In-situ thermal stimulation of gas hydrates

Gas hydrates are increasingly considered a potential energy resource. Various methods of exploiting gas hydrates (thermal stimulation, depressurization, inhibitor injection, etc.) have been proposed so far but each of them has some drawbacks. To overcome some of these drawbacks, we propose a new technology for producing methane from gas hydrates. The method uses in situ thermal stimulation by introducing a specially designed hydrate heating apparatus into a horizontal borehole drilled into a gas hydrate zone (GHZ). Instead of using water or other hot fluid injected from the surface or another location away from the GHZ, an air/gas fuel mixture is introduced into a combustion vessel from the surface via a fuel injection tubing string directly into GHZ. Burning the fuel mixture results in the heat necessary to dissociate gas hydrate. The freed natural gas is then conveyed to the surface via a casing that is lining the wellbore. This technology has the advantage of permitting the dissociated gas to be produced through the same wellbore through which the air/gas fuel mixture is injected, thereby avoiding the need of using two wells (injection and exploitation). An estimated energy gain efficiency of the proposed method shows that only about 1.1 to 1.7% of gas produced will have to be burned to decompose hydrates. Previous estimates of other thermal decomposition methods, for example steam injection, show that about 50% of gas produced will have to be burned just to decompose hydrates. An added advantage is that the produced natural gas is not mixed with the combustion gases, thus avoiding the problem of the resulting gas having a BTU content that is too low. (c) 2009 Elsevier B.V. All rights reserved.