Natural gas hydrates are known to exist in commercial quantities in many places around the world. For many years their potential was concealed by the very specific set of environmental conditions required for them to exist in a stable state. A recent US Department of Energy (DOE) report describes a myriad of projects all related to reaching the ultimate goal of systematically identifying, characterizing and recovering commercially viable quantities of natural gas from subsea hydrate deposits. The report summarizes progress to date.

The DOE report describes ongoing projects offshore Alaska, Louisiana and South Korea, as well as basic research. These include:

• Refinement of 3-D seismic acquisition and interpretation to identify and map subsea structures that potentially contain gas hydrate accumulations.
• Understanding the process whereby natural gas dissociates from hydrates, and the role hydrates play in global climate systems.
• Characterization of reservoir units that could control migration and trap dissociated gas.
• Adaptation of conventional wisdom used by the oil and gas industry to develop drilling and production techniques for exploitation of hydrate deposits.

One thing is clear, the prize is well worth the effort. Dr. Timothy Collet of the USGS estimated in 1998 that as much as 590 Tcf of gas is trapped in clathrate hydrates on Alaska's North Slope, much beneath existing producing reservoirs. The most promising studies so far conclude that when gas hydrate deposits are structurally linked to free gas reservoirs, dissociation of gas from the hydrates can take place under certain conditions as the nearby free gas reservoir depressurizes. Intuitively, one can appreciate that if gas dissociating from hydrate deposits can charge an existing reservoir in a reasonable time-frame, it can be economically produced. However, the question remains, how rapidly can this dissociation occur? Can it take place fast enough to significantly add to the gas saturation of the reservoir? And what about the relative permeability of the link? Will gas dissociation and migration take place over commercial-time or some subset of geologic-time? One modeling program developed at the University of Alaska-Fairbanks (UAF) postulated that a well within the free gas zone could depressurize nearby hydrate deposits creating additional free gas, although inherent adiabatic effects could lower the reservoir temperature, slowing the dissociation process. It is not unlikely that some sort of thermal stimulation might be required to offset this effect. UAF reckons that production of free gas at about 25 MMcfg/d is required to precipitate significant hydrate gas dissociation.

The studies go on to explain that having a reservoir to trap the gas, while fortuitous, is not enough. A method to estimate the quantity and availability of gas reserves, its recovery factor and probable production cost is needed before investors will fund the pursuit of these resources. Among other things, this involves a thorough understanding of the pressure/temperature environment for hydrate stability.
For these reasons, viable production of gas from hydrates seems feasible only from Alaska's North Slope and the Gulf of Mexico at this time. A lot has been written and speculated about huge hydrates deposits off the Carolinas. These have been linked to legends of the so-called Bermuda Triangle. Have violent natural outgassing events there caused ships to founder or aircraft to crash? Questions like these will likely constrain hydrate exploitation to areas with significant existing oil and gas infrastructure.
The task is so daunting it has been subdivided into three distinct phases, starting in Alaska's Milne Point area. Phase 1, scheduled to be completed by next month, will characterize the reservoirs and fluids, leaving recovery/producibility estimates for the Phase 2, to be conducted between November 2004 and December 2005. Also included in Phase 2 will be procedures for drilling, formation evaluation using logs and cores and completion techniques. Phase 3 will take the data from earlier phases and integrate it for the purposes of analysis leading to a decision on the viability of commercial gas hydrate production. Target date for completion of all three phases is December 2006.

Understanding hydrate behavior

Several ongoing projects hope to improve understanding of how hydrates form, the nature of their stability (or instability) and their dissolution. Research vessels sailing the Gulf of Mexico have recovered hydrate samples from 1,640 ft to 1,968 ft (500 m to 600 m) of water in the Bush Hill area offshore Louisiana, at 3,280 ft (1,000 m) in Green Canyon Block 415 and at 9,840 ft (3,000 m) in the Sigsbee Knoll area. Two techniques were used. First a video-controlled "quick grab" sampling device would grab up seabed sediments and dump them on the deck precipitating a frenzy of activity as scientists scrambled to pick out large hydrate clusters for inspection. A more scientific pressurized piston-coring device offered the opportunity to retrieve a hydrate core and keep it in a stabilized state for detailed analysis. A portable CAT scanner borrowed from the medical industry was used to image the core structure. Early success with the portable scanner encouraged further study with a more performant laboratory CT scanner. Accordingly, high-resolution images of sediment grains on the order of 30 µ-in. were obtained. Sample thermal stability was achieved by placing the samples in a stream of nitrogen outgassing from a liquid N2 tank. This stabilized the samples for the 41/2 hours required to make a high-resolution image. Analysis of the images revealed clues of how the hydrates coexist with mud, sediment, free gas and water.

Frozen in Cho-sen

Meanwhile, offshore South Korea, 3-D seismic images of hydrate-bearing formations were obtained during 2001 and 2002. Geophysicists determined that bottom simulating reflectors characteristic of marine hydrate bodies are present in the Ulleung Basin of the East Sea. By comparing their seismic images with those from known hydrate areas, scientists have predicted that commercial quantities of hydrate concentrations (40% or greater) exist in shallow sedimentary traps just below the sea floor. Moreover, images suggest the presence of large quantities of free gas in so-called "chimneys" beneath the hydrate layers. So far, more than 9,652 sq miles (25,000 sq km) have been surveyed in a project expected to last through 2005. Concurrently, 28 deepsea cores were recovered and analyzed showing a wide range of organic carbon content. Gas recovered from the samples was 98% methane. Several promising pull-up structures were identified. These are typical responses for hydrate accumulations.
Encouraged by the early results, the Korean Government has joined the Korea Gas Corp. and Korea Institute of Geology, Mining and Materials to perform more detailed analysis culminating in an exploratory drilling program.

Meanwhile, a multi-well hydrate drilling joint industry program led by ChevronTexaco and slated for the Gulf of Mexico had to be delayed while environmental permits were obtained. It is expected to kick-off this month using the drilling vessel Fugro Explorer.

Hydrate hunt heating up

Never known for its patience, the oil and gas industry is kicking the hydrate hunt up a notch. Several prestigious consortia are forming to benchmark progress on solving the hydrates question. AAPG's Hedberg Research Conference on Gas Hydrates held this month in Vancouver, BC, Canada, will convene world-class experts and hear more than 60 papers on the subject. Goals of the conference are to critically examine the geologic parameters that control the occurrence and stability of gas hydrates, assess the volume of natural gas stored in known hydrate accumulations as well as the exploration methods for assessing their commercial viability and identify gaps in production technology that must be filled before production from hydrates can be attempted. In addition, the conference will assess hazards and risks associated with hydrate instability on steep slope areas and try to determine the effect of hydrates on drilling safety.

A new International Hydrate Consortium is forming, led by the Asia Pacific Economic Cooperation forum. The consortium will focus on Pacific Rim gas hydrates and individual projects.

The DOE Minerals Management Service (MMS) is weighing-in with a methodology for assessment of gas hydrate resources. Their plan is based on petroleum industry practices to map the Gulf of Mexico's most likely areas for hydrate accumulations and indicate their commercial probability. Results are expected by December 2005.

The US Naval Research Laboratory used a Texas A&M University research vessel RV Gyre to probe the deep Atwater Valley this summer. Seismic data will be obtained as well as piston cores from promising seabed outcrops. One goal of the project is to contribute to the MMS's report on understanding drilling hazards specific to hydrate drilling. The cores will be analyzed for pore water chemistry to understand the vertical migration of methane and determine its origin. A third project will obtain heat-flow data from the seabed to correlate with seismic, geochemical and geologic data. In this project, the heat-flow measurements are intended to help refine models of the generation of hydrate mounds, their vertical extent and their potential for rapid dissociation of gas into the sea, a potential risk to drillers. An earlier paper by Dr. Roger N. Anderson of Lamont-Doherty Geological Observatory of Columbia University postulated a direct correlation between seafloor hot-spots and submerged oilfields.

Also probing the Atwater Valley will be the Louisiana Universities Marine Consortium's RV Pelican performing seafloor photography and acquiring resistivity data to verify whether chemosynthetic communities exist around the gas hydrate mounds targeted for drilling.

Knowledge management

With the world's body of knowledge on natural gas hydrates growing exponentially, the scientific community along with the oil and gas industry needs to manage the information process to ensure timely progress toward the ultimate goal of safely and efficiently harvesting the vast energy potential represented by this elusive resource. Patience is a virtue, but time's a-wasting!

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