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.

As a part of an appraisal campaign within a sour gas and condensate field in Abu Dhabi, an extensive coring, logging and testing program was planned for the target reservoir. One of the challenges was to design and implement a suitable water-based polymeric reservoir drill-in fluid (WB-RDF). Optimum lubricity and rheological properties were essential in this 8 -1/2Љ section being drilled through the reservoir at approx. 45°inclination. Another key element of the design process was to ensure that the fluid remained stable during extended periods of coring and logging while being exposed to high levels of H 2 S and bottom hole temperatures of 288 degF.Extensive laboratory testing was undertaken where the behaviour of the RDF was analysed after extended exposure to temperatures and pressures representative of the bottom hole conditions in the well. The mud formulation consisted of polymeric additives, a polymeric alkaline temperature stabiliser and an ammonium bisulfite oxygen and triazine based H2S scavenger. A graded calcium carbonate was included for bridging purposes, aiming to minimse the fluid invasion into the formation. The rheological and fluid loss parameters and stability of RDF were verified during extended periods of static conditions. Specific fluid management procedures were implemented to ensure that the appropriate concentrations of H 2 S scavenger and temperature stabilizer were maintained in the mud system at all times. The Garrett Gas Train and its testing protocol was used to monitor H 2 S levels which, converts the soluble sulfides, bisulfides, H 2 S into gases which, then seperates the gas from the liquid, thereby eliminating effects of the liquid on the drager detection tube. It must be noted that this test converts all soluble sulfides to H 2 S and is therefore not specific to H 2 S alone and this test is a quantative analysis of sulfides. The monitoring process applied was during circulation. The GGT was ran frequently on bottoms up samples and specifically during the periods of circulation where the fluid had remained static due to extended logging.Other fluid parameters, pH and alkalinity measurements were also monitored with basic mass balance and volumetric evaluations taken into consideration.One of the key processes was the spotting of specifically formulated pills across the open hole prior to tripping. The risk of polymer degredation is increased during extended periods without circulation and the consequent elevation of bottom hole temperature. This paper has been prepared to summarise the upfront fluid design and associated laboratory evaluation work performed to ensure fluid stability was maintained in challenging bottom hole conditions. An overview is provided of the fluids management procedures implemented during drilling, coring and logging to ensure a safe and successful operation.

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