Experimental Study On Acoustic Responses Of Gas Hydrate In Vertical Gas Migration Systems

Natural gas hydrate is recognized as a new energy resource with huge reserves and is widely distributed around the world.The oceanic and permafrost gas hydrates are found in China.At present,geophysical exploration technology is still the main methods for explorating marine gas hydrate.As it is difficult to obtain accurate hydrate saturation data in field,the relationship between the wave velocity and hydrate saturation was rarely obtained by field data,but was usually established by simulating experiment.In order to estimate and evaluate the amount of reservoir resources by using the seismic wave parameters and logging data,experimental studies have been conducted on the elastic wave characteristics of methane hydrate reservoirs under static conditions mostly.However,there is little understanding of the acoustic response characteristics under vertical gas migration conditions,which is much closer to natural environment.Therefore,it is important to study the acoustic characteristics of the hydrate reservoir under the vertical gas migration system according to natural gas hydrate geological setting.The results will be of great significance to hydrate geophysical exploration and resource evaluation.In this paper,the process of hydrate formation and subsequent dissociation in sediments were simulated in vertical gas migration system.During the process,the ultrasonic velocity and hydrate saturation were measured by ultrasonic detection technique and time domain reflection technique(TDR),respectively.Later,the influence of different methane fluxes on the hydrate saturation and its acoustic response characteristics were studied by a series of experiments.With the measured data,the acoustic characteristics of the hydrate-bearing sediments under dynamic systems were revealed and compared with the properties during hydrate formation in the closed system,and the velocity models in both conditions were verified.Finally,the hydrate formation and its morphology change was discussed according to the theoretical model and the acoustic properties.The above results provide important experimental basis for understanding hydrate formation behavior under the condition that is much close to the nature.The digital oscilloscope is applied in the acquisition system to obtain the ultrasonic data instead of A/D card.The digital oscilloscope has the same functions of data storage and acquisition compared to the A/D card.What’s more,the waveform can be observed directly during the experiment.In this study a novel method was used to simulate the hydrate formation process under vertical gas migration system.Several novel aspects were incorporated into the existing experimental system: the pressure control system,the microporous sintered plate,the waterproof sand and the heating plate are applied in the system to ensure that gas migrates vertically,and that hydrate can form smoothly.The BROOKS mass flow controller is applied in the gas path to achieve gas flow control of the reaction system.Experiments were carried on to simulate hydrate formation under vertical gas migration system.The real-time temperature,pressure,P-and S-wave velocity and hydrate saturation were obtained simultaneously.The results showed that Vp and Vs measured in the hydrate formation process are higher than that in the hydrate dissociation process at the same hydrate saturation.In the vertical gas migration system,P-wave velocities increase faster with hydrate saturation from 0%-20%,they increase more slowly between 20% and 60% saturation,and then increase faster again above hydrate saturations of 60%.P-wave velocities show a fast-slow-fast response to hydrate saturation in the vertical gas migration system.On the basis of the carried experiments,the methane flux in the inlet side was controlled by the mass flow controller,and then the experiments were carried out under different methane flux supply modes.The effect of different methane fluxes on hydrate formation rate and hydrate production was obtained according to the experiments.The results suggested that the smaller the methane flux is,the less time required to generate the maximum hydrate saturation.In a certain period of time,the larger the methane flux is,the larger the hydrate production is.The variation of the velocity in the methane flux supply mode is similar to that in the vertical gas migration system.P-wave velocities increase faster at the beginning of the hydrate formation process,then the velocities increase at a constant rate,and finally the velocities increase faster again when the hydrate saturation is 50%-60%.In order to compare the formation of hydrates in dynamic system and static system,the simulating experiment in the closed system was conducted in the 2-D experimental device,and the spatial distribution of the hydrate formation was inferred.In the early stage of hydrate formation,the acoustic velocity in the coarse sediment is bigger than that in the fine sediment,while in the end of hydrate formation,the acoustic velocities in different layers have little difference,and hydrates are almost evenly distributed in the reservoir.In the longitudinal direction hydrate formed easily when sediment was close to the gas source,and the acoustic velocity was big in this area.In the transverse direction the hydrate preferentially formed around reaction kettle,the acoustic velocities around the sediments are bigger than that in the internal sediments.The empirical formula of Vp-Vs in hydrate-bearing sediments is obtained,which is matched well with the field data.The results may be useful for the geophysical interpretation of marine gas hydrate and hydrate resource estimation.The velocity results obtained in the gas migration system were compared to the velocity data acquired in the closed system.The result shows that P-wave velocities show a fast-slow-fast response to hydrate saturation in the vertical gas migration system,while velocities increase at a relatively constant rate with hydrate saturation in the closed system.Comparing the measured data in the South China Sea with the experimental results,the relationship between wave velocity and hydrate saturation in gas migration system is consistent with that in the South China Sea.With the measured data,the velocity models in different experimental systems were verified.The results show that the Weighted Equation(WE),the BGTL model and the Effective Medium Theory(EMT)match well with the measured data in the vertical gas migration system.The Weighted Equation is consistent with the measured data by adjusting the weighting factor.It is proved that the Weighted Equation is not suitable for the measured shear wave velocity.When the hydrate saturation is between 20% and 60%,the experimental results are similar to those of the Effective Medium Theory model A.In the methane flux supply mode,the suitable velocity model is the BGTL model and the Effective Medium Theory model.The P-wave velocity predicted by the Weighted Equation has the similar trend with the measured data,but there is a certain difference between the predicted data and the measured data.In the Effective Medium Theory,when the hydrate saturation is 25%-55%,the experimental results are close to the model B.When the hydrate saturation is 60%-70%,the results are consistent with the model A.After the hydrate saturation is greater than 80%,the measured data tends to the predicted data of the model C.It can be seen that the velocity model has different adaptability under different systems.Due to the complex influence of hydrate on the sediments,it is difficult to adapt to different conditions by selecting a single parameter.The BGTL theory and the Effective Medium Theory have good adaptability in different systems.In the high methane flux system,the gas factor needs to be taken into account in the theoretical calculations.The Effective Medium Theory considering the influence of gas can not only estimate the hydrate saturation,but also provide a guiding significance for the micro-distribution of hydrate.According to the theoretical model and combined with the CT data,the micro-distribution pattern of hydrate in different systems was discussed.In the gas vertical migration system,hydrates preferentially form at grain contacts and on the surface of gas bubbles at the beginning of hydrate formation,then hydrates form in the pore fluid(floating hydrate),and lastly hydrates grown into contact with the sediment grains again as they fill up the pore spaces.In the high methane flux system,the hydrate is mainly formed in the grain contact.As the hydrate saturation increases,the hydrate is formed in the pore fluid,and then the hydrate in the fluid comes into contact with the sediment particles,and finally the hydrate is formed in a cemented pattern.