Online Detection And Basic Physical Property Analysis Of Gas Hydrate-bearing Sediment Cores

As natural gas hydrate drilling sampling has made significant progress in recent years,detection and analysis of gas hydrate pressure cores become the bridge which connects hydrate exploration and exploitation.Online detection of natural gas hydrate pressure cores provides the data for the transfer of natural gas hydrate pressure cores to conduct further analysis,which will greatly promote the gas hydrate in-depth study.In this work,a system for natural gas hydrate-bearing sediment cores online detection was improved.A series of experiments for acoustic properties of hydrate-bearing sediment cores were performed,and the field sampling test was carried out.The influence factors,including hydrate saturation,vertical stress,porosity of porous sediments to P-wave velocity were investigated.The experimental results show that the P-wave velocity of the hydrate core will increase gradually with the increase of the hydrate saturation.The effect of vertical on the P-wave velocity is more pronounced when the hydrate saturation increases from 50% to 100%,but this effect becomes smaller as the saturation increases.The influence of the P-wave response signal of the hydrate core was studied taking into account the different sizes of vertical stress.The experimental results show that the P-wave velocity is increasing as the vertical stress increases.When the hydrate saturation is 0,the P-wave velocity increases with the increase of the vertical stress,and when the hydrate saturation is 100%,the P wave velocity increases with the increase of the vertical stress.Therefore,under the condition of high hydrate saturation,the effect of the vertical stress is reduced,and the presence of hydrate can reduce the effect of loss of the vertical stress.The natural gas hydrate fidelity core acoustic wave detection system and the shipborne sampling and transfer system are carried out,taking into account the sea conditions.The field experiment was carried out by using sediment,which could accurately judge the core Water,ice and simulated hydrate.The velocity of the acoustic wave within the core increases with increasing hydrate saturation.Therefore,the P-wave velocity can be used as a criterion for the presence of hydrates.For the natural gas hydrate core samples obtained in the South China Sea drilling,the basic physical properties of the samples were tested.Measurement of acoustic characteristics,CT,particle size distribution,liquid plastic limit and mechanical properties analysis.The experimental results show that the water saturation of the sample is high,and the CT image shows a homogeneous whole,and there is no cracks.It can be predicted that there is no block or flake gas hydrate in the superficial surface of the seabed.As the resolution used in the shooting reached 80?m,still can not see the distribution of the pores,so the sand particle size should be in the nanometer level.The size distribution of sediments in the South China Sea is 0.2?m to 63?m,and the sediments in this area are very fine,mainly including silt and clay.However,previous studies have shown that the smaller particle size of the sediments is not conducive to the formation of hydrates,and the coarse particles are more favorable for hydrate formation and aggregation.High proportion of fine particles will hinder the flow of fluid and hydrate aggregation process.Smaller sediment particle size during the mining process is easy to plug the wellbore,should be based on particle size distribution data layout multiple filters,to prevent the process of mining out of the mud caused by well clogging.