Influence Of Sediment Physical Property Differences On The Heterogeneous Distribution Of Natural Gas Hydrates

Natural gas hydrate is a solid crystalline substance formed under low temperature and high pressure conditions.It is mainly distributed in shallow sea shelves deeper than300 m and polar permafrost regions.With its wide distribution,large reserves and high energy density,it is considered as a clean energy source that can replace fossil fuels in the 21 st century.The distribution of hydrates in both the reservoir and pore scales exhibits non-uniform characteristics.The physical properties of seabed sediments affect the thermodynamic conditions and methane transport of natural gas hydrates,which is an important factor leading to the non-uniform distribution of hydrates at different scales.The capillary effect can increase the methane solubility in the water-hydrate phase and inhibit the formation of hydrates.The pore size of sedimentary deposits is the main controlling factor for capillary action,with smaller pore sizes experiencing stronger capillary action.This is an important reason why hydrates preferentially form in the coarse particle layer of sedimentary deposits.In addition,physical properties such as porosity,thermal conductivity,and pore permeability are all influenced by the nonuniform properties of the seabed,and therefore have a potential impact on the distribution of hydrates.However,research on the effects of thermal conductivity and pore characteristics on hydrate distribution in natural gas hydrate reservoirs is currently lacking.To investigate the influence of sediment properties on hydrate distribution,this study establishes a comprehensive model for hydrate formation that considers pore characteristics,capillary action,and thermal conductivity,and applies it to the U1325 and U1327 sites of the 311 expedition.Numerical simulations are used to study the impact of differences in reservoir properties on hydrate distribution,as well as the effect of methane supply on hydrate formation and distribution.The U1325 site exhibits a high sedimentation rate and a very fast methane production rate,representing a system of in-situ methane generation leading to the formation of gas hydrates.Continuous porosity was inferred from resistivity logging and used to calculate pore size distribution and thermal conductivity,which were applied to simulate the effects of capillary action,porosity and tortuosity,and thermal conductivity differences on gas hydrate formation.Simulations considering each of these factors separately showed that gas hydrate distribution can be influenced by these factors,resulting in a discontinuous thin layer distribution.Considering the combined effects of capillary action,porosity and tortuosity,and thermal conductivity,the simulation results were similar to those considering capillary action alone,but showed a lower gas hydrate saturation of about 10%.Although the influence of pore characteristics and thermal conductivity differences on gas hydrate distribution is much smaller than that of capillary action,they can still reduce gas hydrate saturation by about 10%,indicating that both factors contribute to the formation of gas hydrates at the U1325 site.In addition,gas hydrate distribution under different physical properties,including pore characteristics,thermal conductivity,capillary action,and their combination,is highly sensitive to organic matter degradation rate,indicating that in-situ methane generation is an important way of methane supply in heterogeneous sedimentary environments.In the U1327 hydrate reservoir system,methane is mainly supplied by deep methane sources.By simulating physical properties such as capillary action,porosity,and thermal conductivity,it is shown that various physical property differences can also cause natural gas hydrates to exhibit discontinuous layered distribution.Capillary action can cause the maximum thickness of hydrate layers to be 0.6 m,and hydrate layers formed under the influence of pore characteristics do not exceed 0.5 m in thickness.Comprehensive simulation of multiple factors shows that capillary action plays a dominant role in the distribution of hydrates in the U1327 site.In addition,multiple simulations of pore water flux show that fluid transport affects methane concentration distribution and thus hydrate distribution.Therefore,fluid transport and sediment physical property differences jointly control hydrate distribution in the U1327 site.This article presents a model for the formation of gas hydrates,which takes into account the physical properties of the reservoir such as capillary action,thermal conductivity,and pore characteristics.The model investigates the impact of physical property variations on the non-uniform distribution of gas hydrates and characterizes the evolution of gas hydrates under such conditions.This provides a basis for studying the distribution of gas hydrates and the controlling factors in small-scale reservoirs.Furthermore,the model is combined with an analysis of methane supply from both production and deep sources to examine the influence of methane supply on gas hydrate saturation and distribution.This study provides a reference for further simulation research on gas hydrate accumulation.