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In 2013 the operator initiated a horizontal drilling campaign to increase oil production in Sihil field, offshore Mexico. The new well plan required 14½×17½ and 12¼×14½ Hole Enlargement While Drilling (HEWD) in top hole sections. The challenges were different compared to other top hole applications in the area and include hard formation drilling with low density WBM in a total lost circulation environment (no surface returns). The BHA (bottom hole assembly) would be required to penetrate fractured limestone, shale and dolomite with UCS from 15,000 to 30,000 psi. During the planning phase, engineers realized that neither the 14½×17½ or 12¼×14 ½ HEWD sections had been attempted in a total loss situation in the region. Solving the issues would require accurate BHA modeling and a unique hydraulics system that would effectively clean the hole, provide the necessary pressure drop required to efficiently dive the RSS (rotary steerable system), allow MWD demodulation and enable optimal hydraulic reamer operation. The drillstring would have to be dynamically stable for good directional control despite rollercone TCI pilot bit (IADC517) and 13mm PDC reamer cutters. The first two wells drilled under these conditions experienced reamer nozzle erosion. To temporarily solve the problem three reamer nozzles were blank-out. While this did produce an immediate solution it forced rig personnel to activate the reamer at the surface leading to a dedicated trip for drilling out. Blanking out the nozzles also reduced reamer life by reducing lubrication on the cutter blocks. Having evaluated root causes for all washout cases, a change to extended nozzles was proposed to reduce erosion on the retainer and the area adjacent the nozzle bore. Hardfacing material was added to the nozzle retainers to increase durability. The new nozzle configuration was modeled and compared to the old nozzle design. The results of CFD (computational fluid dynamics) analysis suggested the change would yield positive results. The extended nozzle design was tested on Well-3X and Well-4X and reduced erosion on the nozzle retainer and adjacent area. Cutter block cleaning was improved while eliminating the dedicated drillout trip (48-hours rig-time). Hydraulics were properly adjusted to deliver the required pressure-drop to efficiently operate the RSS/reamer and obtain the optimum choke size for signal demodulation in total mud losses. The operating parameters outlined by the modeling system produced dynamically stable drilling while mitigating S&V during transition events and RPM/WOB. |
