含天然气水合物储层衰减岩石物理研究进展

Review of rock-physics studies on natural gas hydrate-bearing sediment attenuation

衰减是储层的重要基础物性,正确认识天然气水合物储层的吸收衰减机制对于准确预测水合物饱和度具有重要意义.天然气水合物以不同形态赋存在粗粒砂岩或细粒黏土质沉积物中,而这两种水合物储层对应的衰减特征存在显著差异.为了阐明其内部的衰减机理,近年来学者们提出了多种岩石物理模型.其中,砂岩水合物层的衰减模型大致有两类,其中一类以三相Biot理论为基础,模型中包含多种描述水合物颗粒与砂岩颗粒间接触作用的衰减机制,如颗粒胶结、摩擦、微裂隙导致的喷射流等.这类模型可以较合理地再现实际声波测井资料中衰减随水合物饱和度增加而增加的情况,但在地震频段无法产生有效的衰减强度.另一类模型的衰减主要以等效颗粒模型所描述的水合物内部孔隙水与自由孔隙水之间的交换作用为主导,其能够解释砂岩水合物在地震频段的衰减现象.黏土质沉积物中的水合物衰减研究则是首先基于等效颗粒模型模拟黏土矿物附着水的衰减机制建立背景沉积物的衰减模型;接着在其基础上通过量化纯水合物的物性和水合物赋存对背景沉积物性质的改变来完成建模.该模型也已成功应用于地震频段的反射地震资料中.以上衰减模型的提出使水合物的衰减研究取得了实质性进展,使研究人员能够定量化分析观测资料中的衰减现象,并通过衰减约束水合物饱和度.然而这些模型的应用仍存在不同程度的限制:研究发现砂岩水合物储层的衰减模型无法匹配实际资料中观测到的横波衰减强度;以及黏土质水合物储层衰减模型缺少对裂隙参数的表征,在声波频段的模型结果有待验证.为了提升模型的预测精度和应用范围,未来需对水合物赋存形态与衰减的关系加深理解,结合更多实际资料进一步深入细致地开展岩石物理研究.

Attenuation is an essential reservoir property,and understanding its mechanism in gas hydrate-bearing sediments is important for predicting gas hydrate saturation.Natural gas hydrates mainly accumulate in coarsegrained sands or fine-grained clays with different morphologies,and the attenuation characteristics of these gas hydrate-bearing sediments are inconsistent.Many rock-physics models have been proposed in recent years to elucidate the attenuation mechanisms of gas hydrate occurrence.There are two types of attenuation models for gas hydrates in sands.One of these models is based on the three-phase Biot theory and contains multiple mechanisms that describe the contact effects between hydrate and sediment grains,such as grain cementation and squirt flows caused by microcracks.This model can reasonably reproduce the enhanced attenuation observed with increasing hydrate saturation in sonic-logging data.Attenuation described in the effective grain model is dominated by the viscous flow between the water in hydrate pores and free water.These types of models agree with the seismic attenuation observed in gas hydrates in sands.Moreover,the attenuation model of gas hydrates in clay is formulated by first establishing an attenuation model for background sediments,which employs the effective grain model to characterize attenuation in clay minerals;then,the properties of pure gas hydrate and the effects of hydrate occurrence on sediments are quantified.This model has also been successfully applied to seismic reflection data in the seismic frequency band.The above models have achieved significant progress in attenuation studies on gas hydrate-bearing sediments,and they have aided the quantitative interpretation of observed attenuation in field data and attenuationrelated constraints for gas hydrate saturation.However,the application of these models is limited to certain extents:the attenuation predicted by the gas hydrate-bearing sands model failed to match that observed in the field data.Additionally,the gas hydrate-bearing clays model does not consider fracture shapes,and its performance has not been verified at the ultrasonic frequency band.To improve the accuracy and applicability of these models,the relationship between hydrate morphology and attenuation must be elucidated,and further rock-physics studies based on field data must be conducted.