基于分形理论及多尺度方法的页岩体积改造压裂液滤失机理研究

Low flow-back rate and large water consumption of fracturing fluid are a common phenomenon during volume stimulation of shale-gas reservoir. The imbibition of fracturing fluid has been elaborately studied, while the research on the leak-off is relatively less. However, due to shale nano-scale pore, micron-scale crack, macro-scale fracture network with multiple scales and the fractal characteristics, the fracturing fluid leak-off become more complex and still cannot be illustrated well. Thus, there is an urgent need to study the fracturing fluid leak-off in multi scale. This project will use true triaxial stress fracture experimental facility to simulate the treatment for shale reservoir. Focused ion beam scanning electron microscopy test, field emission scanning electron microscope test and isothermal nitrogen adsorption will be conducted to attain the shape and pattern of the fracture networks, fractures, and the pores. A model to characterize these three medium in multi scales will be developed via fractal theory. The multi-scales leak-off model of pore-fracture-network is building based on fractal-like tree networks’ Murray theory, coupling pressure and fracturing fluid volume as the continuity of fracturing fluid leak off process. How these factors affect the leak off will be analyzed: scale parameters (macro-scale fracture network, and micron-scale fracture, nano-scale pore), fractal parameters (describe three different media’s complex morphology), engineering parameters (net pressure and fracturing fluid properties). The main controlling factors will be pointed out, and the leak-off mechanism will be exposed. This work can provide theoretical support for shale gas efficient development.

针对页岩水平井体积改造过程表现的压裂液返排率低、水资源消耗大等现象，现研究多集中于压裂液滞留渗吸作用，对滤失研究较少。页岩纳米级孔隙、微米级裂缝、宏观缝网具有多尺度性和分形特征，使压裂液滤失机理更复杂，迫切需要开展基础研究认清滤失机理。本项目拟采用真三轴应力裂缝装置模拟页岩体积改造过程，利用聚焦粒子束扫描电镜、场发射扫描电子显微镜、等温氮气吸附仪等仪器获取缝网、裂缝、孔隙复杂形态，结合分形理论建立三种介质形态表征模型；以类分形树状分叉网络Murray定律为基础，结合压力与压裂液量的连续性，建立符合分形规律的孔隙—裂缝—缝网多尺度滤失模型；搞清不同尺度参数（缝网宏观尺度、裂缝微米尺度、孔隙纳米尺度）下分形参数（描述三种介质复杂形态）、工程参数（缝内净压力、压裂液性质）等对压裂液滤失行为的影响规律，分析主控因素，揭示多尺度介质复杂形态下压裂液滤失机理，为页岩气高效开发提供理论支撑。

据全国能源工作会议报告，我国2021年页岩气产量230亿方，已成为全球第二大页岩气生产国。页岩气大规模商业开发得益于水平井分段压裂体积改造形成“人造气藏”技术的不断完善，水基压裂液作为页岩体积改造的关键工作液，用量大，应用广泛，但压后低返排率导致大量压裂液滞留在页岩储层中。为诠释滞留压裂液的辩证双重矛盾作用机制（诱导裂缝与储层伤害），需对页岩体积改造形成缝网过程中压裂液滤失行为开展机理性研究。项目采用了真三轴应力裂缝装置模拟受三向不相等主应力状况的页岩水平井页岩体积改造过程，选取页岩岩样来自重庆涪陵地区海相页岩露头，室内加工尺寸为300mm×300mm×300mm的立方体；利用图像分析技术对压后缝网形态、天然裂缝形态、人工裂缝形态进行处理，获取了相关形态表征参数；综合利用聚焦粒子束扫描电镜、氮气吸附方法测试压后块状页岩基质孔径大小，确定了页岩基质孔隙孔径在微介孔尺度下的分布；结合分形理论确定缝网、裂缝、孔隙三种介质的分形表征参数，综合运用类分形树状分叉网络Murray定律、压力与压裂液量的连续性，建立了符合分形规律的孔隙—裂缝—缝网多尺度滤失模型；分析了影响页岩体积改造压裂液滤失的多尺度（缝网宏观尺度、裂缝微米尺度、孔隙纳米尺度）因素和工程因素（缝内净压力、压裂液性质），揭示了页岩体积改造多尺度下压裂液滤失机理。完成的研究成果中标注本基金论文13篇，其中SCI收录2篇、EI收录1篇、中文核心4篇；申请国家发明专利6项，其中已授权1项；参加学术会议4次；培养硕士2名。本项目理论研究成果在中石化涪陵区块的页岩气压裂得到直接应用，对页岩气体积压裂压后闷井制度优化和返排措施优化具有重要指导意义，亦对页岩油密切割体积压裂具有借鉴意义。