液氮低温-高压耦合作用页岩破裂成缝机理研究

Liquid nitrogen fracturing, which uses cryogenic nitrogen as a hydraulic fracturing fluid, can effectively avoid the issues of reservoirs damage and water consumption. It is expected to become one of the significant methods for the efficient stimulation of shale gas. To research the mechanisms of shale cracking and fracturing forming due to the coupling effect of liquid nitrogen cryogenic temperature and high pressure, the macro-meso incorporation method is employed in this project. The detailed research contents are as follows: (1) the extension and evolution characteristics of micro-fissures inside shale attributed to liquid nitrogen super cooling are researched by SEM and CT methods, and then the micro damage evolution mechanisms of shale are analyzed; (2) Based on the acoustic emission source location and CT-scanned means, the dynamic cracking path and failure pattern of shale due to liquid nitrogen cryogenic temperature and high pressure are investigated. Besides, the relationship between the evolution of micro-fissures due to the liquid nitrogen super cooling and the macro fracture failure characteristics of shale is analyzed. On this basis, the shale cracking mechanisms due to the coupling effect of liquid nitrogen cryogenic and high pressure are uncovered; (3) The fractal study on the fracture distributions of fractured shale samples is performed. Then the influences of liquid nitrogen flow rate, cooling time, confining pressure, temperature difference and bedding inclination on the complexity of fractures are inspected, thereby building the method that evaluates the ability of fracture forming for shale due to the coupling effect of liquid nitrogen cryogenic temperature and high pressure. In a word, the research has important significance in exploring the fracture initiation mechanism of liquid nitrogen on shale, which can provided theoretical foundation for the parameter optimization of shale reservoir fracturing with liquid nitrogen.

液氮压裂可有效避免水力压裂存在的储层伤害和水资源过度消耗等问题，有望为页岩气高效开发提供一条新途径。本项目拟采用宏、细观相结合的方法，开展液氮低温-高压耦合作用页岩破裂成缝机理研究。具体内容包括：基于SEM和CT测试方法，研究页岩与液氮接触后的微裂纹扩展特征及力学机制，分析液氮超低温作用下页岩的微观损伤演化机理；根据声发射定位和CT扫描结果，研究液氮低温-高压耦合作用下页岩的动态破裂路径和破坏模式，分析液氮超低温引起的微裂纹扩展演化与页岩宏观断裂破坏的关联性，进而揭示液氮低温-高压耦合致裂页岩机理；研究液氮低温-高压耦合致裂后页岩裂缝的分形特征，探索液氮流量、冷冻时间、围压、温差以及层理方向对裂缝复杂程度的影响规律，建立液氮低温-高压耦合作用下页岩成缝能力的评价方法。通过本项目研究，对于揭示液氮对页岩的造缝机制具有重要意义，可为页岩液氮压裂参数的优化设计提供理论依据。

液氮压裂是一种新型的无水压裂方法，可望为页岩气高效开发提供一条新途径。为揭示液氮低温-高压耦合作用页岩破裂成缝机理并探索高效的液氮压裂工艺，围绕液氮对页岩的低温损伤机制、温压耦合致裂机理及其与岩石之间的对流换热特性等开展研究，主要成果如下：.（1）采用宏、细观力学实验方法，研究了液氮低温作用对页岩微裂纹及力学参数的影响。液氮低温作用能够增加页岩损伤程度，促进页岩内部微裂纹的扩展、微裂纹尺度和裂纹体积的增加，孔隙体积最大增幅为14.8%；微裂纹的扩展能够使页岩渗透率升高、超声波速降低及加载破裂程度增加，这对提高储层改造效果十分有利。.（2）开展了页岩液氮低温-压力耦合致裂实验，得到不同初始温度下页岩的破裂特征和裂缝形态。液氮气化膨胀所产生的压力能够有效压裂页岩，形成宏观裂缝。在液氮低温-高压耦合作用下，页岩的破裂模式主要为沿弱面破坏，裂缝与孔眼轴线呈一定夹角。在液氮压裂时页岩的破裂压力随其初始温度升高而降低，且破裂模式转化为沿孔眼轴线的拉张破裂。.（3）研究了液氮处理方式对页岩低温-压力耦合致裂效果的影响规律。液氮低温作用产生损伤能够增强页岩微观结构连通性，形成新的弱面，导致低温处理岩样的破裂模式以局部裂缝为主。在低温冷冻状态下，页岩的破裂压力平均比原始岩样高11.76%，由于压裂过程中液氮容易沿着张开的裂缝从孔眼内部渗透到岩样外部，导致微裂缝不能充分扩展，页岩破裂模式主要为开度较小的局部裂缝。.（4）模拟了液氮注入过程中的井底流场和温度场，液氮射流流场结构、射流冲击能力以及在地层孔道内的增压效果。液氮能够导致井底温度迅速降低，从而产生显著的热冲击效果；根据流体的相态，可以将液氮射流分为液氮射流区和非液氮射流区，并且射流具有明显的速度边界、温度边界和等速核；液氮射流能够在地层孔道内产生与水射流相当的增压效果。综上，液氮喷射压裂具有明显优势，有望为页岩气开发提供一条新途径。