This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 93492, “Gas-Hydrate Production Technology for Natural-Gas Storage and Transportation and CO2 Sequestration,” by R. Masoudi, SPE, and B. Tohidi, SPE, Heriot-Watt U., prepared for the 2005 SPE 14th Middle East Oil & Gas Show and Conference, Bahrain, 12–15 March.
Gas hydrates (clathrates) are ice-like crystalline compounds that form under low temperature and elevated pressure conditions. Within the gas-hydrate lattice, water molecules form a network of hydrogen-bonded cage-like structures enclosing the guest molecules that generally comprise low-molecular-diameter gases [e.g., methane, ethane, propane, or carbon dioxide (CO2)]. Although hydrate formation can pose serious flow-assurance problems in the oil and gas industry, gas hydrates present a novel means for gas storage and transportation with potential applications in a wide variety of areas.
The need for natural gas at lower costs is the challenge that drives the development of a new process for storing and transporting natural gas in the form of frozen hydrate. An important property of hydrates that makes them attractive for use in gas storage and transportation is their very high gas-to-solid ratio. In addition to the high gas content, gas hydrates are remarkably stable. When the gas hydrate, which is formed at a high pressure and low temperature, is returned to atmospheric pressure, dissociation begins at the surface and an ice film forms over the water that covers the surface and prevents further hydrate dissociation. This “self-preservation” effect causes the gas hydrate to remain stable at atmospheric pressure. These properties make natural-gas-hydrate (NGH) technology very attractive for natural-gas storage and transportation and CO2 sequestration.
For small- to medium-sized gas fields, hydrate technology is an appropriate alternative to the well-established liquefied-natural-gas (LNG) technology. LNG technology is economically feasible only in large-scale projects. Compared to alternative technologies such as LNG and gas to liquids, NGH technology is relatively simple, low cost, and does not require complex processes or pressure and temperature extremes. Capital cost of NGH technology is approximately 25% lower than the capital cost of LNG technology.
There are fundamental interests in continuous production of NGH in a large-scale reactor and effective long-term storage of gas in clathrates. NGH production accounts for more than 60% of the costs of the NGH chain. Development of an efficient technique that is practical and economical for mass production of gas hydrates is vital for realizing NGH technology. There are a number of technical challenges to be addressed in development of a cost-effective NGH production process.