Study On Acoustic Properties Of Hydrate-bearing Unconsolidated Sediments

Methane hydrate is ice-like crystals formed from natural gas and water under high pressure and low temperature conditions.It is like ice and it burns in case of fire,hence it is also called flammable ice.It is mainly distributed on the continental margin and plateau permafrost regions.It has the advantages of clean,high quality,less pollution,and wide distribution of natural gas hydrates.It has been estimated that the organic carbon content of natural gas hydrates is 2 times that of the existing oil,natural gas,and coal.Thereforce gas hydrate exploration and development technology has become a research hotspot.The study of the mechanical and acoustic properties of natural gas hydrates has a guiding role in the exploration and development of natural gas hydrates.It is difficult and costly to take cores in natural gas hydrate formations and to preserve the hydrates in rock samples after core wicking,so this paper simulates the geological conditions of gas hydrate reservoirs in the Shenhu area of the South China Sea.Through the production of artificial natural gas hydrate rock samples,the use of indoor low temperature triaxial and cryogenic transducers to obtain the physical and mechanical parameters of natural gas water strata.On the basis of quantitative analysis,the equation for estimating the wave velocity of natural gas hydrate reservoirs was established.In gas hydrate and reservoir acoustics research:(1)There is the effect of confining pressure on the P-wave velocities of natural gas hydrates under certain saturation,the P-wave and S-wave velocities of natural gas hydrates with different meshes increase with the confining pressure.When the gas hydrates reach a certain confining pressure,the P-wave and S-wave velocities tend to be stable.(2)There is the effect of saturation on the P-wave velocities of natural gas hydrates,at the same pressure,the velocities of P-waves and Swaves of natural gas hydrates with different meshes increase with increasing saturation.However,the increase in P-waves is not the same.(3)Comparing the gas velocity of natural gas hydrates with different mesh numbers.When the natural gas hydrate rock sample saturation is 0%(dry rock sample),with the increase of confining pressure,the wave velocity of the natural gas hydrate rock sample with particle size of 100 is always higher than that of the natural gas hydrate rock sample with particle size of 40 mesh.However,as the confining pressure increases to a certain value,the wave velocity of natural gas hydrates with two different particle sizes tends to be stable.In the gas hydrate reservoir mechanics:(1)Different natural gas hydrate saturations and effective confining pressures,the compressive strength of cemented un-rock gas hydrate reservoirs varies between 4.14 MPa and 12.21 MPa.In general,the stress-strain curve of weakly cemented un-rock-bearing gas hydrate reservoirs changes significantly with changes in natural gas hydrate saturation and effective confining pressure.The type of stress-strain curve for the weakly cemented agglomerate gas hydrate reservoir is obvious elastic-plastic deformation.(2)With the same effective confining pressure(confining pressure-pore pressure),the higher the saturation of natural gas hydrate,the greater the peak strength of the reservoir.The elastic modulus of natural gas hydrate reservoirs is also greater,but confining pressure has no significant related effect on Poisson’s ratio.Through the calculation of various acoustic wave estimation equations,the sound wave estimation equation suitable for the Shenhu area of the South China Sea is optimized,and the influence of the weighting factor W and the parameter n on the weighting equation of Li in the weight equation of Li Weighing is studied.