Research On Seismic Rock Physics Model And Response Characteristics Of Gas Hydrate-bearing Sandstone Sediments

A rock physical model is established for gas hydrate-bearing sandstone,and the critical hydrate saturation parameter is introduced to describe different occurrence modes of gas hydrate,and velocity dispersion and attenuation laws of hydrate-bearing formation related to jet flow mechanism are described.The results show that when the hydrate saturation is lower than the critical saturation,the hydrate only exists in the pore fluid mixture,and the velocity of hydrate-bearing sandstone is not sensitive to the change of hydrate saturation.When the hydrate saturation is higher than the critical saturation,the hydrate exists as pore filler and solid matrix at the same time,and the velocity of hydrate-bearing sandstone increases significantly with the increase of hydrate saturation.Meanwhile,the band range of dispersion and attenuation is controlled by the viscosity coefficient of the mixture of hydrate and water.Based on the rock physics model and propagation matrix theory,the seismic response characteristics of the heterogeneous hydrate-bearing formation model are calculated and analyzed.It is found that the thickness of hydrate-bearing formation is positively correlated with the reflectance amplitude of a bottom simulating reflector(BSR)and its corresponding time-spectrum amplitude energy.An inversion method based on the rock physics model of hydrate critical saturation is established to predict the variation of hydrate saturation with formation depth in Wells SH2 and SH17 in Shenhu area,South China Sea.The calculation results show that the inversion method based on the critical hydrate saturation model in this paper has higher practicability and accuracy compared with the conventional method,and can accurately predict the vertical distribution of hydrate saturation in the well while reflecting the occurrence state of hydrate.Based on the actual seismic interpretation results,a hydrate formation model is established to calculate and analyze its seismic response characteristics and corresponding time-frequency attributes,instantaneous attributes and seismic AVO(Amplitude Versus Offset)attributes.The analysis shows that different time frequency attributes have different sensitivities in identifying the boundary between top and bottom and the variation of internal saturation of hydrate-bearing formation.In addition,the AVO attributes related to S-wave can accurately identify the top and bottom interfaces of hydrate-bearing formation,indicating that S-wave attributes can effectively identify hydrate-bearing formation with solid skeleton occurrence state.The research results can provide a method and technology for seismic description of hydrate-bearing formation.