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Title: Gas Production from Methane Hydrate Bearing Sediments
Keywords: hydrate, dissociation, gas production, bulk conductivity, depressurisation, dissolution
Issue Date: 27-Feb-2012
Citation: SIMON FALSER (2012-02-27). Gas Production from Methane Hydrate Bearing Sediments. ScholarBank@NUS Repository.
Abstract: Natural gas hydrates are solid clathrates of gas and water which are stable at high pressure and low temperature conditions. Estimates suggest that twice the amount of energy presently stored in conventional hydrocarbons is preserved in the form of natural gas hydrates. The vast amount of locally highly concentrated gas hydrate encountered in permafrost regions and deep sea sediments make them an attractive potential energy source for the near future. The required gas extraction method, however, differs from conventional gas reservoirs developments, as gas hydrates must first undergo an in-situ phase change (dissociation) before the freed gas can flow through the porous host sediment and be lifted through wells. This dissociation process is endothermic and thus absorbs energy in the form of heat from the sediment, pore fluid and adjacent non-dissociating regions. A reduction in temperature reduces the dissociation rate, or can even lead to hydrate reformation or pore water freezing. Controlling the temperature regime is therefore expected to be a key component in producing gas from hydrate deposits. This study gives a brief background about the past- and ongoing experimental research on natural gas hydrates. It introduces the methane hydrate testing apparatus designed and built at NUS by describing the components¿ working principles, stating the controlled and measured variables, as well as by giving some recommendations on the work procedures. Repeated small scale production tests show that the gas extraction rate can be increased by 3.6 times on average if the hydrate bearing sediment is dissociated by a combination of depressurised- and heated wellbore (dP+dT), as compared to depressurisation (dP) only. It was further found that under specific circumstances, dP+dT is more efficient in terms of input- to recovered energy than a depressurisation to a lower wellbore pressure. Conductive heat transfer in stable hydrate- and water saturated sediments with a porosity of about 40% can be modeled with a bulk conductivity of 2.59 W/mK, which decreases only slightly under partially gas saturated conditions. The sensible heat of the formation is small compared to the required dissociation energy, and therefore the whole process is governed by the rate of heat supplied into the dissociating zone. A further finding of this study is a temperature increase during pressure reductions in stable gas hydrate conditions. This is caused by two consecutive exothermic reactions: the dissolution of gas from the pore water which subsequently forms hydrate together with the free water. The phenomena results in small increases in hydrate saturation and equilibrium pressure, which implies that hydrate dissociation commences at a higher wellbore pressure than initially assumed.
Appears in Collections:Ph.D Theses (Open)

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