340
Water for Energy and Fuel Production
from hydrate structure. Each of these methods is briefly reviewed in Sections 12.5.1
through 12.5.6. This section also briefly reviews the numerical simulations that have
been carried out for methane recovery from hydrates. Finally, production research
that has been carried out for commercial sites is briefly assessed.
12.5.1 ThermAl STimulATion
The recovery of methane gas from gas hydrates via thermal stimulation has been
examined both experimentally [181–183] and theoretically [184–187]. Technologies
for implementing thermal stimulation include steam injection, cyclic steam injection, fire flooding, hot brine injection and electromagnetic heating. The techniques
of steam injection and cyclic steam injection are very similar to those used in the
recovery of conventional and unconventional oils. Various possibilities for heating
hydrates using steam or cyclic steam injections have been examined in the literature
[182,187]. All of these techniques, however, suffer from high heat losses, and byproducts of fire flooding can dilute the produced natural gas. The energy efficiency
of electromagnetic heating is also low.
A more promising approach is to inject a saline aqueous solution at an elevated
temperature into gas hydrate-bearing geological reservoir. In this method, the
sensible heat carried by the brine solution is discharged to the gas hydrates by a
convective heat-transfer mechanism. The dissolved salt depresses the dissociation
temperature of the gas hydrate. The experimental evidences indicate that with the
injection of brine, the hydrates become colloidal and migrate convectively with the
brine [188–190]. Tang et al. [181,191] showed that the energy efficiency of the hot
brine injection process is dependent on the brine temperature, injection rate, and
initial hydrate saturation.
The energy efficiency is defined as the ratio of combustion heat of produced gas
over the heat input of the brine. The study showed that a better energy efficiency
was obtained at higher initial hydrate saturation and lower temperature and injection
rates [181–188]. This higher energy efficiency is, however, accompanied by lower
production rates. For moderate to high temperature and injection rate, about 50% of
the recovered energy from methane is used to heat the brine solution. A modification of this approach was suggested by Chatterji and Griffith [78] who proposed an
injection of two aqueous fluids that react and produce the heat required to release
methane from the hydrates. This type of acidic and basic solutions reactions will
yield a hot salt solution, and this will not require the external heating of brine solution, thereby improving the energy efficiency.
12.5.2 dePreSSurizATion
Gas hydrate production via depressurization is considered to be the most economically
promising technology [190,192–200]. This method has been adopted in Messoyakha
field in northern Russia, which contains both free natural gas and hydrates. This
reservoir has been constantly producing natural gas because of dissociation of
gas hydrates into gas due to depressurization. The production rate in this field is,
however, controlled by the heat transfer toward the hydrate dissociation region.