Temperature Increase during the Depressurization of Partially Hydrate-Saturated Formations within the Stability Region

Abstract

Depressurization experiments of methane-hydrate-bearing sediments were conducted to measure changes within the hydrate stability region before dissociation started. Pore-filling hydrate with a saturation of about 40% was formed in water-saturated silica sand samples with a porosity of 0.4. The initial pressure was kept constant at around 14.5 MPa, and the initial temperature was varied between 281.3 and 283.7 K. When the samples were depressurized at rates ranging from −1.2 to −3.9 MPa/min, temperature increases of 0.26–0.37 K on average were measured. These were caused by a decreasing aqueous methane solubility as well as the liberation of isolated free gas, leading to additional heat-releasing hydrate formation (ΔH MH‑f = −51.86 kJ mol–1 of CH4 at 280 K). The results suggest that the solubility obeys Henry’s law as long as free methane is present in the pore space. The temperature changes during depressurization shift the equilibrium pressure, leading to an anticipated dissociation during the process. The increase in hydrate saturation, in turn, reduces the effective permeability of the formation, reducing the extent to which a formation can be depressurized by a single vertical wellbore. Sensitivity studies for the above state sediment conditions show that the induced increase in the temperature raises the equilibrium pressure of up to 0.7 MPa and raises the hydrate saturation by 7%, for an initial in situ pressure of 35 MPa and varying temperatures.

Size Effect of Porous Media on Methane Hydrate Formation and Dissociation in an Excess Gas Environment

“…As such, care should be taken in selecting suitable grains to form representative synthetic hydrates for methane hydrate production studies, especially for studies conducted with the conventional P–V–T calculation without a sighting window, to ensure the usefulness of data reported in the literature. A representative approach would be to employ the excess water method by which water is added in excess to ensure that synthetic hydrates are formed between the interstitial pore spaces of the chosen porous media. ,,,, …”

Review of natural gas hydrates as an energy resource: Prospects and challenges

“…thermal stimulation, for economical gas production [125,139]. However, one recent work reported a temperature increase of 0.26-0.35 K during depressurization from a high pressure (14.5 MPa) [140]. The authors reasoned that the presence of free gas resulted in reformation of methane hydrate during gas production.…”

“…However, this is highly probable in the natural environment given the fact that ambient conditions in nature will not be as easily manipulated as in a laboratory reactor and chances for hydrate re-formation always exist should the appropriate conditions exist at a different location during recovery [140].…”

Enhanced Gas Production from Hydrate Reservoirs with Underlying Water Layer

“…The guest gases, such as methane molecules, are trapped in the water cavities at a low temperature and high pressure. − With the increasing demand of oil and nature gas resources, there is an urgent need to search for an alternative energy. Nature gas hydrates (NGHs) found in permafrost and deep ocean sediments are of significant potential due to their enormous reserves. − Different exploitive methods of nature gas hydrate from hydrate deposits have been proposed, namely, depressurization, − thermal stimulation, , as well as thermodynamic inhibitor injection. , The depressurization method is considered the most promising in terms of economic efficiency. , …”

“…12,13 The depressurization method is considered the most promising in terms of economic efficiency. 14,15 The deposits of methane hydrate are generally divided into three classes, Class I (with underling free gas), Class II (with underling free-water), and Class III (single hydrate zone), 16,17 as shown in Figure 1. The hydrate dissociation of Class I and Class II has been studied by MRI; 18 it was found that liquid water hindered the output of methane gas during gas production.…”