Natural Gas Hydrates as an Energy Source - Revisited
Authors: Wang Xiuli , Economides Michael John 
Published: March 2015

Abstract
The success of shale gas, mostly resulting from the deployment of technology to an energy source that was known but considered until recently inaccessible, has rekindled, by analogy, an interest in natural gas hydrates whose volume dwarfs any other energy source on earth.
Natural gas hydrates sediments are reputed to have the potential of providing a huge amount of natural gas for human use. Gas hydrate-bearing sediments have been found worldwide on slopes of continental shelves and arctic permafrost where the required temperature and pressure conditions have been satisfied and natural gas and water are available. A lot of research has been carried out and is ongoing as to the characterization, composition and location of known deposits of natural gas hydrates.
Resource exploitation requires the safe dissociation of the gas hydrate sediments. Basic depressurization techniques and thermal stimulation processes have been tried in pilot efforts to exploit the resource leading to a series of findings.
This work analyzes most of the issues relating to the subject of natural gas hydrates by investigating the history and properties of natural gas hydrates, establishing reserve estimates from available data which suggest that hydrate exploration is amenable to proven exploration methods, and presenting the technologies and processes necessary for drilling and production. This paper also discusses ongoing research in support of exploration and production. Current technologies show that natural gas hydrates are meaningfully abundant and that cost effective production, although not commercially demonstrated already, is more practical than commonly thought possible.

Introduction
Natural gas hydrates are "clathrates" where molecules of gas no larger than 0.83 nm are included in a crystalline lattice formed by hydrogen-bonded water molecules. They form in oil and gas production and transportation systems as well as in nature.
One cubic meter (m3) of gas hydrate contains from 70 to 210 standard m3 of gas. Hydrate crystals differ greatly and depend on gas and water composition, pressure and temperature (Wang and Economides, 2009, Max et al., 2006).

Hydrate Formation in Geological Structure
Four conditions must co-exist for hydrates to form in a geological structure: presence of reservoir waterfree or dissolved gashigh pressure and, especially,lower temperature.A geothermal gradient is very important for hydrate formation or hydrate dissociation. In offshore environments the gradient can vary from 1.2 to 4.2 oC/100 m (0.65 to 2.3 oF/100 ft.) Figure 1 (Jones et al., 2010) shows a general scheme for determination of a hydrate formation zone, HFZ (Makogon, 1974; 1981). The thickness of a HFZ depends on the gas composition and can be from 100 m (330 ft) to 700 m (2,300 ft) and more. Figure 2 is a well-known graph showing the formation of hydrates (McKetta and Wehe, 1958) at different pressures and temperatures. For example, assuming a sea depth of 1,000 ft with a pressure gradient of 0.43 psi/ft the pressure at the bottom of the sea would be 430 psi. From Figure 2 hydrate formation will happen if the temperature is 50oF or less (gas gravity is 0.6).
From Figure 3 at the same conditions and the same gravity the temperature for hydrate formation is 47oF. If the water depth is just 100 ft with a pressure of just 43 psi from Figure 3 the temperature should be 36oF or less. All of these temperatures are plausible and hydrates can readily form practically anywhere at the bottom of the ocean. On land, beneath permafrost layers natural gas hydrates have been postulated to exist and, indeed, may be acting as the caprock for other hydrocarbon accumulations, including natural gas in the gaseous state.