Sediment pore-structure and permeability variation induced by hydrate formation: Evidence from low field nuclear magnetic resonance observation
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摘要:
含水合物储层的宏观物性表现是由储层沉积物的微观孔隙特征所控制的。理解沉积物在水合物生成过程中微观孔隙结构特征变化对于其物性特征的预测和分析有重要意义。本文利用低场核磁共振（LFNMR）技术监测了不同砂样中氙气水合物的生成过程，利用横向弛豫时间（T2）谱对生成过程中的微观孔隙结构及水相渗透率演化规律进行了分析。研究表明，水合物优先生成于沉积物较大孔隙中，在半径较小的孔隙中水合物很难生成；生成前期水合物的生长速率较快，后期逐渐减缓；水合物的生成导致沉积物孔隙尺寸和分布的变化，表现为随着水合物的生成，沉积物水相孔隙空间的最大孔隙半径和平均孔隙半径逐渐减小，孔隙空间的分形系数逐渐增大；沉积物水相渗透率随水合物生成过程中水合物饱和度的增加，先迅速减小后缓慢减小；具有不同孔隙结构特征的样品水相渗透率变化规律存在差异；相较于SDR模型和Kozeny-Carman模型，分形方法能够更好地体现孔隙结构变化对渗透率的影响。
Abstract:The macro-scale physical properties of hydrate-bearing sediments is in fact controlled by their micro-scale pore-structures. Understanding the changes in pore-structure characteristics of the sediments during the process of hydrate formation is essential to the analyzing and predicting of the sediment properties. In this paper, the formation processes of Xenon hydrate in different sandy samples are measured with the low-field nuclear magnetic resonance (LFNMR) method. The obtained transverse relaxation time (T2) spectra are interpreted for study of the changes in pore-structure and physical properties of the sediments during the hydrate formation. The results show that Xenon hydrates preferentially form in larger pores and only little amount of hydrates formed in smaller pores; the forming rate of hydrate is higher at the early stage of formation but decrease slowly at the later stage; the hydrate formation process also leads to the changes in pore size and pore-size distribution patterns, for examples, the maximum radius and mean radius of the water-phase pores decrease with increasing hydrate saturation, while the fractal dimension of the effective water-phase pores increases with the increasing hydrate saturation; the water-phase permeability decreases rapidly in the early stage of hydrate formation, but slowly decrease since then; the changes of water-phase permeability during hydrate formation are affected by the pore-structures of the sediment; compared to the SDR model and the Kozeny-Carman model, the fractal model of permeability performs better in showing the influences of pore-structure characteristics on the changes of water-phase permeability during the hydrate formation.
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表 1 测量沉积物样品基础物性参数
Table 1. Physical properties of sedimentary samples for testing
样品编号 样品组成 样品粒径分布/μm 样品尺寸/cm2 样品1 不规则石英砂颗粒 150～250 ϕ 2.5×4.2 样品2 不规则石英砂颗粒 100～150 ϕ 2.5×4.9 样品3 不规则石英砂颗粒 75～250 ϕ 2.5×4.6 
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为横向表面弛豫率，T2LM为T2特征谱的对数平均值。
为样品有效孔隙度
为迂曲度系数，
为平均迂曲度，
为平均孔隙半径，
为样品的长度
图(9)
表(2)
计量
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