煤层压裂缝网内支撑剂的运移与压嵌规律研究

The coal seam fractures network formed by hydraulic fracturing leads to the complex migration of the proppant. Coal seam is soft, the difference of the strength between the coal and proppant is big, and the drainage period is long, which caused the proppant easily embedded in coal seam fractures. Therefore, the migration and embedment of proppant in coal seam fractures network is one of the major common problems in the exploitation of coalbed methane. On the basis of applicant’s early studies on the fractures propagation law of coal seam hydraulic fracturing, this project takes the fracture network conceptual model of coal seam hydraulic fracturing as an research pointcut, takes the spatial distribution and evolvement rule of the fracture’s opening degree as an research bond, the large size（500 mm×500 mm×500 mm）true triaxial experimental system simulating the whole process of coal seam fracturing is utilized to truly represent the whole process of migration and embedment of proppant in coalbed methane well fracturing. To research the fracture network and opening degree model, migration and settlement rules and embedment characteristics of proppant. Building the fracture network conceptual model of coal seam hydraulic fracturing, mastering the spatial distribution and evolvement rule of the opening degree of network fractures during the fracturing stages of sand-carrying fluid. On this basis, revealing the flow state of the two-phase flow at the site of fractures bifurcation and confluence, migration characteristics of proppant, the formation mechanism of sand plugging and the mechanism of partial proppant migration return caused by flowback of fracturing fluid after fractures, expounding the mechanism and time effect of proppant embedment in coal seam fractures. This will lay the foundation for the coalbed methane fracturing technology and the optimization and development of proppant.

煤层压裂形成的裂缝网络导致支撑剂的运移复杂，煤层软、煤与支撑剂强度差异大、排采期长等导致支撑剂易压嵌；因此，压裂缝网支撑剂的运移与压嵌是煤层气开采的重大共性难题之一。项目在申请人前期研究煤层压裂水压裂缝扩展规律的基础上，以煤层压裂水压裂缝网络概念模型为研究的切入点，把裂缝张开度的空间分布与演变规律作为研究的纽带，采用大尺寸（500mm×500mm×500mm）真三轴煤层压裂全过程实验系统等真实再现煤层气井压裂支撑剂的运移与压嵌全过程，研究煤层压裂缝网与张开度模型、支撑剂运移沉降规律与压嵌特性。构建煤层压裂的水压裂缝网络概念模型，掌握携砂液阶段水压网络裂缝张开度的空间分布与演变规律；在此基础上，揭示两相流在裂缝分叉与汇合处的流态及支撑剂运移特征、砂堵的形成机制、压裂后返排导致部分支撑剂返移的作用机制，阐明煤层裂缝内支撑剂压嵌的作用机制与时间效应。为煤层气压裂工艺与支撑剂的优选及研制奠定基础。

研究背景：煤层压裂形成的裂缝网络导致支撑剂的运移复杂，煤层软、煤与支撑剂强度差异大、排采期长等导致支撑剂易压嵌；因此，压裂缝网支撑剂的运移与压嵌是煤层气开采的重大共性难题之一。. 主要研究内容：以煤层压裂水压裂缝网络概念模型为研究的切入点，研究煤层支撑剂运移沉降规律与压嵌特性，阐明煤层裂缝内支撑剂压嵌的作用机制与时间效应。. 重要结果：煤体压裂裂缝内支撑剂在嵌入过程中存在着嵌入应力和支撑应力两个临界应力，将支撑剂嵌入煤的过程分为支撑剂压密、初始嵌入和嵌入支撑3个阶段。支撑剂在嵌入煤的过程中，存在支撑剂簇整体嵌入煤体现象，单支撑剂或支撑剂簇作用于煤体表面形成压嵌裂缝和压嵌坑。煤岩裂隙面破坏形式从中心向外依次为压破坏、拉剪破坏和拉破坏。支撑剂嵌入煤体裂隙面形成四个区域：中心压实区、层状裂隙区、裂隙发育区和原始状态区。在含裂缝-支撑剂的煤的蠕变过程中，不仅存在煤的硬化和损伤，还存在支撑剂嵌入损伤。蠕变硬化损伤模型能够更好地描述支撑剂压裂后煤的减速蠕变、稳定蠕变和加速蠕变的全过程。通过多级蠕变的正交试验可有效分析各参数优先级，铺砂浓度对支撑剂嵌入煤的影响最大，粒径次之，时间最小。. 关键数据：发表论文8篇，其中SCI收录6篇，EI收录1篇。授权加拿大发明专利1项、澳大利亚发明专利1项、中国发明专利2项。获中国安全生产协会安全科技进步二等奖1项。培养了博士研究生2名、已毕业1人，硕士研究生6名、已毕业3名。. 科学意义：阐明了支撑剂的压嵌特性，揭示了接触应力对煤体的破裂影响规律，建立了含裂缝-支撑剂的蠕变模型，给出了支撑剂的优选方法。研究结果为煤层气压裂的支撑剂优选以及评价煤层压裂裂缝的导流能力奠定了基础。