345
Methane from Gas Hydrates
in gas hydrate sediments is thermodynamically favorable. This net exothermic
process allows the dissociation of hydrates to be carried out with only minimal requirement of an additional heat source. (2) Once CO 2 -rich fluid fills pores
vacated by methane, the subsequent formation of carbon dioxide hydrate would
mechanically stabilize the formation, eliminating subsidence concerns in some
production environment, and (3) the overall process is carbon neutral since methane is permanently replaced by carbon dioxide as gas hydrate. Produced water
can also be used to form the emulsion, eliminating a problematic disposal issue
in arctic settings [73,77,81].
12.5.6 ComPuTer SimulATion
There are some reported computer simulation studies of commercial production
methods for gas hydrates, and most of them have examined conventional production concepts of depressurization coupled with some form of thermal stimulation
[83,201–214,224–226]. An EGHR process that utilizes a microemulsion of liquid
CO 2 and water to decompose methane hydrate in situ and produce free gas described
earlier has been successfully demonstrated in laboratory-scale experiments with gas
hydrate-bearing sediments. Since these laboratory-based studies were extremely
encouraging, a reservoir modeling assessment that compared and contrasted the
EGHR process with conventional methods of gas hydrate production was carried
under a Department of Energy (DOE) project [73,210,212].
Within the DOE project [73,210], STOMP-HYD simulator was applied to a
series of one- and two-dimensional simulations that investigated the production of
CH 4 hydrates in geologic media using CO 2 injection. Effectively, the project considered two approaches to producing CH 4 hydrate in geologic media using CO 2 injection: (1) hydrate dissociation and reformation and (2) direct molecular exchange.
In the hydrate dissociation and reformation approach, the injected CO 2 first dissociates CH 4 hydrate. This stage is followed by reformation of a mixed gas hydrate,
which predominately comprises CO 2 . In the direct molecular exchange approach,
the injected CO 2 exchanges with the CH 4 in the hydrate structure, maintaining
the hydrate integrity. The dissociation–reformation approach has the advantage of
releasing CH 4 in both the small and large cages. In the direct-exchange approach,
only the CH 4 in the large cages is released. Co-injection of CO 2 and N 2 has been
shown to allow molecular exchange of CH 4 in both the small and large cages.
Because the STOMP-HYD simulator did not track small- and large-cage occupancies, it is currently limited to CO 2 exchange with CH 4 in large cages. The principal
conclusion from this series of simulations was that both CO 2 exchange approaches
yielded faster production times, but lower CH 4 recoveries over pure water injections. Without consideration of the cage occupancies, the direct exchange yielded
faster production times over the dissociation–reformation approach, with nearly
equivalent CH 4 recoveries. The CO 2 -to-water ratio in the injecting fluid primarily
affected production rates, with higher ratios yielding faster productions.
STOMP-HYD simulation results also showed the following conclusions
[73,213,224]:
Methane from Gas Hydrates
in gas hydrate sediments is thermodynamically favorable. This net exothermic
process allows the dissociation of hydrates to be carried out with only minimal requirement of an additional heat source. (2) Once CO 2 -rich fluid fills pores
vacated by methane, the subsequent formation of carbon dioxide hydrate would
mechanically stabilize the formation, eliminating subsidence concerns in some
production environment, and (3) the overall process is carbon neutral since methane is permanently replaced by carbon dioxide as gas hydrate. Produced water
can also be used to form the emulsion, eliminating a problematic disposal issue
in arctic settings [73,77,81].
12.5.6 ComPuTer SimulATion
There are some reported computer simulation studies of commercial production
methods for gas hydrates, and most of them have examined conventional production concepts of depressurization coupled with some form of thermal stimulation
[83,201–214,224–226]. An EGHR process that utilizes a microemulsion of liquid
CO 2 and water to decompose methane hydrate in situ and produce free gas described
earlier has been successfully demonstrated in laboratory-scale experiments with gas
hydrate-bearing sediments. Since these laboratory-based studies were extremely
encouraging, a reservoir modeling assessment that compared and contrasted the
EGHR process with conventional methods of gas hydrate production was carried
under a Department of Energy (DOE) project [73,210,212].
Within the DOE project [73,210], STOMP-HYD simulator was applied to a
series of one- and two-dimensional simulations that investigated the production of
CH 4 hydrates in geologic media using CO 2 injection. Effectively, the project considered two approaches to producing CH 4 hydrate in geologic media using CO 2 injection: (1) hydrate dissociation and reformation and (2) direct molecular exchange.
In the hydrate dissociation and reformation approach, the injected CO 2 first dissociates CH 4 hydrate. This stage is followed by reformation of a mixed gas hydrate,
which predominately comprises CO 2 . In the direct molecular exchange approach,
the injected CO 2 exchanges with the CH 4 in the hydrate structure, maintaining
the hydrate integrity. The dissociation–reformation approach has the advantage of
releasing CH 4 in both the small and large cages. In the direct-exchange approach,
only the CH 4 in the large cages is released. Co-injection of CO 2 and N 2 has been
shown to allow molecular exchange of CH 4 in both the small and large cages.
Because the STOMP-HYD simulator did not track small- and large-cage occupancies, it is currently limited to CO 2 exchange with CH 4 in large cages. The principal
conclusion from this series of simulations was that both CO 2 exchange approaches
yielded faster production times, but lower CH 4 recoveries over pure water injections. Without consideration of the cage occupancies, the direct exchange yielded
faster production times over the dissociation–reformation approach, with nearly
equivalent CH 4 recoveries. The CO 2 -to-water ratio in the injecting fluid primarily
affected production rates, with higher ratios yielding faster productions.
STOMP-HYD simulation results also showed the following conclusions
[73,213,224]: