Microscopic pore and fracture evolution characteristics and influencing factors during imbibition process of shale reservoirs: a case study of the first section of the first member of Longmaxi Formation, western Chongqing area, Sichuan Basin
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摘要: 水力压裂已成为页岩气开采的重要手段，明确渗吸过程页岩储层孔隙、微裂缝的演变特征与影响因素，对指导页岩气井压后增产措施优化具有重要意义。为此，选取四川盆地渝西地区大足区块主力产层龙马溪组龙一1亚段底部黑色页岩为研究对象，开展渗吸水过程的氩离子抛光场发射扫描电镜(FE-SEM)定点观察实验，明确了渗吸水不同时间页岩储层微观孔缝演变规律。研究表明：①页岩储层渗吸水7 d后，有机质边缘有机孔出现不同程度的减小，而内部孔隙形态、大小基本不变；②粒内溶蚀孔和粒间孔会出现明显的扩溶现象，引起矿物颗粒溶蚀、脱落，增大页岩气泄气面积；③页岩储层渗吸水后不会大量萌生新的微裂缝，仅在原有微裂缝的基础上进行扩展，在吸水14 d后缝宽扩展为原来的5~10倍；④页岩储层面孔率在渗吸水后7 d达到最大值，大于7 d后微裂缝缝宽受黏土矿物持续膨胀影响出现不同程度的减小；⑤页岩储层增孔扩缝强度主要受矿物组成与孔渗性质影响，不稳定矿物与脆性矿物含量越高、粒径越大，增孔现象越明显，越有利于压后页岩气的渗流。Abstract: Hydraulic fracturing has become an important means for shale gas exploration. Understanding the evolution characteristics and influencing factors of pores and micro-fractures during the imbibition process in shale reservoirs is crucial for optimizing post-fracturing production enhancement measures. This study focuses on the black shale at the base of the first section of the first member of the Longmaxi Formation (Long 1-1 sub-member), the main production layer in the Dazu area, western Chongqing area of the Sichuan Basin. Argon ion polishing and field-emission scanning electron microscopy (FE-SEM) experiments were conducted at fixed sites to observe the evolution pattern of microscopic pores and fractures in shale reservoirs at various stages of water imbibition process. The findings revealed: (1) After water imbibition for 7 days, organic pores at the edges of organic matter exhibited varying degrees of reduction, while the internal pore shapes and sizes remained largely unchanged. (2) Intragranular dissolution pores and intergranular pores exhibited noticeable dissolution effects, resulting in mineral particle dissolution and detachment, which increased the leakage area for shale gas. (3) The water imbibition did not induce a significant amount of new micro-fractures. Instead, it extended existing micro-fractures, with the fracture width expanding by 5 to 10 times after imbibition for 14 days. (4) The surface porosity of the shale reservoir reached its peak value at day 7 of water imbibition. After 7 days, due to the continuous swelling of clay minerals, micro-fracture widths experienced varying degrees of reduction. (5) The intensity of pore and fracture expansion in shale reservoirs was primarily affected by mineral composition and pore permeability properties. Higher contents of unstable minerals and brittle minerals with larger particle sizes led to more pronounced pore expansion effects, which were conducive to post-fracturing shale gas seepage.
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图 1 四川盆地构造区划分及研究区地理位置(a)与典型井五峰组—龙马溪组龙一1亚段地层柱状图(b)
Figure 1. Tectonic division of Sichuan Basin and geographic location of study area (a) and stratigraphic histogram from Wufeng Formation to Long 1-1 sub-member in a typical well (b)
图 2 四川盆地渝西地区龙一1亚段页岩样品面孔率计算过程
a.Z203H2-1号样品渗吸7 d后电镜下孔隙分布；b.软件识别后红色部分代表孔隙；c.对识别后的色块排序，统计孔径及面积(统计色块近似标准圆直径作为孔径)。
Figure 2. Calculation process of surface porosity for shale samples from Long 1-1 sub-member in western Chongqing area, Sichuan Basin
图 3 四川盆地渝西地区龙一1亚段页岩样品原始状态(自发吸水前)孔缝特征
a-d.Z203-1号样品有机孔(a)、无机孔(b-c)、微裂缝(d)特征；e-h.Z207-1号样品有机孔(e)、无机孔(f-g)、微裂缝(h)特征；i-l.Z203H2-1号样品有机孔(i)、无机孔(j)、微裂缝(k-l)特征；m-p.Z208-1号样品有机孔(m-n)、无机孔(o)、微裂缝(p)特征。
Figure 3. Pore and fracture characteristics of shale samples in their original state (before spontaneous water imbibition) from Long 1-1 sub-member in western Chongqing area, Sichuan Basin
图 4 四川盆地渝西地区龙一1亚段页岩样品自发吸水不同时间有机孔隙演变特征
a.Z203-1号样品自发吸水前有机质特征；b.Z203-1号样品自发吸水1 d后有机孔特征；c.Z203-1号样品自发吸水14 d后有机孔特征；d.Z208-1号样品自发吸水前有机质特征；e.Z208-1号样品自发吸水1 d后有机孔特征；f.Z208-1号样品自发吸水14 d后有机孔特征。
Figure 4. Evolution characteristics of organic pores at different times of spontaneous water imbibition for shale samples from Long 1-1 sub-member in western Chongqing area, Sichuan Basin
图 5 四川盆地渝西地区龙一1亚段Z203-1号样品自发吸水不同时间粒内溶蚀孔隙演变特征
a.自发吸水前矿物分布特征；b.自发吸水前粒内溶蚀孔特征；c.自发吸水1 d后粒内溶蚀孔特征；d.自发吸水3 d后粒内溶蚀孔特征；e.自发吸水7 d后粒内溶蚀孔特征；f.自发吸水14 d后粒内溶蚀孔特征。
Figure 5. Evolution characteristics of intragranular dissolution pores at different times of spontaneous water imbibition for sample Z203-1 from Long 1-1 sub-member in western Chongqing area, Sichuan Basin
图 6 四川盆地渝西地区龙一1亚段Z203-1号样品自发吸水不同时间矿物粒间孔隙演变特征
a.自发吸水前矿物分布特征；b.自发吸水前矿物粒间孔隙特征；c.自发吸水1 d后矿物粒间孔隙特征；d.自发吸水3 d后矿物粒间孔隙特征；e.自发吸水7 d后矿物粒间孔隙特征；f.自发吸水14 d后矿物粒间孔隙特征。
Figure 6. Evolution characteristics of intergranular pores of minerals at different times of spontaneous water imbibition for sample Z203-1 from Long 1-1 sub-member in western Chongqing area, Sichuan Basin
图 7 四川盆地渝西地区龙一1亚段Z208-1号样品自发吸水不同时间基质矿物间微裂缝演变特征
a.自发吸水前矿物分布及微裂缝位置；b.自发吸水前微裂缝特征；c.自发吸水1 d后微裂缝特征；d.自发吸水3 d后微裂缝特征；e.自发吸水7 d后微裂缝特征；f.自发吸水14 d后微裂缝特征。
Figure 7. Evolution characteristics of micro-fractures between matrix minerals at different times of spontaneous water imbibition for sample Z208-1 from Long 1-1 sub-member in western Chongqing area, Sichuan Basin
图 8 四川盆地渝西地区龙一1亚段Z207-1号样品自发吸水不同时间黏土矿物—有机质微裂缝演变特征
a.自发吸水前微裂缝及矿物分布特征；b.自发吸水前微裂缝发育特征；c.自发吸水1 d后微裂缝特征；d.自发吸水3 d后微裂缝特征；e.自发吸水7 d后微裂缝特征；f.自发吸水14 d后微裂缝特征。
Figure 8. Evolution characteristics of micro-fractures between clay minerals and organic matter at different times of spontaneous water imbibition for sample Z207-1 from Long 1-1 sub-member in western Chongqing area, Sichuan Basin
图 9 四川盆地渝西地区龙一1亚段页岩样品自发吸水前后微观孔隙变化特征
a.Z203-1号样品自发吸水前孔隙及矿物分布特征；b.Z207-1号样品自发吸水前孔隙及矿物分布特征；c.Z203H2-1号样品自发吸水前孔隙及矿物分布特征；d.Z203-1号样品自发吸水14 d后孔隙发育特征；e. Z207-1号样品自发吸水14 d后孔隙发育特征；f.Z203H2-1号样品自发吸水14 d后孔隙发育特征。
Figure 9. Variation characteristics of microscopic pores before and after spontaneous water imbibition in shale samples from Long 1-1 sub-member in western Chongqing area, Sichuan Basin
图 10 四川盆地渝西地区龙一1亚段页岩样品微裂缝缝宽(a)和面孔率(b)与自发吸水时间变化关系
Figure 10. Relationship between micro-fracture width (a) and surface porosity (b) and spontaneous water imbibition time in shale samples from Long 1-1 sub-member in western Chongqing area, Sichuan Basin
图 11 四川盆地渝西地区龙一1亚段不同页岩样品吸水过程中微观结构演变特征
a.样品Z203-1吸水过程中孔隙面积、累计面孔率与孔径关系；b.样品Z207-1吸水过程中孔隙面积、累计面孔率与孔径关系；c.样品Z203H2-1吸水过中程孔隙面积、累计面孔率与孔径关系。
Figure 11. Evolution characteristics of microscopic structure during water imbibition in different shale samples from Long 1-1 sub-member in western Chongqing area, Sichuan Basin
图 12 四川盆地渝西地区龙一1亚段页岩吸水第7 d较吸水前面孔率增幅与岩样孔隙度(a)和渗透率(b)的相关性
Figure 12. Correlation between increase in surface porosity and sample porosity (a) and permeability (b) in shale samples from Long 1-1 sub-member in western Chongqing area, Sichuan Basin on the 7th day of water imbibition compared to before imbibition
表 1 四川盆地渝西地区龙一1亚段页岩样品TOC含量、物性及矿物组成统计
Table 1. TOC content, physical properties, and mineral composition of shale samples from Long 1-1 sub-member in western Chongqing area, Sichuan Basin
样品编号 孔隙度/ % 渗透率/ 10-3μm2 ω(TOC)/ % 矿物含量/% 石英 斜长石 方解石 白云石 黄铁矿 黏土矿物 Z203-1 4.23 0.32 4.8 70.1 3.7 4.3 7.0 3.2 11.7 Z207-1 4.10 0.28 4.5 53.0 6.0 4.0 20.0 4.0 13.0 Z203H2-1 6.05 0.58 5.6 42.5 4.0 12.0 22.0 3.5 16.0 Z208-1 4.90 0.62 4.3 30.4 7.2 5.7 17.7 4.6 34.4 
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