Abstract

In April and May of 2009, the Gulf of Mexico Gas Hydrate Joint Industry Project (JIP) conducted its second field program (Leg II) with the semi-submersible Helix Q-4000 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. 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 core samples were collected in Leg II. In the two wells drilled at the Walker Ridge 313 (WR 313) site, gas hydrate in high saturations was found in the primary target sand intervals. A stratal-bound, clay-prone, fracture fill gas hydrate unit was found at shallow depths at both wells. Gas hydrate was indicated by log data in numerous thin sands and silts at both wells. The two holes in WR 313 were drilled where low velocity geophysical events showed phase reversals across the base of gas hydrate stability (BGHS), caused by high velocity gas hydrate pore fill updip of the BGHS. Pore filling gas hydrate with saturations up to 80% in a 40 ft net sheet sand was found at the primary target at the WR 313-G location. Likewise, 33 net feet of highly gas hydrate-saturated sand was found in a deeper sequence that served as the primary target sand at the WR 313-H well location. The JIP's discovery of thick gas hydrate-bearing sands at the WR 313 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 and fine-grained marine sands worldwide.

Introduction

In April 2009 the Gulf of Mexico Gas Hydrates Joint Industry Program (JIP) conducted Leg II logging-while-drilling (LWD) operations at sites in Alaminos Canyon block 21 (AC 21), Walker Ridge block 313 (WR 313), and Green Canyon block 955 (GC 955) in the northern Gulf of Mexico (Figure 1). The primary objectives of the JIP Leg II drilling program were 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. Initial summaries of JIP Leg II operations, scientific results, and logging-while-drilling data collection methods are provided by Collett et al. (2009), Boswell et al. (2009), and Mrozewski et al. (2009). This paper describes the scientific rationale and initial results for the LWD program conducted at the WR 313 site. Hutchinson et al. (2009) provides a review of the analyses conducted to select the targets to be permitted for possible drilling. A detailed review of logging-while-drilling (LWD) operations and acquired data from the WR 313 site is provided by Cook et al. (2009). The JIP's selection of sites for drilling in Leg II was based on a prospecting approach that sought to combine the elements needed to support a gas hydrate petroleum system (appropriate temperatures and pressures, gas and water sources, and migration pathways) with the likely presence of sand reservoirs (to concentrate gas hydrate in a porous and permeable media) within the gas hydrate stability zone (Hutchinson et al., 2008). Furthermore, the site selection process also incorporated the expectation that gas hydrate-charged sand reservoirs of sufficient thickness should be amenable to direct detection and quantification from seismic data (Dai et al., 2008). All of these elements were seen in the WR 313 area, providing a multitude of high value sites for the JIP program (Hutchinson et al., 2009).

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