Formation and thermal based dissociation of methane pore space hydrate has been studied at various initial hydrate saturations and heating rates utilizing a 1.3 cm 3 glass bead sample pack. Initial hydrate formation occurred rapidly after pressurization and proceeded at a higher rate for systems with the lower initial water saturation. Peak initial formation rates for 21%, 41%, and 60% initial water saturation was 4.3 × 10 −6 , 1.44 × 10 −6 , and 1.7 × 10 −6 mol/s, respectively. Three distinct stages of formation were observed for hydrate growth in porous media; a rapid initial formation regime determined by the enclathration reaction followed by a slow mass limited diffusion regime again followed by a second rapid and discrete formation period. Peak efficiency rates ranged from 74% to 81%. Experimental and numerical results showed slightly higher efficiency rates and maximum cumulative efficiency values for lower heating rate conditions. Lower heating rates produce a higher cumulative efficiency at the trade-off of less total hydrate being dissociated. Cumulative production efficiencies were greater for conditions of higher initial hydrate saturation, with values at 62% for 18% hydrate saturation and 72% for 50% hydrate saturation.
Natural gas hydrates represent a potentially substantial unconventional natural gas resource and the recovery of permafrost hydrates has seen significant attention over the past decade. Laboratory study of different growth and dissociation methods is an important step in the development of gas hydrate production methods. The formation and dissociation behavior of gas hydrates in quartz sand sediment is investigated on a large laboratory scale reactor with a sample volume of 59.3 L. Hydrate saturations of 10% and 30% pore space volume are dissociated via a point source thermal stimulation method using both a low heating rate of 20 W and a high heating rate of 100 W. Hydrate growth via gas invasion method resulted in nonhomogenous hydrate formation. Secondary hydrate formation was observed during prolonged hydrate formation periods in a quasi-repeatable manner. Peak efficiency rates of gas production ranged from 91% to 72% and net "end of test" efficiencies from 86% to 41%. Higher initial hydrate saturations resulted in better production performance while greater heating rates resulted in higher peak efficiency rates. Higher hydrate saturations displayed heater temperature spikes followed by a transition zone where heater temperatures stabilized due to the onset of increased convective heat transport.
The largest amount
of methane gas is trapped in less-conventional
natural gas resources, such as methane hydrates. It is estimated that
these reserves of methane gas, in the form of hydrates, are larger
than all of the conventional resources of methane gas combined. [
U.S. Energy Information Administration (EIA), Independent
Statistics and Analysis, Potential of Gas Hydrates Is Great,
but Practical Development Is Far off,
]. Methane extraction
from hydrates can be coupled with carbon dioxide sequestration to
make this process carbon-neutral. A large-scale laboratory reactor
is used to simulate the conditions existing in permafrost hydrate
sediments to study the hydrate formation and dissociation processes.
The dissociation process occurs via a cartridge heat source (to simulate
the down-hole combustion) and carbon dioxide injection, to study the
CO2 sequestration behavior. The hydrate sediment studied
was formed with 50% saturation of hydrate by pore volume and the dissociation
of this sediment was done using different combinations of high and
low heating rates (100 W and 20 W) and high and low CO2 injection rates (1000 and 155 mL/min). Two baseline
tests were conducted without any addition of heat at CO2 injection rates of 155 and 1000 mL/min, for comparison. The results
indicate that, at a constant heating rate, the number of moles of
methane recovered decreases with an increasing flow rate of CO2 injection, whereas the number of moles of CO2 sequestered
increases as the CO2 injection flow rate increases. At
50% initial hydrate saturation (S
H) and
a heating rate of 100 W, the number of moles of methane recovered
decreased from 96 to 58 when the CO2 injection rate was
increased from 155 mL/min to 1000 mL/min, respectively. Whereas, at
50% initial saturation and a heating rate of 100 W, the number of
moles of CO2 sequestered increased from 13 to 40 when the
CO2 injection rates were increased from 155 mL/min to 1000
mL/min. The recovery efficiency improved from 18% to 22% to 60% when
the heating rate was increased from 0 to 20 W to 100 W, respectively,
at 1000 mL/min CO2 injection.
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