|Gas hydrate accumulations at one of the drill sites (Walker Ridge). Source: JIP. Click to enlarge.|
The US Department of Energy’s National Energy Technology Laboratory (NETL), US Geological Service (USGS), and US Minerals Management Service (MMS), in collaboration with an industry research consortium led by Chevron, on 6 May completed a landmark gas hydrates drilling expedition that demonstrated the occurrence of gas hydrates at high saturations in reservoir-quality sands in the Gulf of Mexico (GOM).
The Gulf of Mexico is thus the first offshore area in the United States with enough information to identify gas hydrate energy resource targets with potential for gas production.
Gas hydrates are a unique substance comprising natural gas (almost exclusively methane) in combination with water. Gas hydrates are known to exist in great abundance in nature and have significant implications as a potential energy source. However, prior to this expedition, there was little documentation that gas hydrates occurred in resource-quality accumulations in the marine environment, according to NETL.
|The research consortium used the Helix drilling vessel Q4000. Click to enlarge.|
The expedition conducted logging-while-drilling (LWD) operations at multiple sites to test a variety of geologic/geophysical models for the occurrence of gas hydrate in sand reservoirs in the deepwater Gulf of Mexico. The target formations were 3,000 feet below the mud line in 7,000-8,000 feet of water.
The LWD tool combination was designed to provide unprecedented information on the nature of the sediments and their pore fill constituents. The program featured full research-level LWD data on formation lithology and porosity, and included 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 featured 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.
Select LWD data was analyzed by the onboard science team in real time, and that information was 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 well was drilled, the drill string was raised to clear the seafloor, and the ship moved a short distance to the next hole location. After completion of two or three holes at a site, the full drill string was retrieved and laid down on deck, and the ship steamed to the next site.
Other participants in the field program included the US Geological Survey, the Minerals Management Service, the borehole research group at the Lamont Doherty Earth Observatory, Schlumberger, and AOA geophysics.
The objective of the 21-day expedition was to confirm that gas hydrates occur at high saturations within reservoir quality sands in the Gulf of Mexico. This objective was fully met, with highly-saturated gas hydrate-bearing sands discovered in at least in two of three sites drilled.
The Gulf of Mexico Gas Hydrate Joint Industry Project (JIP) Leg II expedition follows a 2005 JIP Leg I drilling program that focused on possible drilling hazards related to gas hydrate in fine-grained sediments. Leg II was designed to expand the understanding of gas hydrate in the Gulf of Mexico by specifically targeting systems thought to include high-quality (thick, porous, and permeable) sands.
The field program was led by NETL’s Dr. Ray Boswell and the USGS’s Dr. Timothy Collett. It was managed by Chevron’s Rana Roy and Dr. Emrys Jones. The program was completed on time, with zero safety incidents, and below the planned $11.2 million budget.
The results from this effort confirm the ideas that lay at the foundation of the DOE’s efforts to assess the resource potential of gas hydrates. Gas hydrates were found at saturations ranging from 50% to more than 90% in high quality sands near the base of the gas hydrate stability zone. The deposits were also found in close accordance with the projects pre-drill predictions, providing increased confidence in our gas hydrate exploration and appraisal technologies.—Ray Boswell
We have also found gas hydrate in a range of settings, including sand reservoirs, thick sequences of fracture-filling gas hydrates in shales, and potential partially saturated gas hydrates in younger systems. These sites should provide a wealth of opportunities for further study and data collection that should provide significant advances in understanding the nature and development of gas hydrate systems.—Timothy Collett