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

Abstract

A thermodynamicm odel for hydratef ormationi s usedt o computet he solubility of methanei n pore wateri n equilibriumw ith gaseousm ethaneo r methaneh ydrateo r both.F ree energyo f water in the hydratep hasea nd of methanei n gasb ubblesa re correctedto accountf or salt effects and capillary effects. Capillary effects increase the solubility of methane in fluid in equilibriumw ith eitherh ydrateo r gas.N aturals edimentsh avea broadd istributiono f pore sizes, and the effective pore size for capillary effects is a function of the fraction of the pore space filled by hydrateo r gas( phasef raction).T he equilibriumc onditionsfo r hydrate+water+gaesq uilibrium thusd ependo n hydratea ndg asp hasef raction.D ata acquiredo n Blake Ridged uringO ceanD rilling Program Leg 164 show that the base of the hydrate stability there is shifted by -2øC or more with respectt o the expectedt emperaturea nd this shift hasb eena ttributedt o capillarye ffects.W e show thatt hise xplanationw ouldr equirea very smalle ffectivep orer adius( 20 nm at 30 MPa). Mercury porosimetryin dicatest hat the percolationth resholdf or Blake Ridge silty claystoneis reacheda t 20-25% phasef ractiona ndc orrespondtso a 100 nm porer adius.H ydratea nd gasp hasef raction determinedw ith severali ndependenmt ethodsa re all lower thant hisp ercolationth reshold, 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) shifta s an equilibriump henomenonC. apillarye ffectsm ay, however,e xplaino ther observationssu cha sl argev ariationso f the gash ydratec ontentin the sedimenwt ith lithologya nd porositya nd the distributiono f hydrateb etweeni nterstitialh ydratea nd segregatemd asses. Capillary effects should also oppose the migration of gas bubbles when gas phase fraction is less thant he percolationth resholda ndm akeu nnecessartyh e assumptioonf a hydrates eali mpermeable to fluids. Alternatively, we can go some way to explaining the offset position of the BSR by relaxingt he assumptionth at the systemi s in thermodynamice quilibrium.N ucleationk ineticso f hydratea nd/orf ree gasb ubblesm ay be inhibitedb y confinemenot f them ethane-bearinfglu id in smallp ores.E quilibrationm ay alsob e limitedb y possibler ateso f diffusionalt ransporot f gas, water,a nd salt componentos r be perturbedb y significantf lows of fluid or heatt hrought he sediments.