陆相页岩井周天然裂隙力学活动性评价方法基础研究

The evaluation of mechanical activity of the natural fissure is an important basis for analyzing the mechanical properties of the limnetic shale reservoir and guiding the design of fracturing selection. However, the existing natural fissure mechanics theory and evaluation method can neither reflect the structural characteristics of natural fissure in limnetic shale, nor reveal the mechanical behavior mechanism of natural fissure during fracturing process. From the perspective of engineering application basic research, based on imaging logging data and using Fractal-Topology theory, the characterization method of natural fissure around hole will be established. On the basis of natural fissure characterization, the distribution of stress around hole is depicted by finite element method. Based on the characterization and stress characterization of natural fissure, a mechanical mechanism model of natural fissure during hydraulic fracturing is established. Combined with core fracture slip diversion experiment, the mechanism of natural fissure mechanical behavior will be explored. According to studying of the relationship between the characteristic parameters of the natural fissure mechanic and the conventional logging data, the evaluation method of the mechanical activity of the natural fissure around hole in limnetic shale formation will be formed. This research project is expected to solve the uncertainty of evaluation result of natural fissure characterization and mechanical activity around hole in limnetic shale reservoir, and research results can not only enrich the shale reservoir fracability evaluation method, but also promote the application of natural fissure mechanics theory and method of activity evaluation and development, which will be of great significance to provide scientific guidance for limnetic shale development efficiently.

天然裂隙力学活动性评价是分析陆相页岩储层力学性质和指导压裂选层设计的重要基础，但现有的天然裂隙力学理论和评价方法既不能反应陆相页岩天然裂隙结构特征，也无法揭示压裂过程中天然裂隙的力学行为机理。本研究将从解决工程应用基础研究角度出发，以成像测井为基础，运用分形拓扑理论，建立井周天然裂隙表征方法；基于天然裂隙表征，运用有限元方法实现含裂隙井周应力场分布刻画；基于天然裂隙表征和应力刻画，建立水力压裂条件下天然裂隙力学机理模型，并结合岩心裂隙滑移导流实验，探究揭示天然裂隙力学活动行为机理；通过研究天然裂隙活动性表征参数与常规测井曲线关系，形成陆相页岩井周天然裂隙力学活动性评价方法。本项目的研究有望解决陆相页岩储层井周天然裂隙表征和力学活动性评价不确定性问题，研究成果不仅可丰富页岩储层可压性评价方法，也推动了天然裂隙力学活动性评价方法和理论的应用与发展，对于科学指导陆相页岩油气高效开发具有重要意义。

陆相页岩储层天然裂缝类型多种、数量庞大且分布随机，造成勘探过程三维地质建模的不确定性、钻井过程井壁的失稳性和压裂开采缝网形成的复杂性。因此，本项目采用理论分析、数值模拟和物理模拟等，从天然裂缝定量表征、含天然裂缝井周应力场刻画、应力场-渗流场耦合下天然裂缝力学行为及对天然裂缝力学活动性修正等进行了研究。结果表明：.（1）共分为I、II和III型节点天然裂缝3类，分别与最大水平主应力方向夹角10°、5°、5°分布，分形特征较差，由I型节点主导，连通性较差；.（2）II和III型节点天然裂缝对近井井周周向应力、径向应力和切向应力影响最大，井壁最不稳定；.（3）仅含天然裂缝储层水平井井壁水力压裂裂缝能在最大水平主应力方向起裂，在I型节点天然裂缝发育储层起裂位置呈对称式，其它呈非对称式；.（4）水力裂缝未能天然裂缝穿过天然裂缝，在相交点处开启天然裂缝并沿其转向延伸，II和III型节点天然裂缝开启分支多于I型节点天然裂缝；.（5）天然裂缝与水力裂缝相交发生剪切滑移，张性起裂瞬间天然裂缝滑移速率高、持续时间短，而剪切起裂相反，最大滑移距离约为张性起裂2~4倍；.（6）较大偏应力和围压及较小的粗糙度有利于天然裂缝发生剪切滑移，逼近角为30°时天然裂缝剪切滑移距离最大；.（7）II型节点天然裂缝对水力裂缝扩展的诱导能力最强，压裂后形成的裂缝网络最为复杂，III型节点天然裂缝次之，I型节点天然裂缝诱导能力最低；.（8）压裂液排量是影响裂缝力学行为的主要因素，而压裂液黏度和地应力差对力学行为的影响较小；.（9）基于裂缝扩展长度和缝网复杂性及正交试验，确定不同天然裂缝发育储层水平井横向和垂向水力压裂最优压裂液排量、黏度和地应力差及最优压裂施工参数组合；.（10）物理模拟不同节点天然裂缝发育储层水力压裂实验，裂缝力学行为与数值模拟结果契合，且所建立的地应力大小模型结果与测井结果契合。.本研究可为陆相页岩油气安全、高效开发方案的制定提供理论和技术指导。