碎软煤层顶板水平井穿层压裂裂缝扩展机理及形态控制研究

Layer-penetrating fracturing is an effective measure to increase the production of horizontal well within roof as well as coalbed methane recovery, yet the fracture propagation mechanism on layer-penetrating fracturing is not fully clear at current stage, this situation leads to following predicaments: lack of supportive technological engineering theory, and absence of reliable quantitative reference for construction parameters, as a result the reformations of layer-penetrating fracturing turn out to be varied. Hence, this research proposes to adopt a method that is to combine both experiment and theory, researches the characteristics of fracture open and propagation and distribution law of fracture morphology, then explores the mechanism of fracture propagation of layer-penetrating fracturing. By establishing a theoretical model on fractures open and propagation of horizontal well within roof by layer-penetrating fracturing on the basis of different rupture criteria, the mechanical conditions of fracture propagation and extension are analyzed, thereupon then criterions on fractures penetrate interface are proposed, then the dynamic evolution mechanism of fracture propagation in roof-interface-coal seam is revealed, the propagation rule of layer-penetrating fracturing, as well as the fracture propagation and formation mechanism on the same subject. And last, by establishing a control model on the influence factors of fracture propagation shape with principal component analysis as its foundation, thereupon then draws out the control mechanisms of fracture propagation shape, studies on the quantitative design method on shape control of fracture propagation, and completes the corresponding design on parameters. The findings of this research will provide theoretical basis and technical support for construction design and effect evaluation of layer-penetrating fracturing in horizontal well within roof in different strata conditions.

穿层压裂是提高顶板水平井煤层气产量和采收率的有效手段，目前关于穿层压裂的裂缝扩展机理尚未理解清楚，致使工程技术理论支撑不足、施工参数设计缺乏定量可信依据，导致穿层压裂改造效果差异明显。因此，本项目拟采用实验与理论相结合的研究手段，研究裂缝的起裂、扩展特征与裂缝形态分布规律，探究穿层压裂裂缝扩展与形成机制；建立基于不同破裂准则的顶板水平井穿层压裂裂缝开启和延伸的理论模型，分析裂缝扩展延伸的力学条件，提出裂缝穿越界面的判别准则，研究穿层压裂裂缝扩展规律，揭示裂缝在“顶板-界面-煤层”中扩展延伸的动态演变机制，探究穿层压裂裂缝扩展与形成机理；最后，建立基于主成分分析法的裂缝扩展形态影响因素的控制模型，揭示裂缝扩展形态的控制机制，研究裂缝扩展形态控制的量化设计方法，实现对施工参数的定量设计。本项目研究成果可以为不同地层条件的顶板水平井穿层压裂施工设计与效果评价提供理论依据与技术支持。

顶板水平井是开发碎软煤层煤层气的一种有效方法，而穿层压裂是提高顶板水平井煤层气产量的有效手段。目前关于穿层压裂裂缝扩展规律尚未理解清楚，致使压裂施工参数设计缺乏定量可信的科学依据。为此本项目采用室内实验与数值模拟研究相结合的方法，建立了基于岩石力学强度的“顶板-煤层”胶结物理模型，开展了穿层压裂物理模拟实验和裂缝穿界面数字散斑测试，建立了顶板水平井穿层压裂数值模型，开展了穿层压裂裂缝扩展的数值模拟，研究了顶板水平井穿层压裂裂缝扩展机理，并进行了工程验证。研究结果表明：（1）水平井与煤层的距离、注入排量、垂向与水平应力差和射孔方式是影响顶板水平井穿层压裂缝扩展形态的4个重要因素；（2）裂缝穿越煤岩界面存在4种破坏模式和三种裂缝形态，裂缝穿层主要受三轴应力、煤层与顶板的界面强度以及煤层的抗拉强度三个因素共同控制，揭示了裂缝在符合穿层力学条件下具有形成裂缝更长、改造煤层效果更好的增产机理；（3）提出了射孔模型逐级尺度依次递进的模拟方法，构建了单个射孔局部、射孔区局部和整个地层的射孔模型；开发形成了基于扩展有限元（XFEM）和粘聚单元（cohesive elements）相结合的顶板水平井穿层压裂数值模拟技术；（4）查明了顶板水平井穿层压裂效果的6个主控因素及控制规律，得到穿层压裂改造效果随着射孔方位角（与竖直方向的夹角）、水平井与煤层距离的增加而变差，随着射孔深度、界面强度、煤层弹性模量、垂向与水平应力差的增加而增加，研究形成了顶板水平井穿层压裂裂缝形态控制的量化设计方法；（5）顶板水平井穿层压裂理论在山西余吾井田和贵州新田井田工程应用取得了较好的产气效果，进一步验证了穿层压裂裂缝扩展理论的正确性与可靠性。研究结果可为顶板水平井穿层压裂施工参数优化设计提供理论依据。