基于龟裂物理模拟实验的页岩成岩收缩裂缝发育规律与地质模式

In recent years, study, exploration and development about shale oil and gas are booming in China. Shale reservoir fracture is becoming an important study subject. Diagenetic shrinkage fracture in shale is one of main types of shale fractures, and has great influence on storage, transfusion of shale oil and gas and hydraulic fracturing of shale. However, it is weak that diagenetic shrinkage fracture in shale is studied at home and abroad. According to the similarity of morphology and genesis between desiccation crack of modern muddy sediment and diagenetic shrinkage fracture in shale, this project will use experiments of desiccation crack to study diagenetic shrinkage fracture in shale. It is of important significance in both theory and practice. Multi-group experiments of desiccation crack in different conditions will be made in this project. The cracking location, critical conditions and forming process will be studied, and analogy analysis will be done to research dynamic forming process of diagenetic shrinkage fracture in shale. The static geometricl characteristics of diagenetic shrinkage fracture, including geometric parameters, morphology and their distribution rule, will be studied. Then we will analyze the development rule of diagenetic shrinkage fracture and their controlling factors. Finally, the geological model of diagenetic shrinkage fracture development will be established in different typical conditions of shale. We hope that the above study could afford significant reference for research on diagenetic shrinkage fracture and non-tectonic fracture in shale.

近年来，我国页岩油气的研究、勘探和开发获得迅速发展，页岩储层裂缝日益成为一个重要的研究课题，页岩成岩收缩裂缝是一种重要的（非构造）裂缝类型，对页岩油气的储集、渗流，尤其后期压裂具有重要影响，但由于重视不足和研究手段限制，研究相对薄弱。本项目根据现代泥质沉积物龟裂与地下页岩成岩收缩裂缝在形态和成因上的相似性，拟以泥质沉积物龟裂物理模拟实验为基础，对页岩成岩收缩裂缝进行探索性研究，具有重要的理论和实践意义。通过进行多组不同条件的龟裂物理模拟实验，研究龟裂的开裂位置、临界条件和形成阶段，类比分析成岩收缩裂缝的动态形成过程；进而分析龟裂的几何参数、几何形态及其分布规律，类比总结成岩收缩裂缝的静态几何特征；然后总结页岩成岩收缩裂缝的发育规律，分析其影响因素；最后建立典型页岩条件下的成岩收缩裂缝发育地质模式。本项目研究可为页岩成岩收缩裂缝等非构造裂缝的研究提供参考。

页岩成岩收缩裂缝是一种重要的非构造裂缝类型，对页岩油气的储集、渗流，尤其后期压裂具有重要影响，但由于重视不足和研究手段限制，研究相对薄弱。本项目根据现代泥质沉积物龟裂与地下页岩成岩收缩裂缝在形态和成因上的相似性，利用泥质沉积物龟裂物理模拟实验，对页岩成岩收缩裂缝发育规律（动态形成过程和静态几何参数）、主要影响因素（厚度、粘土矿物含量、水矿化度、加热温度和含沙量）和发育地质模式进行探索性类比研究，具有重要的理论和实践意义。研究表明，页岩成岩收缩裂缝开始在力学较薄弱的地方开裂并形成主裂缝，如较大颗粒处、凹陷处、已经存在的裂缝和孔隙处；生长过程中存在主裂缝形成、次级裂缝形成、稳定阶段和固定阶段等4个发育阶段。厚度越大，开裂的起始时间越晚，临界含水率越高；而裂缝的长度和宽度越大，面密度和裂缝率越小。水矿化度与成岩收缩裂缝关系较小，不同水矿化度的试样，开裂时间和临界含水率相近，随着水矿化度的增大，成岩收缩裂缝的宽度、长度、面密度（条数）、裂缝率均呈现减小或降低的趋势，但减小或降低的幅度较小。较高的粘土矿物含量造成页岩收缩开裂的时间偏晚，临界含水率偏大；裂缝的长度和裂缝率增大，裂缝宽度更是急剧变大，而裂缝面密度减小。温度越高，开裂的时间越早，临界含水率越高；裂缝的面密度越大，而长度和宽度越小，裂缝率呈现先增大后降低的趋势。含沙量越大，开裂时间越晚、开裂临界含水率越低，裂缝长度、宽度、裂缝密度和裂缝率均显著降低。在以上研究基础上，建立了孔洞或大颗粒控制下、层理或原有裂隙控制下和随机发育3种页岩成岩收缩裂缝发育地质模式。