Encountering an Unexpected Tar Formation in a Deepwater Gulf of MexicoExploration Well (2007)

Encountering an Unexpected Tar Formation in a Deep Water Gulf of Mexico Exploration Well Michael Harris Weatherl Michael Harris Weatherl Chevron Corp. Search for other works by this author on: This Site Google Scholar Paper presented at the SPE/IADC Drilling Conference, Amsterdam, The Netherlands, February 2007. Paper Number: SPE-105619-MS https://doi.org/10.2118/105619-MS Published: February 20 2007 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Get Permissions Search Site Citation Weatherl, Michael Harris. "Encountering an Unexpected Tar Formation in a Deep Water Gulf of Mexico Exploration Well." Paper presented at the SPE/IADC Drilling Conference, Amsterdam, The Netherlands, February 2007. doi: https://doi.org/10.2118/105619-MS Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentAll ProceedingsSociety of Petroleum Engineers (SPE)SPE/IADC Drilling Conference and Exhibition Search Advanced Search AbstractIndustry experience reveals that tar formations can present significant challenges in deep water drilling operations (1–4). In a case where several feet of active tar is present at depth, substantial trouble time often results. Tar can even prevent reaching programmed depth in some cases. These difficulties are further compounded in a high cost environment with limited rig availability where project appraisal and viability can be impacted (5).These events came to pass for Chevron and partners during late 2005 and early 2006 on the 'Big Foot' Prospect in the Gulf of Mexico at Walker Ridge Block 29 (see Fig. 1 Walker Ridge Area). Challenges on this sub salt prospect included loop currents, an active hurricane season, and numerous sub-surface issues related to geologic uncertainty, narrow pore pressure/fracture gradient margins, and wellbore stability.This paper will focus primarily on issues related to the tar formation which was encountered below salt, its impact on operational strategy, lessons learned, and steps taken to ultimately achieve the well objectives.BackgroundThe ultra deep water drilling unit 'Cajun Express' had been mobilized during July 2005 to drill the 'Big Foot' exploration well to 27,000' MD in 5,300' of water depth. The first evidence of trouble related to tar occurred while drilling ahead at 19,980' MD with 12 ppg synthetic oil based drilling fluid (SOBM) in a 12–1/4″ hole section. Since there was no prior history of significant tar in this area, the implications of the troublesome zone were not immediately recognized. Tight hole was encountered on a connection and back reaming was employed while circulating to stabilize and clean up the hole. On bottoms up sticky tar was observed 'blinding' the shakers and other solids control equipment. A series of oil sands had been drilled over the preceding 800' of open hole with significant gas shows. Even with gamma ray/resistivity logging while drilling (LWD) and mud log data thru the interval the characterization of the bottom 40' of hole containing tar was initially inconclusive. Over subsequent days attempts were made to stabilize the hole with mud weight, short trips, and reaming to enable drilling ahead. Ultimately, however, the hole section was abandoned when it became clear that further drilling, casing, or logging operations below the tar zone were not likely in a reasonable amount of time.Well Objectives and DesignThe objectives for the 'Big Foot' Prospect were developed around the evaluation of Middle Miocene and Eocene/Wilcox targets between 19,000' and 25,000' TVD in Block 29. The directional design was necessitated to penetrate both targets at the desired structural position and included building angle below salt to 30º inclination at a 130 degree azimuth. In addition to the salt section from 8,000' to 12,000' TVD, another salt body to the east overhung the deeper targets. The directional design enabled the structural objectives without having to drill two separate salt intervals.The combination of an overlying salt section, poor seismic resolution, uncertain pore pressure/fracture gradient below salt, and increasing pore pressure below the Miocene dictated a 'large bore' casing configuration. (See Wellbore Diagram Fig 2).Geologically comparable offset data was sparse to non-existent, therefore a significant amount of subsurface uncertainty was recognized early in the planning phase.The original Big Foot wellbore reached TD during December 2005 and was announced as a discovery shortly thereafter. The well was drilled and evaluated in 141 days for $70.8 million. The scope of the project with the Cajun was then expanded to incorporate an appraisal sidetrack to the Middle Miocene +/-4,000' north from the original location. The tar interval was encountered while drilling the sidetrack. Keywords: drilling operation, drilling fluid, Upstream Oil & Gas, Big Foot, liner, sidetrack, tar interval, tar formation, unexpected tar formation, hyd 513 Subjects: Wellbore Design, Drilling Operations, Wellbore integrity This content is only available via PDF. 2007. SPE/IADC Drilling Conference You can access this article if you purchase or spend a download.