Pre-Drilling Expedition Overview

The primary goal of the Gulf of Mexico JIP is to evaluate the occurrence, nature, and implications of gas hydrates in the Gulf of Mexico. Prior work by the JIP has contributed significantly to the development of remote sensing and field sampling technologies, wellbore stability modeling, and an extensive database of experimental data on the impact of gas hydrate on the physical properties of sediments of various grain sizes. In 2005, the JIP completed Leg I drilling, logging, and coring operations designed primarily to assess hazards related to drilling through gas hydrate within the clay-dominated sediments that typify the shallow sub-seafloor in the deepwater Gulf of Mexico (see Ruppel et al., 2008). The current Leg II program is designed to extend that knowledge base to sand-dominated systems by evaluating multiple targets through LWD operations. Leg II results will be then be used to assess the best sites for additional data acquisition (through both conventional and pressure coring) in Leg III (currently planned to occur as early as 2010).

Leg II Preparations
In preparation for Leg II, the JIP and its collaborators (see sidebar) in 2006 began detailed geologic and geophysical evaluations of numerous potential sites, seeking evidence for active petroleum systems (gas sources and migration pathways) co-located with sand-prone lithologies (Hutchinson et al., 2008). By 2008, the JIP had developed geologic interpretations; conducted pre-stack, full-waveform 3-D inversions for gas hydrate saturation; delineated and prioritized specific drilling targets; assessed drilling hazards; and developed operational plans for sites in Alaminos Canyon (AC) 818, Green Canyon (GC) 955, and Walker Ridge (WR) 313. Leg II LWD operations were originally planned for spring 2008, but drilling was postponed when delivery of the contracted drill rig was delayed until after the start of the hurricane season.

The delay in delivery of the rig allowed additional site selection and evaluation activities to continue through the rest of 2008 and early 2009. In addition to wellbore modeling within the JIP, several controlled source electromagnetic (CSEM) surveys were conducted by Scripps Institute of Oceanography over the planned drill sites in AC818, GC955, and WR 313 in Fall 2008 . The JIP effort also benefitted greatly from continuing work within the Minerals Management Service’s ongoing assessment of Gulf of Mexico resources (see Frye, FITI, Spring 2008 and Recommended Reading) that revealed additional opportunities to target gas hydrates in coarse-grained sediments in East Breaks (EB) 992, GC 781/825, and AC 21/65. The USGS continued to coordinate the site selection process and develop additional interpretations of gas hydrate occurrence at selected existing drill holes. All sites were then fully reviewed by the site selection team in order to refine and prioritize the final drilling targets.

Throughout early 2009, the JIP has pursued permitting and hazards analysis for five sites (AC21/65; EB 992, GC 955; WR 313; GC 781/825; ). The site in AC 818 had earlier been deemed too risky for Leg II drilling due to expected reservoir overpressure and was dropped from the Leg II program. However, the site remains a candidate for GoM JIP Leg III coring activities if the drilling hazard issues can be addressed. As of this writing, ongoing industry activity at two of the locations (EB 992 and GC 781) renders the JIP’s ability to drill at those locations uncertain.

Other issues, including weather, ocean currents, and unexpected operational complications, may further affect which sites will be visited during the program. However, the site selection team has proposed a total of 23 potential drill holes amongst the five sites, allowing for flexibility in dealing with these unknowns.

Leg II Operations
Leg II operations will place high priorities on drilling safety and adherence to budget constraints. All sites have undergone extensive hazards review by the AOA Geophysics team and will be drilled only to those depths that are deemed free of safety and operational hazards, particularly free gas accumulations. The budget for the Leg II field program is approximately $11.2 million with 60% of the cost associated with operating the rig and the remainder primarily committed to use of the LWD tools. At roughly $500,000 per day, the expedition will be limited to ~22 days of operational time, which will include initial mobilization, transit between holes and between sites, and final demobilization. Both mobilization and demobilization will occur at sea.

GoM JIP Leg II will feature a state-of-the-art LWD tool combination that will provide unprecedented information on the nature of the sediments and their pore fill constituents. The program will feature full research-level LWD data on formation lithology and porosity, and will include Schlumberger’s MP3 (quadrapole sonic tool) and PeriScope (3-D high-resolution resistivity) tools. These tools will provide full 3-D information on the both acoustic (both compressional and shear wave) and electrical properties of the sediment enabling the improved evaluation of gas hydrate in both pore filling and fracture-filling modes.

Given the different (smaller) diameter of the MP3 tool compared to the rest of the tool string, the JIP Leg II bottom hole assembly will feature a second “hole opener” located above the MP-3 tool. This innovation will enlarge the hole to allow logging with the other tools after the passage of the MP3.

Current plans call for Leg II to proceed first to WR 313, where five holes have been permitted. The initial LWD run will target the hole judged by the site selection team, the JIP, and geohazards specialists as having the greatest potential for success in finding gas hydrates within coarse-grained lithologies with the lowest drilling risk. The Helix and JIP teams will then drill and collect LWD data from this first site in “high resolution” mode, which is characterized by reduced drilling penetration rates.

Select LWD data will be analyzed by the onboard science team in real time, and that information will be used to evaluate the hole being drilled as well as decision-making regarding the location and drilling parameters for the next hole. Once the first WR well is drilled, the drill string will be raised to clear the seafloor, and the ship will move a short distance to the next hole location. After completion of two or three holes at the WR site, the full drill string will be retrieved and layed down on deck, and the ship will steam to the next site. Under optimal conditions, it is believed that up to three sites, with up to three holes at each site, could be drilled during Leg II.

Gulf of Mexico gas hydrates JIP Leg II is enabled by the contributions of dozens of scientists from the JIP, collaborating federal agencies, and other participating companies. Please look for operational and science reports to be posted regularly on both this site and the JIP website [external site]. Additional data, including final drilling and logging reports, and site evaluations will be also be posted and announced in FITI.