The Influence Of Natural Organic Matter On Hydrate Formation Characteristics In The Hydrate Enrichment Area Of The South China Sea

Climate change is a global problem facing mankind.The strategic goals of emission peak and carbon neutrality have been put forward by our country.Among that,CO₂ geological storage is an important method for reducing carbon emission.On account of the unique cagelike structure and huge gas compression ability,gas hydrate is considered as a potential storage media of CO₂,therefore,marine gas hydrate reservoirs are expected to become carbon dioxide storage sites.The marine sedimentary environment is extremely complex and riched in organics and nanoclay,thus the phase equilibrium condition and formation kinetics of hydrates in marine sediments environments are the key issues for the safe and efficient storage of carbon dioxide hydrates.In this work,the fidelity samples from the China South Sea Shenhu hydrate occurrence area were investigated to clarified the law of phase equilibrium changes of marine multi-component hydrates,and further illuminated the mechanism of the influence of pore organics and nanoclay particles on hydrate formation.The specific results are as follows:（1）There are abundant pore organic carbon sources in the occurrence environment of hydrate in the South China Sea,and the main components are lignin,protein and lipid molecules.Organic carbon source could be deprotonated to form organic anions by hydrogen-rich functional groups in weakly alkaline marine environment.Organic anions tend to form organic colloid by complexing with cations in solution in order to achieve electric neutrality.A large number of Ca²⁺and Na⁺ions are distributed inside the colloid,while Mg²⁺and Cl^-ions are distributed around the colloid.The salt ions coordination sites were occupied by organics,and the electrostatic effect of salt ions on water molecules decreases accordingly,thus reducing the energy barrier of hydrate formation.（2）A total of 2,058 kinds of metabolic molecules and 10 types of proteins were identified in the fidelity samples from the South Sea hydrate occurrence area.50%of the metabolic molecules are lipid molecules,possessing properties similar to surfactants,and thus could reduce the the surface tension of pore water and significantly promote the formation of hydrates.The protein mainly comes from Methanococcus jannaschii and thermophilic bacteria,with molecular weight and isoelectric point varied from 21.3-48.3 k Da and 4.92-9.25,respectively.The protein molecules could form hydrogen bonds with pore waters,to further reduce pore water activity to inhibit hydrate nucleation.（3）The abundant polar functional group in organics on nanoclay surface provide wider isoelectric point,and further protect particle surface charge density from acidification due to carbon dioxide injection.Meanwhile the charge carried by the organic matter on the surface of the clay could contribute to maintain its stability in the solution to prevent the particles from adsorbing and agglomerating.The water molecules on clay surface could be dipolarized under the action of static electricity of the clay,and the weakly depolarized outlayed water molecules could be arranged regularly to promote nucleation of hydrate.（4）The ocean storage process of carbon dioxide will be accompanied by potential local acidification,which will lead to the dissolution of acidic soluble organic matter on the clay particles surface.By comparing the spectra and energy spectra of water-soluble organics and acidic organics in clay,it was found that the type of organic matter in acidic organic matter is more than twice than that of water-soluble organic matter,and the sulfur-containing organic compound in acidic organic matter has increased significantly.Acid dissolving organics,which has the properties of surfactants,not only significantly promote the nucleation of hydrates,but also its organic carbon can disrupt the growth of hydrate perfect crystals in large pores,breaking the barrier function of mass transfer from large pore hydrates to small pore guest molecules,enhancing the contact between air and water in small pores,so that more water molecules could be converted into hydrates.