天然气水合物储层吸收衰减机制及岩石物理理论研究进展

似海底反射(BSR)特征作为天然气水合物存在的重要标志,虽能指示水合物的底部位置,却难以用于水合物含量的定量解释。近年来天然气水合物勘探技术的迅速发展,使人们认识到BSR上方存在的地震振幅“空白”带与地震波吸收衰减直接相关,故可将其作为天然气水合物分布及量化的一项指标。本文首先回顾了世界不同水合物探区(加拿大Mallik冻土带、日本南海海槽、阿拉伯海Makran增生楔、墨西哥湾、中国南海神狐海域)以及人工含水合物岩样的地震波吸收衰减特征,发现对于不同的水合物样品、使用不同的资料,地震波表现出不同的衰减特征。进而总结了水合物储层可能存在的衰减机制及相关岩石物理理论,主要包括：全局流动衰减(Leclaire模型)、喷射流(改进的Leclaire模型、亚微米水合物颗粒喷射的HEG模型或微米流体喷射的HBES模型)、骨架摩擦衰减(改进的Leclaire模型)等。目前存在的主要问题在于,虽然多个地区的水合物地层呈现较明显的吸收衰减特征,但由于不同地区水合物生成条件和地质环境存在差异,水合物在沉积物中的赋存状态不同,导致吸收衰减随水合物饱和度的变化关系尚不明确。另外,目前的实际观测资料和岩石物理实验测试的频带范围有限,难以全面地体现衰减随频率变化的特征。因此,需要进一步地开展岩石物理实验研究,并充分结合实际探区资料和人工岩心的制作及实验测量结果,深入研究水合物储层微观结构对衰减机制的附加作用,明确水合物储层地震波衰减的原因,以此为基础研发水合物饱和度的定量地震解释方法。

Research progress of absorption and attenuation mechanism and petrophysical theory for gas hydrate reservoir

The bottom simulating reflection (BSR) characteristics of reflected seismic waves are an important sign of gas hydrate. Although they can indicate the bottom of hydrate,they can hardly be used for quantitative interpretation of hydrate content. The rapid development of the gas hydrate exploration technology in recent years results in an understanding that the "blank" zone of seismic amplitude above BSR,directly related to the absorption and attenuation of seismic waves,can be used as an indicator of gas hydrate distribution and quantification. This paper reviews the seismic wave absorption and attenuation characteristics of various hydrate exploration areas around the world (the Mallik permafrost area in Canada,the Nankai Trough in Japan,the Makran accretionary wedge in the Arabian Sea,the Gulf of Mexico,and the Shenhu area in the South China Sea) and artificial hydrate-bearing rock samples. The results show that for different hydrate exploration areas,hydrate-bearing samples,and data used,seismic waves show diffe-rent attenuation characteristics. Then,the possible attenuation mechanisms and related petrophysical theories are summarized for hydrate reservoirs,mainly including global flow attenuation (the Leclaire model),squirt flow (the improved Leclaire model,the hydrate effective grain (HEG) model for submicron hydrate particle squirt,or the hydrate-bearing effective sediment (HBES) model for micron flow squirt),skeleton friction attenuation (the improved Leclaire model). At present,the main problem is that although the hydrate-bearing strata in many areas demonstrate obvious absorption and attenuation characteristics,the relationships of absorption and attenuation variation with hydrate saturation remain unknown due to the varied hydrate formation conditions and geological environments and different occurrence states of hydrate in sediments of different areas. In addition,the frequency band ranges of the current measured observation data and those petrophysical experiments test are limited,so the characteristics of attenuation variation with frequency are not fully reflected. Therefore,petrophysical experimental studies need to be further conducted,and available data from actual exploration areas and the making and experimental measurement results of artificial cores shall be well utilized,thereby studying the additional effect of the microstructure of the hydrate reservoir on the attenuation mechanism in depth. After the reasons of seismic wave attenuation in hydrate reservoirs are clarified,a quantitative seismic interpretation method for hydrate saturation can be developed.