【作者】 胡高伟；

【导师】 王家生； 业渝光；

【作者基本信息】 中国地质大学 ， 海洋地质， 2010， 博士

【摘要】 天然气水合物是一种极具潜力的能量资源,在世界各地海洋和永冻土中都有广泛分布,我国也在南海海底和祁连山冻土带中发现了水合物。目前,地球物理勘探仍是水合物勘探和资源评价的重要手段,各种高分辨率地震调查技术被应用于获取储层的纵横波速度等参数,同时,学者们建立了多种水合物饱和度与弹性波速度之间的关系模型,以期根据获取的地震波速度能准确地预测沉积层中是否含有水合物,或估算沉积物中水合物的饱和度,从而对储层的资源量进行评估。然而,在应用过程中发现不同的理论模型在同一地区得出的结果具有很大的差别,由于缺乏实测的水合物饱和度与声波速度之间的关系数据,难以检验这些理论模型的适用性。利用模拟实验技术研究水合物饱和度与声波速度(Vp和Vs)的关系,在实验的基础上检验前人模型、或提出新的更为合适的模型,是一种经济而又实用的办法。因此,本文在模拟海底真实的温度、压力条件下,模拟了固结沉积物和松散沉积物中水合物的生成和分解过程,并在此过程中同时采用超声探测技术和时域反射技术(TDR)实时探测了沉积物的纵横波速度和水合物饱和度的变化情况,建立了固结沉积物和松散沉积物中声波速度和水合物饱和度(Sh)之间的关系,检验了BGTL理论(Biot-Gassmann Theory by Lee)等七种理论模型在固结沉积物和松散沉积物中的适用情况。随后,在掌握实验技术和经验的基础上,对南海沉积物的水合物声学特性进行了研究,获取了南海沉积物中水合物饱和度与声速之间的关系,并检验了BGTL理论等七种模型在南海沉积物中的适用性。通过上述实验研究,不仅在技术方面有所创新,而且在水合物对沉积物声学特性影响、模型及其参数的选取方面取得了一些新的认识。在技术方面,将弯曲元技术引进到测量含甲烷水合物松散沉积物的声学特性研究中,不仅制造了新型结构的弯曲元换能器,而且采用小波分析和频谱分析的方法提高了获取声波速度的能力,使弯曲元技术在松散沉积物实验和南海沉积物实验中得到了有效的应用。实验发现了已有的TDR技术不能用于盐度高于0.5%wt沉积物的含水量和水合物饱和度测量,通过对TDR探针进行改进,并对改进后的探针进行适当的标定,使TDR技术具有了测量(高盐分)海洋沉积物中含水量和水合物饱和度的能力。建立了固结沉积物中实测的纵横波速度与水合物饱和度之间的关系。对固结沉积物的水合物声学特性研究表明,相同饱和度的水合物在水合物生成过程和分解过程中对沉积物声速的影响具有一定差别。在同一饱和度下,在水合物分解过程中测量的纵波速度(或横波速度)明显高于水合物生成过程中测量的纵波速度(或横波速度)。在同一饱和度下,将生成过程中测量的纵横波速度和分解过程中测量的纵横波速度进行了平均,用平均值作为实测的纵横波速度值与水合物饱和度建立了关系。结果表明,当Sh<10%时,纵横波速度变化不明显；当Sh>10%后,纵横波速度随Sh增加而快速增大,且在10%<Sh<30%间纵横波速度增长最快。这一结果表明,在固结沉积物中水合物先在孔隙流体中生成,随后逐渐向沉积物骨架靠拢,当饱和度大于30%后,水合物开始依附于骨架生成,而且,水合物可能堵塞了孔隙间的通道,使孔隙内圈闭的一部分流体因得不到气源的补充而无法进一步生成水合物,因此固结沉积物中水合物饱和度最终仅达到65.5%。在引进弯曲元技术后,获取了松散沉积物中实测的纵横波速度值与水合物饱和度之间的关系。对粒径为0.09～0.125mm天然砂的水合物声学特性研究表明,在同一饱和度下,在水合物生成过程中测得的纵横波速度高于水合物分解过程中所测得的纵横波速度。用两过程中获取的纵横波速度平均值作为实测值与水合物饱和度建立了关系。结果表明,当Sh<25%时,纵横波速度增长较快,25%-60%期间增长较为缓慢,随后声速随着水合物饱和度增加又快速增长。水合物在0.09～0.125mm天然砂中生成时,可能先胶结沉积物颗粒,随后可能与沉积物颗粒呈接触关系或继续胶结沉积物颗粒生成。在掌握弯曲元技术和改进的TDR技术基础上,研究了南海沉积物中实测的纵横波速度与水合物饱和度之间的关系。利用弯曲元技术和改进的TDR技术,测量了水合物生成过程中南海沉积物的纵横波速度等参数随水合物饱和度的变化情况。结果表明,水合物的生成对超声信号造成了一定的影响。当Sh<14%时,超声信号随着Sh的增加而逐渐减弱；Sh>14%后,信号随Sh增加而逐渐增强。纵横波速度随Sh的增加而增大,且横波速度在Sh>14%后增长速度加快。这些现象表明,在南海沉积物中水合物可能先在孔隙流体中以微粒子的状态生成,这些微粒子对超声造成了较大的散射衰减,导致超声信号变弱；当Sh高于14%后,微粒子数量的增多使水合物聚集在一块,并与沉积物颗粒接触,从而增快了横波速度的增长。同时,微粒子的减少对超声的散射衰减变小,且水合物接触沉积物颗粒加强了沉积物骨架,使超声信号逐渐增强。总结上述实验,主要得到了如下认识：(1)水合物的微观分布模式是水合物对沉积物声学特性影响最为重要的因素之一,水合物胶结沉积物颗粒的分布方式对沉积物声速影响最为明显,而水合物在孔隙流体中存在的方式则对声速影响较小；(2)水合物对不同类型沉积物的声学特性具有不同影响；固结沉积物和松散沉积物的骨架受水合物的影响不同,导致水合物对沉积物声速的影响具有差异：当水合物在孔隙流体中生成时,水合物对固结沉积物中超声波传播的影响不大,但对松散沉积物中超声波的传播却造成较大的衰减；(3)沉积物粒度对含水合物松散沉积物声学特性的影响主要表现在粒度制约水合物的生成方面,在细粒沉积物中,甲烷的渗透力很弱,水合物偏向于在孔隙流体中生成,虽然对声速的影响较小,却对超声信号的影响较大；(4)南海沉积物因具有较细的粒度,增加了甲烷在孔隙流体中渗透的难度,当水合物饱和度低于一定值时,水合物偏向于在孔隙流体中生成,不会对沉积物孔隙通道造成堵塞,有利于流体的运移,因而使水合物具有进一步生成的条件,当气源供给充足时,在地质时间尺度容易形成高饱和度的水合物,这可能是南海沉积物中水合物饱和度较高的原因之一此外,本文在实验的基础上对BGTL理论等七种曾用于水合物饱和度预测的理论模型进行了验证,结果表明：(1)在固结沉积物中,当水合物饱和度低于40%时,利用权重方程预测的纵波速度与实测纵波速度一致,结合权重方程与BGTL中的Vp/Vs公式,预测的横波速度与实测值也较一致；当水合物饱和度大于30%时,利用BGTL理论预测的纵波速度和横波速度均与实测值一致。(2)在粒径为0.09～0.125mm的松散沉积物中,当饱和度小于90%时,权重方程预测的纵、横波速度与实测值接近；在饱和度大于20%时,BGTL理论预测的纵横波速度与实测值较为一致；等效介质理论模式B预测的纵横波速度在20%-70%间与实测值接近；伍德方程预测的纵波速度接近于实测值；K-T方程预测的速度值在饱和度～40%-～90%间与实测值一致。(3)在南海沉积物中,权重方程预测的纵波速度值与实测值结果吻合很好；伍德方程预测的纵波速度值与实测值较为接近；BGTL理论预测的纵、横波速度值与实测值比较接近。综上可得,权重方程和BGTL理论在多种沉积物中均具有较好的适用性,而且,在不同沉积物中,两模型各有其适用范围。因此,建议将权重方程与BGTL模型结合起来应用于各种类型沉积物的声波速度预测。对于权重方程中参数W和n的选择,建议通过调节W来定位无水合物时的沉积物速度,然后通过调节n值来适应实际情况；对于BGTL理论中参数G和n,建议以自由取值的方式使预测的结果适应实测值。由于仅对南海沉积物的水合物声学特性进行了单点研究,对两模型中的参数选择尚不能给出更为具体的建议。更多还原

【Abstract】 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.更多还原