页岩储层渗吸过程微观孔缝演变特征及影响因素——以四川盆地渝西地区龙马溪组龙一亚段为例

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

钱计安 蒋裕强 罗彤彤 杨一骁 付永红 陈维铭 孙超亚 王占磊

QIAN Ji'an JIANG Yuqiang LUO Tongtong YANG Yixiao FU Yonghong CHEN Weiming SUN Chaoya WANG Zhanlei

西南石油大学 地球科学与技术学院, 成都 610500 中国石油 非常规油气重点实验室 储层评价实验室, 成都 610500 重庆页岩气勘探开发有限责任公司, 重庆 401121 中国石油 西南油气田分公司 页岩气研究院, 成都 610051

School of Geoscience and Technology, Southwest Petroleum University, Chengdu, Sichuan 610500, China Reservoir Evaluation Laboratory, Unconventional Oil and Gas Key Laboratory, CNPC, Chengdu, Sichuan 610500, China Chongqing Shale Gas Exploration and Development Co., Ltd., Chongqing 401121, China Research Institute of Shale Gas, PetroChina Southwest Oil and Gasfield Company, Chengdu, Sichuan 610051, China

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。

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.

国家自然科学基金面上项目 42272171;中石油—西南石油大学创新联盟体项目 2020CX020104

https://doi.org/10.11781/sysydz2024061336