives, the necessity to perservere through adversity becomes of yet higher importance.