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During the second half of 2018, an operator faced significant downhole issues during exploration drilling on a semisubmersible rig in ultra-deepwater Nova Scotia. While these challenges would have made this well a candidate for the use of traditional surface backpressure managed pressure drilling (MPD) technology, the predrill forecast did not indicate the need for MPD, and during execution, it was not practical to install MPD because of the required time and detailed engineering to retrofit the rig.
During the original attempt to drill this well, the operator was forced to abandon the 12 1/4- × 14 1/2-in. hole section after mud losses, followed by wellbore breathing and associated gas events from the mud flowback. The well was undrillable with conventional methods, and the wellbore was plugged back and sidetracked. On the sidetrack, continuous circulation technology was implemented to help maintain a more constant bottomhole pressure, navigate the narrow drilling margin between pore pressure and fracture gradient, and help prevent wellbore breathing and associated gas on connections.
The planning and execution of this technology that allowed the operator to successfully drill through previously difficult targets are discussed, along with lessons learned for future wells. Overall, more than 30 connections were performed, with an average of 600-gal/min connection flow rate with synthetic oil-based mud over the two hole sections.
The focus of this study is to improve our technical understanding of anticipated drilling hazards in the Arctic Circle, especially hazards relating to drilling into and adjacent to evaporitic (salt) structures and associated tectonics. We explore current drilling technologies available to us to mitigate any anticipated drilling hazard. We demonstrate applicable operational experiences from other areas similar to drilling in the Arctic.
The Arctic's vast oil and gas potential has spurred exploration since mid-20th century. Government institutions such as the Geological Survey of Canada and historic companies such as Panarctic provide critical information on geology and petroleum discoveries. U.S. Geological Survey (2008) published Arctic mean estimated undiscovered technically recoverable conventional oil and gas resources at a total of 412 billion barrels of oil equivalent (BBOE).
Exploration in the Arctic varies in complexity mainly based on the depth drilled and hazards encountered. The remoteness of drilling anywhere in the Arctic makes both onshore and offshore operations generally more complex than drilling elsewhere in the world. To put it in perspective, our research into drilling time in deepwater Nova Scotia show for the majority of high complexity wells, non-productive time (NPT) can exceed 24% of total drilling time, and half of documented NPT is contributed to formation related problems.
Our geological analysis has found that Arctic petroleum basins and margins such as the Sverdrup Basin and East Canada and show comparable salt tectonics to Nova Scotian continental margin, offshore Brazil and Angola. Salt diapirs, salt domes, and thicken salt sections are common occurrences. Associate structures such as anticlines, extensional growth faults, wrench faults are observed in these basins. Extensional growth faults, listric normal faults, thrust faults, flank-salt shears, and brecciated fault zones are associated with salt bodies. These structures are planes of weakness. Depending on effective in-situ stress conditions these faults and intense natural fractures can become critically stressed and induce slip on plane.
Salt rheology and geochemistry pose higher drilling risk than drilling through other rocks. Salt creeps towards borehole during drilling, and plastic yielding around borehole is unavoidable when drilling through salt body. Boundary zone tends to be heavily naturally fractured, brecciated, or sheared, and rock may become unconsolidated and lose its cohesiveness. Taking heavy losses in naturally fractured boundary zone may occur. Abnormal pressure exists and taking a kick while drilling out of salt body is not uncommon.
Public domain documentation available for Arctic region support the hazards identified by our geological analysis and also suggest that a great deal of downhole uncertainty exists during early exploration. In analogous setting outside of the Arctic Circle, drilling problems related to pressure uncertainty, tight windows and wellbore stability are referenced throughout and the lessons learned suggest limiting the uncertainty when possible and the use of contingency planning.
Based on the similarities in the structural geometry of petroleum basin in Arctic and select basins in other parts of the world, it seems logical that lessons learned from these areas away from the Arctic, e.g., offshore Nova Scotia, Brazil, and Angola should provide some assistance with the planning and execution of Arctic drilling activities.
All information collection during this study has been referenced throughout. This information will be beneficial for continued support of drilling in salt tectonic structural provinces in the Arctic and anywhere else in the world.