Formation of natural gas hydrates in marine sediments: 2. Thermodynamic calculations of stability conditions in porous sediments

Abstract

A thermodynamic model for hydrate formation is used to compute the solubility of methane in pore water in equilibrium with gaseous methane or methane hydrate or both. Free energy of water in the hydrate phase and of methane in gas bubbles are corrected to account for salt effects and capillary effects. Capillary effects increase the solubility of methane in fluid in equilibrium with either hydrate or gas. Natural sediments have a broad distribution of pore sizes, and the effective pore size for capillary effects is a function of the fraction of the pore space filled by hydrate or gas (phase fraction). The equilibrium conditions for hydrate+water+gas equilibrium thus depend on hydrate and gas phase fraction. Data acquired on Blake Ridge during Ocean Drilling Program Leg 164 show that the base of the hydrate stability there is shifted by -2øC or more with respect to the expected temperature and this shift has been attributed to capillary effects. We show that this explanation would require a very small effective pore radius (20 nm at 30 MPa). Mercury porosimetry indicates that the percolation threshold for Blake Ridge silty claystone is reached at 20-25% phase fraction and corresponds to a 100 nm pore radius. Hydrate and gas phase fraction determined with several independent methods are all lower than this percolation threshold, implying that gas and hydrate fill pores larger than 100 nm. We conclude that additional inhibition factors other than pore size effects must be involved to explain the -2øC bottom-simulating reflector (BSR) shift as an equilibrium phenomenon. Capillary effects may, however, explain other observations such as large variations of the gas hydrate content in the sediment with lithology and porosity and the distribution of hydrate between interstitial hydrate and segregated masses. Capillary effects should also oppose the migration of gas bubbles when gas phase fraction is less than the percolation threshold and make unnecessary the assumption of a hydrate seal impermeable to fluids. Alternatively, we can go some way to explaining the offset position of the BSR by relaxing the assumption that the system is in thermodynamic equilibrium. Nucleation kinetics of hydrate and/or free gas bubbles may be inhibited by confinement of the methane-bearing fluid in small pores. Equilibration may also be limited by possible rates of diffusional transport of gas, water, and salt components or be perturbed by significant flows of fluid or heat through the sediments. 1Now at 1988] that may play a significant role in global climate change, both present [Paull et al., 1991] and past [Dickens et al., 1997a]. The distribution of hydrate with depth in the sediment has been reliably determined at very few sites [Dickens et al., 1997b] and processes by which hydrates accumulate and dissociate in the sediments are not yet well understood. Occurrence of gas hydrate in the sediment is generally recognized from the presence of a bottom-simulating reflector (BSR) and this reflector is generally assumed to represent the thr...

Low heat flow from young oceanic lithosphere at the Middle America Trench off Mexico

“…Hyndman et al [15] concluded that in situ data favour a base of the hydrate stability field controlled by the pure methane/pure water phase boundary. However, other authors (e.g., [12,16]) have favoured the pure methane/seawater boundary, and this boundary better fits the data from recent Ocean Drilling Program (ODP) legs [12] (Fig. 4).…”

“…Thin solid and dashed lines are empirical fits to these data [13]. Thick solid and dashed lines are phase boundaries predicted by the theoretical approach of Tohidi et al [14] Here seawater is approximated by 560 mMol NaCl solution, and the dashed curve shown, which is the one used in this study, matches closely the seawater curve of [16]. Arrows mark the range of inferred temperatures at the BSR in our study area.…”

“…4). The remaining discrepancies between the computed phase boundary and ODP calibrations may be explained by capillary effects in fine-grained sediments [16]. These effects are expected to be minor on the case of the western Mexico margin, where the predominant sediment input is likely to turbidites.…”

Defining the updip extent of the gas hydrate stability zone on continental margins with low geothermal gradients

“…The accumulation of hydrate within the HSZ is controlled by geologic factors [e.g., Bünz et al, 2003;Riedel et al, 2006], pore fluid dynamics [e.g., Gorman et al, 2002;Hornbach et al, 2004], methane saturation levels in pore fluids [Gering et al, 2000], and anaerobic oxidation of methane [Borowski et al, 1996;Xu and Ruppel, 1999]. Additionally, the extent of the HSZ itself will be affected by such factors as pore water salinity and the composition of the gas (assumed in our case to be methane but possibly consisting of other natural gases) Henry et al, 1999]. It follows that the occurrence and concentration of hydrates within the HSZ are likely to be irregular and variable.…”

Water Transfer Characteristics during Methane Hydrate Formation Processes in Layered Media

“…Therefore, knowledge about the thermodynamic mechanism(s) of formation and dissociation of hydrates in porous media is significant for research on gas hydrate storage and exploitation [4]. Many researchers have done much work based on this knowledge [5][6][7][8] and many study results have presented the fundamental pressure/temperature conditions properties of hydrate formation and dissociation in media [9][10][11][12][13][14][15].…”