Gulf of Mexico Gas Hydrate Joint Industry Project Leg II: Initial Results from the Green Canyon 955 Site
Authors: McConnell Daniel Russell , Collett Timothy Scott , Boswell Ray , Frye Matthew , Shedd William W. , Dufrene Rebecca Surcouf , Godfriaux Paul Doucette , Mrozewski Stefan , Guerin Gilles , Cook Ann , Jones Emrys 
Published: July 2010

In April and May of 2009 the Gulf of Mexico Gas Hydrate Joint Industry Project realized its second field program (Leg II) with the semi-submersible Helix Q4000 drillship. The three week expedition drilled seven logging-while-drilling (LWD) holes at three sites that tested a variety of geologic/geophysical models for the occurrence of gas hydrate in sand reservoirs in the deepwater Gulf of Mexico. At the GC 955 site, high saturation gas hydrate deposits in sands were found, where predicted, at two of the three holes. The full research-level LWD assembly deployed for Leg II collected data on formation lithology and porosity, and included quadrapole acoustic and high-resolution 3-D resistivity logs. No samples, only LWD data were collected in Leg II. The three holes in GC955 were drilled where a wide and thick late Pleistocene channel complex has been raised and fractured by salt uplift. A four-way closure with numerous amplitude anomalies at the base of gas hydrate stability is near-to but west of the channel axis. The first well (GC955-I) was drilled very close to the channel axis in a location with muted geophysical indications of gas hydrate. More than 300 ft of porous sands were encountered as predicted; however the sands contained primarily water - with only modest indications of gas hydrate. The next hole (GC955-H) targeted sands higher in the four-way closure. Fracture filling gas hydrate was detected above the deeper sand target, and, at the target, 98 ft of sand fully saturated with gas hydrate with little to no gas beneath. A third well (GC955-Q) also encountered at least 35 ft of fully saturated gas hydrate sand at the target depth, but drilling was aborted because of a gas hydrate dissociation event or penetration of free gas and subsequent gas flow. The JIP's discovery of thick gas hydrate-bearing sands at the GC 955 site validates the integrated geological and geophysical approach used in the pre-drill site selection and provides increased confidence in assessment of gas hydrate volumes in the Gulf of Mexico.

Introduction

The Gulf of Mexico Gas Hydrate Joint Industry Program (JIP) is a cooperative research program run by an international consortium of energy companies managed by Chevron in partnership with the U.S. Department of Energy. The primary objectives of the JIP Leg II drilling program was to determine the occurrence of gas hydrate within sand reservoirs in the Gulf of Mexico, to assess current approaches for interpreting gas hydrate occurrence from geologic and geophysical data, and to determine the most suitable sites for additional drilling and coring in future phases of the JIP program. Gas hydrate is a unique type of chemical substance in which a lattice of water molecules encase, without chemical bonding, a gas molecule. Gas hydrates, if found in high concentration, are a dense energy resource. A cubic ft of methane hydrate contains 184,000 Btu of energy. Gas hydrate can be present in deep marine sediments where pore fluids are highly saturated with gas and the pressure temperature conditions are favorable. Hutchinson et al. (2009)1 provide a review of the JIP's site selection process and of the analyses conducted to select the targets to be permitted for possible drilling. The final drilling program included three sites, Alaminos Canyon block 21, Walker Ridge block 313, and Green Canyon block 955 (GC 955) in the northern Gulf of Mexico (Figure 1). Initial summaries of JIP Leg II operations, scientific results, and logging-while-drilling data collection methods and operations are provided by Collett et al. (2009)2, Boswell et al. (2009)3, Mrozewski et al. (2009)4, McConnell et al. (2009a) 5, McConnell et al. (2009b) 6and Guerin et al. (2009) 7 respectively. This paper describes the scientific rationale and initial results for the LWD program conducted at the GC 955 Site.