Formation of natural gas hydrates in marine sediments 1. Conceptual model of gas hydrate growth conditioned by host sediment properties

Abstract

The stabilityo f submarineg ash ydratesis largelyd ictatedb y pressurea ndt emperature, gasc ompositiona,n dp orew aters alinity.H owever,t he physicapl ropertiesa nds urfacec hemistry of deepm arines edimentms ay alsoa ffectt he thermodynamisct ate,g rowthk ineticss, patial distributionsa,n dg rowthf ormso f clathratesO. ur conceptuaml odelp resumesth at gash ydrate behavesin a way analogousto ice in a freezings oil. Hydrateg rowthi s inhibitedw ithin finegraineds edimentbsy a combinatioonf reducedp orew atera ctivityi n the vicinityo f hydrophilic minerals urfacesa, ndt he excessin ternale nergyo f smallc rystalsc onfinedi n pores.T he excess energyc anb e thoughto f asa "capillaryp ressurei"n the hydratec rystalr, elatedt o the pores ize distributiona ndt he stateo f stressin the sedimentf ramework.T he baseo f gash ydrates tabilityi n a sequencoef fine sedimentiss predictedb y our modelt o occura t a lowert emperatur(en earert o the seabed)t han would be calculatedf rom bulk thermodynamice quilibrium.C apillary effectso r a buildu p of salti n the systemc ane xpandth e phaseb oundaryb etweenh ydratea ndf ree gasi ntoa divariantf ield extendingo ver a finite depthr anged ictatedb y total methanec ontenta ndp ore-size distributionH. ysteresisb etweent het emperatureosf crystallizationan dd issociatioonf the clathrateis alsop redictedG. rowthf ormsc ommonlyo bservedin hydrates amplesre coveredfr om marines ediments(n odules,a nd lensesi n muds;c ementsin sands)c an largelyb e explainedb y capillarye ffectsb, utk ineticso f nucleationa ndg rowtha rea lsoi mportantT. he formationo f concentrategda sh ydratesin a partiallyc loseds ystemw ith respectto materialt ransporto, r where gasc anf lusht hroughth e systemm, ayl eadt o waterd epletionin theh osts edimentT. his" freezedrying" m ayb e detectablteh roughp hysicacl hangetso thes edimen(tl ow waterc ontenat nd overconsolidationa)n d/orc hemicala nomaliesin the porew atersa nd metastablep resenceo f free gasw ithint he normalz oneo f hydrates tability