Study On P-wave Acoustic Properties Of Methane Hydrate Formation And Depressurization In Sediments

Natural gas hydrates have attracted widespread attention due to their high energy density,non-polluting combustion and huge reserves,but it is impossible to accurately estimate the resource reserves of natural gas hydrates,also lack the effective technical means to monitor the process of natural gas hydrate mining up to now.The major work of this paper is to research the technology for natural gas hydrate exploration and depressurization.The variations of physical parameters such as P-wave velocity of hydrate-bearing sediment were systematically studied.The main contents of this thesis are as follows:(1)A series of methane hydrate were formed in sediment with an experimental apparatus,and P-wave velocity and amplitude were experimentally measured during the methane hydrate formation to investigate the effects of initial pure water saturation,initial brine saturation and the axial pressure on sediment on the acoustic properties of P-wave.The experimental results show that during the formation of hydrate,the hydrate saturation is on the rise with the prolonging of reaction time,and hydrate saturation tends to be stable after reaching the maximum value.The P-wave velocity increased linearly with the increase of hydrate saturation.The saturation of methane hydrate increases as initial water saturation rises,and the salt concentration of water in the sediment pores and the axial pressure of the sediments have little effect on the saturation of methane hydrate.(2)Using the methane hydrate samples prepared in the laboratory,the variation of temperature,pressure and P-wave velocity of hydrate depressurization process was studied in gas-rich and water-rich environment.And the influences of mining pressure,hydrate saturation,formation pressure were also investigated.The experimental results show that during the hydrate depressurization process,the temperature in the reactor was dropped first and then increased,and the temperature drop of the hydrate depressurization process in the gas-rich environment is greater.During the depressurization process,the P-wave velocity decreases stepwise with time,and the P-wave velocity and hydrate saturation have no longer kept a significant linear correspondence.In this study,it is found that the P-wave velocity increases linearly with the increase of hydrate saturation during the formation of methane hydrate,which indicates the P-wave velocity can be used to predict the natural gas hydrate resource reserves.During the depressurization of methane hydrate,the P-wave velocity decreases stepwise with time,and the P-wave velocity decreases sharply after the sediment skeleton strength decreases.This characteristic of P-wave can be used to monitor the change of formation stability during hydrate depressurization,which can ensure the safety operation of hydrate mining.