Experimental Study On Acoustic Responses Of Gas Hydrates To Sediments From South China Sea

Gas hydrates, recognized as a potential of energy resources, are distributed in oceanic seabed or permafrost all over the world. Both oceanic and permafrost gas hydrates are found respectively in South China Sea (SCS) and Qilian Mountain, China. So far, geophysical prospecting method still plays an important role in gas hydrate explorations and quantifications. Various high-resolution seismic techniques are developed to obtain elastic velocities (Vp, Vs) of gas hydrate reservoirs. Meanwhile, many velocity-models are constructed to relate elastic velocities with hydrate saturations of the hydrate-bearing sediments, with which we can predict the presence of gas hydrate in sediments, or even obtain the amount of gas hydrates. Unfortunately, it is found that the results predicted by various models are quite different. Obviously, observations on relationship between gas hydrate saturation and elastic velocities are needed to validate these models.Since there is rare gas hydrate saturation data in field exploration, experimental methods to obtain the relation between hydrate saturation and acoustic properties of hydrate-bearing sediments are thought to be economically and effectively. In this paper, acoustic properties of gas hydrate-bearing sediments are investigated experimentally. Gas hydrate was formed and subsequently dissociated in both consolidated sediments and unconsolidated sediments. In the whole process, ultrasonic methods and Time Domain Reflectometry (TDR) are simultaneously used to measure the acoustic properties and hydrate saturations of the host sediments, respectively. With the measured data, we verified seven velocity models (e.g. BGTL, Biot-Gassmann Theory by Lee) in predicting velocities of both consolidated and unconsolidated hydrate bearing sediments. After that, the similar experimental processes are conducted on sediments from SCS, with the results we may understand the acoustic properties of hydrate-bearing sediments in SCS, or give suggestions on the usage of various velocity-models in field gas hydrate explorations.Some improvements have been achieved in detecting acoustic velocities of hydrate-bearing sediments. The bender elements technique was introduced into measurement of ultrasonic waveforms of the hydrate-bearing sediments. Also, a new method (we called FFT-WT method hereafter), which combined Fast Fourier Transform (FFT) and wavelet transform (WT), is proposed to obtain both Vp and Vs of the hydrate-bearing unconsolidated sediments. Another technique improvement is made on the TDR probe. In our experiments, we found that the conventional TDR probes are not able to measure water contents of a sample when the salinity of pore fluid is higher than about 0.5%wt. A coated TDR probe was then developed to solve water content measurement problem in high salty sediments. With the coated TDR probe, water content and hydrate saturation are successfully measured in both high salty sediments and marine sediments.Some understandings on the acoustic properties of hydrate-bearing sediments are also gotten basing on the experiments.The relationship between hydrate saturations and acoustic velocities of the consolidated sediments was established basing on the experiments. For the consolidated sediments, the compressional (or shear) wave velocity measured in the hydrate-dissociation process is much higher than that measured in the hydrate-formation process at the same saturation degree. Because it's difficult to judge whether in situ gas hydrates are in the process of formation or dissociation during gas hydrate exploration, it uses the average Vp (or Vs) of the compressional (or shear) wave velocities obtained in the two processes as the measured velocity to relate with gas hydrate saturations. The result shows that acoustic velocities are insensitive to low hydrate saturations (Sh,0-～10%). However, the velocities increase rapidly with hydrate saturation when saturation is higher than 10%, especially in the range of 10-30%. This suggests that when Sh is less than 30%, the hydrate locates in the pore fluid, or partly adheres to the sediment frame. However, gas hydrate may be treated as a component within a matrix of consolidated sediments when hydrate saturation exceeds 30%. As a result, the pore throat may be blocked by the cemented hydrates and a part of pore fluid cannot convert to hydrate.In the unconsolidated sediments, the bender elements are successfully used in measuring both Vp and Vs of the hydrate-bearing sediments, and the relationship between gas hydrate saturation and acoustic velocities was gotten subsequently. The result shows that the compressional (or shear) wave velocity measured in the hydrate-dissociation process is much lower than that measured in the hydrate-formation process at the same saturation degree. With the average Vp (or Vs) of the compressional (or shear) wave velocities obtained in the two processes, we obtained the relationship between gas hydrate saturation and acoustic velocities of hydrate-bearing unconsolidated sediments. The result shows that Vp and Vs increase rapidly vs. hydrate saturations although they increase relatively slow in the range of saturation 25%-60%. It indicates that gas hydrate may first cement grain particles of the unconsolidated sediments, when hydrate saturation is higher, gas hydrate may contact with the sediment frame, or continue cementing sediment particles.The bender elements technique and the improved TDR probe were successfully used in measuring acoustic properties of hydrate-bearing sediments from SCS. As gas hydrate forming in sediments from SCS, the acoustic signals decreases at the first stage of hydrate formation (Sh, 0-14%), after that the signals increase slowly with the growth of gas hydrate. Acoustic velocities of hydrate-bearing sediments from SCS increase with hydrate saturations. Observations show that the shear wave velocity increase slowly at the first stage of hydrate formation (Sh,0-14%), after that it increase much fast with the hydrate saturation (Sh>14%). The result may reveal that gas hydrate is firstly located in the pore fluid of the SCS sediments. The small hydrate particles have significant attenuation on acoustic signals. When Sh is higher than 14%, hydrate begins to contact with the sediment frame, the attenuation decreases and the shear wave velocity increase more rapidly.The initial experimental results indicate that:(1) the morphology (or called micro-models) of gas hydrate in the sediments has significance on the acoustic properties of the sediments. Generally, a cement model has largest impact on acoustic properties, while the pore model has less. (2) the acoustic responses of gas hydrate to consolidated sediments and unconsolidated sediments are quite different. (3) the grain size of the unconsolidated sediments appears to influence the hydrate formation mechanism, that is, the small particle may prevent gas dissolving in the pore fluid, as a result gas hydrate may not form, or forms in the pore fluid and has a less impact on acoustic properties of the sediments. (4) for the fine-grained SCS sediments, gas hydrate may prefer to form in the pore fluid when saturation is low. This may not block the pore throat of the sediments. In the geological time scale, an amount of high saturation hydrate may be formed provided there are sufficient gas resources. It may be a possible reason that why hydrate saturation in SCS sediments is very high.With the experimental data, seven velocity models were validated. The results indicate that:(1) in the consolidated sediments, the Weighted Equation (WE) predicts corresponding compressional velocity with the measured data when Sh<40%. A combination of the WE and the Vp/Vs ratio in the BGTL model predicts consistent shear velocity with the measured data (Sh<40%). When Sh>30%, both Vp and Vs predicted by the BGTL model are consistent with the measured data. (2) in the unconsolidated sediments (particle size,0.09～0.125mm), Vp and Vs predicted by the WE model are consistent with the measured data when hydrate saturation is less than 90%, while Vp and Vs predicted by the BGTL model are consistent with the measured data when hydrate saturation is higher than 20%. The Effective Medium Theory (EMT) also shows good agreements with the measured data when hydrate saturation is in the range of 20%-70%. The compressional velocity predicted by Wood's equation is close to the measured data, while Vp and Vs predicted by the K-T equation is corresponding to the measured data (Sh,～40%-～90%). (3) in the SCS sediments, the elastic properties predicted by the WE model, the BGTL model and the Wood's equation are consistent with the measured data. The validation results of the above velocity models indicate that the WE model and the BGTL model are more flexible in velocity predictions in various types of sediments. Moreover, it shows that the results predicted by the two models are respectively consistent with the measure data for a different range of hydrate saturations. A combination of the two models may be more suitable to predict both Vp and Vs for a wide range of sediments. With regard to the parameters W and n in the WE model, it suggests that the parameter W can be obtained with data of the hydrate-free sediments. After W is fixed, the parameter n can be adjusted to qualify the WE model predict consistent velocities with the measured data. For the parameters G and n in the BGTL model, there are no better choose than treat them as free parameters because there are rare data to formulate an empirical equation to correctly get them. Thus, further works are needed on investigating acoustic properties of SCS sediments containing gas hydrate to give rigorous suggestions for gas hydrate exploration in SCS.