Study On Pore Structure Characteristics And Hydraulic Properties Of Hydrate-bearing Sediments

Natural gas hydrate has been treated as potential unconventional resource with wide distribution,cleanness and huge reserves.Developing and utilizing natural gas hydrate actively is of great significance to adjust energy structure,control climate variation and alleviate energy shortage.However,the exploitation of hydrate is a challenge in worldwide.In particular,the complex engineering geological characteristic leads to low efficiency and high risks,among them,reservoir permeability is a critical parameter that determines the economic feasibility of gas production from therein,and it is also the basic parameter for the optimization of hydrate exploitation.Revealing the hydraulic properties and its mechanism is a key scientific problem to be solved for the efficient exploitation,and it is also one of the difficult topics in hydrate community.There,permeability tests,microstructure observations and theoretical analysis were comprehensively used to study the pore structure of hydrate-bearing sediments and its implications for permeability.The microstructure of hydrate-bearing sediments was comprehensively characterized by X-CT,LF-NMR and SEM.Based on experimental results,the relationships between fractal parameters(pore-size fractal dimension,area maximal diameter and tortuosity fractal dimension)and hydrate saturation were established,and fractal permeability models were proposed to predict the permeability of hydrate-bearing sediments.More,the effects of effective stress and hydrate saturation on the effective permeability and relative permeability were evaluated,and experimental data were used to verify the feasibility of these fractal models.Main research contents and conclusions are drawn:(1)In situ hydrate formation/dissociation experiments were conducted on sandy sediments,and the effects of different formation methods on hydrate growth habits and pore structure parameters were studied by combined X-CT and LF-NMR tests.The results showed that hydrate distribution in sediments was uniform,and the contact relationships between hydrate and particle were the one of most important factors affecting surface area,pore size distributions and interface relaxation characteristics.The pore structure of hydrate-bearing silty clay sediments recovered from the South China Sea was characterized by combined X-CT,LF-NMR and SEM tests,and the relationship between hydrate growth habits and pore size was analyzed.The results showed that hydrate accumulated preferentially in the pores with the diameter larger than 1 ?m.The presence of foraminifera significantly enhanced the effective porosity,promoted the migration of natural gas and the accumulation of hydrates.(2)The fractal parameters were extracted from X-CT images and LF-NMR curves.The effects of hydrate saturation on the fractal parameters were systematically analyzed,and these parameters were further extended to develop fractal theory?based models for predictions of effective and relative permeability.The results showed that the pore space occupied by pore fluids within hydrate?bearing sandy sediments is inherently fractal,and the pore-size fractal dimension and area maximal pore diameter decreased with the increase of hydrate saturation,while the tortuosity fractal dimension changed a little.The proposed fractal models for permeability had a good match with previous experimental data and logging data,which verified the feasibility of fractal models for permeability in hydrate-bearing sandy sediments.(3)The effective permeability of hydrate-bearing silty clay sediments during triaxial loading was measured,and the effects of hydrate saturation and effective stress on effective permeability were systematically investigated.The fractal model for the effective permeability of hydrate-bearing sandy sediments was evaluated and modified with own experimental data.The results showed the effective permeability decreased exponentially with the increase of the effective porosity,and the effective permeabilitystress sensitivity index increased with increasing hydrate saturation.The experimental permeability data were matched well with the fractal model modified by replacing parameters related to maximal pore diameter.(4)The water retention curves of hydrate-bearing silty clay sediments were tested,the effects of hydrate saturation on water retention curves were systematically discussed,and these parameters were further used to estimate the relative permeability to gas and water.The feasibility of the fractal model for the relative permeability of the hydratebearing sandy sediments was verified by own experimental data.The results showed the shape parameters m-values decreased with the increase of hydrate saturation,and the relative permeability calculated by van Genuchten model was in good agreement with the fractal model considering critical capillary radius.