页岩储层渗吸过程中水的微观分布及其气测渗透率动态响应特征

在页岩油气储层水力压裂过程中，压裂液的滤失行为会增加储层的含水饱和度、降低储层渗透率。研究压裂液侵入对储层渗透率的影响，对于页岩油气的高效开发具有重要意义。为此，针对渤海湾盆地沾化凹陷沙河街组三段下亚段灰质页岩和混合质页岩岩相样品，通过自发渗吸实验获得了样品不同含水饱和度行为，利用核磁共振表征水在孔隙中的动态运移，结合覆压渗透率定量表征气测渗透率动态变化。实验结果表明：①在含水饱和度为5%~40%的渗吸过程中，以微孔（核磁共振T₂<1 ms）吸水为主，T₂谱累积信号呈上升趋势，占比逐渐增加，而中孔和大孔的T₂累积信号呈缓慢上升和波动状态且占比逐渐降低。当含水饱和度达到40%时，微孔累积信号占比高达87%以上。②水化改善作用与水锁和水敏损害相互制约，影响渗透率的动态变化，主要分为3种制约状态（水化远大于、大于、小于水锁和水敏）。结合矿物组分和孔隙结构分析发现，在黏土矿物含量为25%~43%的页岩中，渗透率损害率与黏土矿物和石英含量呈正相关，与脆性指数和碳酸盐矿物含量呈负相关性，灰质页岩水化改善作用优于混合质页岩，且大于1 μm孔喉的发育既能促进水化改善作用，又能降低水锁和水敏对孔喉的损害。

Microscopic distribution of water in the imbibition process of shale reservoir and dynamic response characteristics of its gas logging permeability

In the process of hydraulic fracturing in shale oil and gas reservoirs, the leakage behavior of fracturing fluid can increase the water saturation of reservoirs, and reduce the permeability of reservoirs. A study on the influence of fracturing fluid invasion on the effective permeability of reservoirs is of great significance for the efficient development of shale oil and gas. Therefore, in view of the calcareous and mixed shale lithofacies samples in the lower sub member of Member 3 of Shahejie Formation, Zhanhua sag, Bohai Bay Basin, different water saturations of samples were obtained through spontaneous imbibition test in the laboratory, the dynamic migration of water in pores was characterized by nuclear magnetic resonance (NMR), and the dynamic changes in gas permeability was characterized quantitatively using overburden permeability. The results shows that:(1) When the water saturation ranges from 5% to 40%, the imbibition process is dominated by small pores (NMR T₂<1 ms), and the cumulative signalof T₂ spectrum shows an upward trend and its proportion is gradually increased; the cumulative signal of T₂ spectrum in the mesopores and macropores slowly rises or fluctuates up and down, respectively, and their proportions are gradually decreased. When the water saturation reaches 40%, the cumulative signal of small pores accounts for as high as 87% and more. (2) The hydration improvement effect and the damage of water blocking and water sensitivity are interacting, and thus affect the dynamic changes in permeability, which can be mainly divided into three states, i.e., hydration improvement is far greater than, greater than, and less than water-locking and water-sensitivity. Further, through the analysis of mineral composition and pore structure, it is found that in the shale with a clay mineral content of 25%-43%, the damaging rate of permeability is positively correlated with clay mineral and quartz contents, while negatively correlated with brittleness index and the carbonate content; calcareous shale has a better hydration improvement than mixed shale, and the development of the pore-throat greater than 1 μm not only can promote hydration improvement, but can reduce the damages of water blocking and water sensitivity to the pore throat.