gas hydrate - CCOP

INTRODUCTION
Clathrate hydrates are solid crystalline compounds that form when water (host molecule)
under low temperatures and high pressures is exposed to small molecules (guest molecule),
such as methane (CH 4 ), ethane (C 2 H 6 ), hydrogen sulfide (H 2 S), and carbon dioxide (CO 2 )
(Englezos, 1993; and Sloan 1998). Geologic deposits of gas hydrates mainly have methane
guests and occur in settings where the necessary low temperatures and high pressures exist for
gas hydrate formation and stability; on-shore beneath the permafrost, and off-shore in deep
ocean sediments. Kvenvolden (1993) estimates the amount of methane in geologic deposits at
1015 m 3 , which equals a carbon mass that exceeds all conventional fossil energy reserves by a
factor of two.
Almost without exception, gas hydrates crystallize into three types of structures depending on
the guest molecules; sI, sII, or sH. The sI and sII structures were identified via Xray
diffraction by von Stackelberg and Müller (1954), and 33 years later Ripmeester et al. (1987)
proposed the sH hydrate structure. The sI and sII structures have both small and large cavities
for the guest molecules and the sH structure has small, medium, and large cavities. Naturally
occurring gas hydrates generally have guest molecules that are hydrophobic, such as CH 4 .
Gas hydrates of H 2 S and CO 2 , however, are water soluble acid gases, as classified by Jeffery
(1984). Although chemically quite different, CH 4 and CO 2 form sI structures as simple
hydrates, occupying both the small and large cavities of the sI structure.
Conventional production methods for methane hydrates in geologic formations involve
dissociating the gas hydrate by altering the system conditions to a point outside the stability
region, producing CH 4 gas and water. Gas hydrate production via thermal stimulation
involves adding heat to the geologic formation, which initially raises the formation
temperature outside the stability region, causing the gas hydrate to dissociate. Once the
dissociation process is underway, added heat is used to overcome the endothermic heat of
dissociation. Production via depressurization typically involves reducing the system pressure,
causing dissociation of the gas hydrate. However, the endothermic heat of dissociation
decreases the reservoir temperature, which may reduce gas production rates or halt production
if water ice forms and reduces formation permeability. The third conventional production
approach involves injecting an inhibitor into the reservoir, which effectively shifts the gas
hydrate equilibrium curve, causing dissociation. As with depressurization, the endothermic
heat of dissociation decreases the reservoir temperature, lowering or halting gas production.
An unconventional method that has been discussed for gas hydrate production involves the
injection of CO 2 . The idea of swapping CO 2 for CH 4 in gas hydrates was first advanced by
Ohgaki et al. (1996) and then for ethane hydrate by Nakano et al. (1998). Their concept
involves injecting CO 2 gas, which is then allowed to equilibrate with methane hydrate along
the three-phase equilibrium boundary (Smith et al., 2001). Because of the difference in
chemical affinity for CO 2 versus methane in the sI hydrate structure, the mole fraction of
methane would be reduced to approximately 0.48 in the hydrate and rise to a value of 0.7 in
the gas phase at equilibrium. McGrail et al. (2004) have proposed a more advanced concept
involving injection of a microemulsion of liquid CO 2 and water into the gas hydrate formation.
This approach is intended to provide additional sensible heat in the emulsion and heat of
formation of the CO 2 hydrate as a low grade heat source for further dissociation of methane
hydrate away from the injectate plume, which has been reported to enhance production rates
by as much as a factor of 30. The additional benefits of this approach include 1) gas hydrate
remains in the pore space during production thus maintaining mechanical stability of the
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New Energy Resources in the CCOP Region - Gas Hydrates and Coalbed Methane