超临界CO2穿层致裂煤岩界面区裂纹扩展机理及规律研究

Traditional hydraulic fracturing has many problems in the drilling and fracturing process of the coal seam. This project takes low-strength, low-permeability and high-gas coal strata as the research object, and takes the gas extraction through fracturing increases permeability to control gas as the research background. This project intends to investigate the mechanism and crack propagation characteristics of coal-rock interface area by supercritical CO2 fracturing through the roof rock layer to coal seam. 1) The mechanical properties of coal-rock and the evolution characteristics of pore-fracture structure under the long-term infiltration of supercritical CO2 will be investigated by the experimental study combined with micro-CT scanning method, revealing the weaken mechanical properties of coal and rock and the evolution of pore-fracture structure with temperature and pressure. 2) Acoustic emission monitoring and micro-CT scanning method will be used to study the supercritical CO2 fracturing crack initiation, penetration expansion conditions and morphological characteristics by the true triaxial fracturing seepage simulation device. 3) Based on the interface mechanics and fracture mechanics theory, the influence of mechanical properties of coal-rock interface on the complex cracks propagation by supercritical CO2 fracturing will be studied, and the condition criterion for discriminating crack propagation across the interface will be established. And the project will use the "phase field method" fracturing program of the research group to numerically simulate the crack penetration propagation, and verify the results with the experimental results. Finally, it will reveal the mechanism and the law of supercritical CO2 fracturing crack propagation through rock strata to coal seam. It provides theoretical and technical guidance for formation of fracture system by high-efficiency fracturing coal seam, improving gas drainage efficiency, controlling the coal seam gas, and the development and utilization of coalbed methane resources.

传统水压致裂煤层的钻孔及压裂过程中存在诸多问题。本项目以低强度、低渗、高瓦斯煤系地层为研究对象，以压裂增渗治理瓦斯为研究背景，拟开展从顶板实施超临界CO2穿层致裂煤体的界面区裂纹扩展机理及规律研究：1）通过实验结合显微CT扫描方法，研究超临界CO2长时间浸润作用下煤岩力学特性及孔裂隙结构演化特征，揭示煤岩力学特性损伤弱化及孔裂隙结构演化随温度及压力变化规律；2）采用声发射监测及显微CT扫描方法，通过真三轴压裂模拟装置研究超临界CO2穿层致裂裂纹起裂、扩展条件及形态特征规律；3）基于界面力学、断裂力学理论，研究煤岩界面力学特性对超临界CO2复杂形态裂纹扩展的影响，建立判别裂纹跨越界面扩展的判据，利用课题组自编“相场法”压裂程序进行穿层压裂数值模拟，最终阐明超临界CO2穿层致裂煤体机理及规律。为高效压裂煤层形成缝网、提高瓦斯抽采效率、有效治理煤层瓦斯、煤层气高效开发利用提供理论及技术指导。

本项目以新兴ScCO2压裂技术压裂增渗治理及利用煤层瓦斯为研究背景，进行从顶板实施超临界CO2穿层致裂煤体的界面区裂纹扩展机理及规律研究。项目围绕ScCO2“高密度、低粘度、易扩散”的特点，开展裂纹从煤层顶板覆岩起裂穿层扩展进入煤层过程中，裂纹在界面区域的扩展机理及特征规律。开展ScCO2作用下煤岩物理力学特性及其孔裂隙结构演化规律、ScCO2穿层致裂煤体裂纹扩展条件及形态规律、煤岩界面力学特性对ScCO2致裂复杂形态裂纹扩展的影响规律等内容的研究。为高效压裂煤层形成缝网、提高瓦斯抽采效率、有效治理煤层瓦斯、煤层气高效开发利用提供理论及技术指导。经过三年努力，取得主要研究成果如下：.（1）研究探明天然煤与泥岩组合体界面黏聚力及ScCO2浸泡作用下的界面弱化力学特性；发现了ScCO2浸泡对煤体孔裂隙结构损伤能力，揭示了温度-应力-ScCO2耦合作用下力学特性劣化特征，探明了ScCO2浸蚀对天然煤岩组合体弱化力学特征。.（2）研究发现天然煤岩组合体ScCO2压裂的起裂压力低、更容易沟通天然层理等弱面结构进而产生复杂缝网缝的特点，掌握了不同界面强度煤岩裂缝扩展贯通形态特征。.（3）提出了ScCO2压裂裂缝跨界面穿层扩展的临界差应力与界面粘聚力的影响关系。.（4）掌握了天然煤岩体压裂及其天然组合体穿层压裂裂纹扩展特征，揭示了煤体及天然煤岩组合体穿层压裂前后渗透率特征，进一步研究了压裂效果。.在本项目资助下，共发表SCI、EI收录论文4篇，核心期刊论文1篇，申请国家发明专利2项（公布）；培养在读硕士生2名；获得山西省技术发明一等奖1项（排名第四）。