Study On The Acoustic Properties Of Multitype Natural Gas Hydrate During The Formation And Decomposition

The relationships between the hydrate-bearing sediments with physical properties such as wave velocity are of importance for the exploration and exploitation of natural gas hydrate.The natural gas hydrates(NGHs)extensively exist in permafrost regions and sea-floor and the distribution modes of naturally occurring gas hydrates in sediments are diverse.Meanwhile,it may possibly induce geologic hazard such as subsea landslides or mudslides in the existing hydrate exploitation technology.Therefore,multi-type hydrate-bearing sediments were prepared and the acoustic properties were studied in this paper.Moreover,a novel technology to CH₄hydrate exlploitation and CO₂ sequestration was proposed from the view of exploiting hydrates safely and efficiently.Related basic research works have been conducted and the main contents are as follows:(1)In response to the diverse distribution models of naturally occurring gas hydrates,a novel method to prepare the hydrate-bearing sediments with heterogeneous distribution including layered distribution was proposed by controlling the temperature,water migration,and water saturation distribution in this paper.Velocity and amplitude of P-wave are investigated to evaluate the physical properties of hydrate-bearing sediments with homogeneous or heterogeneous distribution.The results showed that the hydrate formation process apparently consists of unconsolidated and consolidated steps according to the amplitude curve.The P-wave velocities of the hydrate deposits with homogeneous and heterogeneous distribution are different when under a same hydrate saturation.This was also verified from the experiments performed on hydrate-bearing sediments with layered distribution.This difference of the P-wave velocity between heterogeneous and homogeneous hydrate-bearing sediments would provide a guidance in the future development of natural gas hydrate.(2)To simulate the diverse naturally occurring gas hydrates in sediments,a novel method to prepare the hydrate-bearing sediments with dispersed,layered or massive distribution model.V_P was measured to simulate the physical properties of the hydate-bearing sediments with diverse distribution models.The results showed that the final water conversion rate and hydrate saturation are increased with the increasing initial water saturation.The V_P of hydrate-bearing sediments is not only related with the hydrate saturation,but also affected by the distribution models of the gas hydrate and the water and gas concentration.(3)To improve the application scope of the model to simulate the relationships between the hydrate saturation and V_P,the traditional empirical models were modified by weighting the three-phase and four-phase media theory.Deviation factor was proposed to modify the weight equation in order to adopt the hydrate-bearing sediments with diverse distribution mode.A method to predicte the hydate saturation of hydrate-bearing sediments with diverse distribution mode was proposed in this paper.(4)To improve the stratum stiffness and prevent the geological disaster,a new approach by executing CH₄ hydrate exploitation and CO₂ sequestration below the freezing point of water was presented to maintain the stratum stiffness during and after hydrate exploitation in this paper.Series of experiments were conducted to simulate the CH₄ exploitation and CO₂sequestration process below and above the freezing point of water.The stratum stiffness was found to be maintained during the whole hydrate exploitation process when below the freezing point.Moreover,the presence of ice and the CO₂hydrate formation would further enhance the mechanical properties.Compared with hydrate exploitation above the freezing point,the CH₄ recovery efficiency is a little lower when under below the freezing point.(5)A gas swap approach to hydrate exploitation below the freezing point was proposed according to analyzing the reason of the low CH₄ recovery efficiency.The hydrate exploitation technologies of depressurization+CO₂ swap,depressurization+N₂swap or N₂ swap were simulated to improve the CH₄ recovery efficiency below the freezing point.The results showed that gas swap method could improve the recovery efficiency when under below the freezing point and the ecovery efficiency of N₂ swap method is more efficient than the CO₂ swap method.The variations of V_P showed that the stratum stiffness of the hydrate-bearing sediments would further be enhanced during the CO₂ swap process,could be maintained during the lower pressure(1.0MPa)N₂swap,and was reduced during the higher pressure(2.8MPa)N₂swap.