343
Methane from Gas Hydrates
thermodynamic properties of hydrates. In geologic media that have distribution of
pore sizes, hydrates would form and dissociate over a range of temperatures and
pressures according to the distribution of pore radii and the impact of salts in the
residual pore water [191]. Goel [67] and Goel et al. [209] indicated that in order
to understand gas-exchange technology in porous media, quantitative estimates of
formation and dissociation processes in a typical geologic media core samples are
needed.
12.5.5 eghr meThod
As shown above, a strict gas exchange of CO 2 for CH 4 in bulk methane hydrate is
slow by several orders of magnitude to be considered as an effective method of gas
hydrate production. An EGHR process that involves injecting a two-phase emulsion
of liquid CO 2 and water at proper volumetric ratio can considerably enhance (three
times or higher) the production rate over injecting cool water (15°C) alone [76–83].
It is important to know the range of reservoir conditions where EGHR technique
can be applied. Collett and coworkers [204,222,223] calculated these conditions for
Alaska Northern slope (ANS) and concluded that EGHR method can be applied
over a large fraction of ANS. They also found that CO 2 hydrate would be stable
under almost any conditions on the ANS short of very near the ground surface. They
also suggested that typical ANS reservoir conditions would inject liquid CO 2 with
a density ~82%−94% of the water phase. ANS well log temperature data as well as
carbon dioxide hydrate and vapor–liquid equilibrium data are described by Collet
et al. [204,222,223].
The laboratory studies indicated that there are no signs of coagulation into macrodroplets as the emulsion moves away from the injector—a conclusion that needs to
be tested at reservoir scale [73]. Another important restriction is that the temperature
of the water–CO 2 emulsion remains above the equilibrium point where CO 2 hydrate
could form in the wellbore or near the wellbore. Interruption of the supply of emulsion fluid during production for an extended period could result in the premature
formation of CO 2 hydrate and plugging [73,77]. Provisions for temporary injection
of heat may be needed to allow for flow interruptions, which are important for well
maintenance.
The EGHR method has been tested in laboratory for continuous production
of a suitable liquid carbon dioxide and water emulsion [73,76–82]. This test is
largely one dimensional. A suitable downhole tool that can work in actual field
needs to be developed. The injector tool design should be compatible with downhole conditions typical of gas hydrate formations. Wellbore completion requirements such as open hole, uncased, or perforated casing influence the design
parameters of the injection tool. Injection of the liquid carbon dioxide and water
emulsion in the target formation is the most important requirement. A new design
to fit these requirements is depicted in Figure 12.6 [73]. Here, emulsion outlets
are located on the side. Surface-warmed liquid carbon dioxide and water can
be directed into such an injector from the high-pressure lines. Use of produced
water to form emulsion would eliminate issues associated with disposal of these
fluids in arctic conditions. Both rate and distance of formation penetration can