Thermally Assisted Dissociation of Methane Hydrates and the Impact of CO₂ Injection

Abstract

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.

Characterising thermally controlled CH4–CO2 hydrate exchange in unconsolidated sediments

“…The effect of sediment fraction on hydrate exchange and CH4 recovery was explored by performing a series of experiments on hydrate cores of varying sediment fraction, as detailed in Table 1. There are only a limited number of experiments reported 17,21,33 that investigate the effect of sediment on CH4 recovery through exchange with CO2, all of which have looked at cementing and pore-filling hydrate sediments. In all experiments presented here, the hydrate sediment formed was of a load-bearing nature, which allowed us to investigate the efficiency of exchange in the presence of the sediment phase without mass-flow limitations imposed by pore connectivity, as may be expected in a cemented structure.…”

“…Secondly, incremental thermal stimulation is shown to be an effective means of progressively enhancing CH4 recovery in the presence of sediment. Consequently, the energy input required to drive recovery, 33 for example through direct heating, catalysis or combustion, could potentially be reduced. Thirdly, the observed uptake of CO2 in the hydrate phase was consistent with the CH4 produced and is not adversely affected by the presence of a sediment phase or thermal stimulation for the temperature and pressure conditions investigated.…”

“…This is in contrast with other experiments that reported a dependence on heating and flow rate. 21,33 Looking forward, the development of a generalised and quantitative model of enhancement relative to simple exchange associated with thermal stimulation and sediment structure will require a new range of experiments. These are likely to include the use of direct Raman imaging of hydrate sediments to quantify both the exchange process and morphological changes in hydrate composition and distribution within the sediment matrix.…”

“…The experiments indicate that higher temperature CO2 injection results in higher recoveries. Tupsakhare et al 33 investigated CO2 exchange in CH4 hydrate sediments, over a range of heating rates and CO2 injection rates. In general, they demonstrated that higher heating rates and higher flow rates typically resulted in higher recoveries; however, no clear trend was established.…”

An Application of the Results from the Large-Scale Thermal Stimulation Method of Methane Hydrate Dissociation to the Field Tests

“…At a higher hydrate saturation, the amount of free water available may be significantly reduced which changes the heat transfer characteristics of the hydrate dissociation process. We have shown previously that understanding heat transfer during the hydrate dissociation process is the key to achieving higher values of recovery efficiency and thermal efficiency. − In an attempt to focus on these parameters, several laboratory-scale tests that focus on thermal and depressurization methods have been reported. , Most of these dissociation tests require a small thermal input to initiate the gas recovery. In actual field tests, steam or hot fluid is injected to increase the temperature of the sediment .…”