Experimental Research On The Effects Of Particle-size And PoreSize Distribution On Hydrate Formation And Its Acoustic Responses

Natural gas hydrateare widely distributed in continental slopes of natural oceans and permafrost areas on land,with great resource value.Geophysical prospecting is an important method for the prospecting of natural gas hydrate and its resource evaluation.The characteristics of sediment particles have a great influence on the formation and occurrence of hydrate,thus affectingacoustic responses of reservoirs.Under the same experimental systemthe relationship between hydrate saturation and sound velocityin sediment with different partcle sizes was established by simulation experiments in this study.On this basis,the effects and mechanisms of particle size and specific surface area on the acoustic characteristics of saturation were revealed by combining with particle size and specific surface area analysis of sediment.The particle size and specific surface area of arenaceous sediment,arenaceous and silty sediment,sediment from South China Sea samples were measured using a laser particle size analyzer and a specific surface area and pore size analyzer.Simulation experiments of hydrate formation were carried out in a multi-particle size experimental apparatus under the same conditions.It's show that the acoustic parameters and saturation of hydrate were real-time detected using bending elements and time domain reflectometry.The results demonstrated that the shape of particles in porous media was irregular,and no strict mathematical quantitative relationship was found between particle size and specific surface area,but the particle size of sediment was negatively correlated with their specific surface area.When the gas hydrate has not started to formin the saturated sediments,thecompressional-wave and shear-wave velocitiesin sediment with different partcle sizes are basically the same.As the hydrate saturation increases,thecompressional-wave and shear-wave velocities increase continuously in the sediment.The trend is basically the same,indicating that the hydratesaturation is the main factor affecting the compressional-wave and shear-wave velocity.The particle size and specific surface area of sediment particles was closely related to the saturation of natural gas hydrate.With the particle size decreasing and the specific surface area increasing,and the maximum saturation of hydrate decreased gradually,which is resulted from that the large pore space provided by sediment with large particle size facilitates the formation of more hydrate,thus affecting hydrate saturation.The larger specific surface area,the more preferential formation of hydrate,but the larger particle size of the sediment,the greater maximum hydrate saturation.The formation of hydrate in sediment with small particle size was suppressed by capillary pressure from pore radius,which reduced the saturation of methane hydrate.The pure silty sediment has a larger grain size than the sediment from South China Sea,and the specific surface area is smaller than that sediment from South China Sea.However,the hydrate formed in the sediment of the South China Sea presented high saturation.It may be related to the rich foraminifera shells in the sediment of South China Sea.It is found that the paleobiology shells not only serve as a coarse gift part in sediment but also increases the pore space of the sediment by the porous structure,thus providing a favorable growth environment and convenient occurrence space for the enrichment of natural gas hydrate.By comparing the relationship between sound velocity and hydrate saturation,With the decrease in sediment particle size and the increase in specific surface area,the increase of compressional-wave and shear-wave velocities was gradually not obvious from the beginning of hydrate formation to complete formation,which is related to that hydrate formation can increase bulk modulus and shear modulus,decreases density and affect acoustic velocity of sediment.Small particle size and large specific surface area led to low saturation of formed hydrate and not obvious increase in compressional-wave and shear-wave velocities.