Encountering an Unexpected Tar Formation in a Deepwater Gulf of Mexico Exploration Well

Abstract

Industry 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. Background The 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 Design The 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.

A USD 100 Million "Rock": Bitumen in the Deepwater Gulf of Mexico

“…Many field developments, such as ConocoPhilips' Spa Prospect at Walker Ridge (Rohleder et al 2003), BP's Mad Dog field at Green Canyon (Romo et al 2007), and Chevron's Bit Foot at Walker Ridge (Weatherl 2007), have been affected. Many field developments, such as ConocoPhilips' Spa Prospect at Walker Ridge (Rohleder et al 2003), BP's Mad Dog field at Green Canyon (Romo et al 2007), and Chevron's Bit Foot at Walker Ridge (Weatherl 2007), have been affected.…”