The presence of hydrates has primarily been a nuisance or a well control issue when drilling for conventional oil and gas offshore and in onshore permafrost regions. However, methane hydrates could very well become the new source of clean and affordable gas supplies in the United States by 2030. For example, if several large "sweet spots" of hydrates could be defined and developed in U.S. waters, the ultimate recoverable hydrate resource could range from 1,500 to 2,000 Tcf of gas. This is close to the current U.S. domestic natural gas recoverable resource, yet it may be less than 1% of the total in-place methane hydrate resources of the U.S.
To achieve the long term goals of the Federal Government in regard to energy supplies for the U.S.A., the Department of Energy is committed to developing the knowledge and technology base to allow commercial production of methane from domestic hydrate deposits by the year 2015.
Hydrates or clathrates are a crystalline lattice material consisting of molecules of water that have formed an open, cage-like solid lattice that encloses molecules of methane. Hydrates of a given composition exist under particular pressure-temperature conditions caused by geological conditions. The variability of the pressure-temperature phase diagrams for hydrates is caused by changes in geological conditions during hydrate evolution and by the chemical composition of the hydrate, which may have been enriched with ethane. Under changing geological conditions hydrates can dissociate and be released gradually or explosively, depending on how rapidly the pressure drops or the temperature increases. In addition to hydrates being only quasi-stable, the crystal structure of a hydrate packs methane so efficiently, depending on the purity of the hydrate, it can contain between 70 to 164 times the volume of free gas at standard temperature and pressure vs. the volume of the hydrate prior to dissociation.
Methane hydrates contain enormous volumes of natural gas and are now known to accumulate worldwide on the slopes of continental shelves and below the artic permafrost where pressures and temperatures are suitable and methane and water are available. The existence and thickness of hydrates zones are determined by the influx of gas and water through shallow sediment, the rapid accumulation of organic rich sediments, the geothermal gradient, the water depth and sea-bottom temperature, and the gas mixture of the hydrate. Analysis indicates hydrates can occupy as much as 500 m of sediments.
Because hydrates will dissociate or release free gas upon a decrease of pressure, increase of temperature, or combinations thereof; premature dissociation around the wellbore must be avoided during the drilling process to minimize wellbore instability and well control issues resulting from changes in the mechanical and physical properties of the sediments when hydrates dissociate. In addition, the hydrates dissociation into gas and fresh water will create a gas cut mud with a lower mud density causing associated changes in mud rheology that reduce hole cleaning capacity, which may cause wellbore instability and potential hole collapse or a pack off and stuck pipe.
There is an enormous amount of information available in the public domain as to the characterization, composition, and the location of known deposits of methane hydrates. However, the know-how to drill, complete, and produce hydrates in commercial quantities is just beginning to be developed and has received little attention from the oil and gas industry.
Although the focus of this paper is upon drilling for methane hydrates in marine environments, specifically in deep water, the issues are applicable to drilling hydrates on land as well.
Methane hydrates drilling-related challenges include:
Narrow margins between pore pressure and fracture gradient in ocean surface sediments and within the hydrate reservoir.
Surface hole instability.
Subsidence caused by hydrate production.