甲烷燃爆压裂冲击下页岩动态破裂响应规律实验研究

Experimental study on the dynamic fracture response law of shale under the impact of methane combustion fracturing

甲烷原位燃爆压裂是通过地面投放的助燃剂与地层产出的甲烷气,在页岩气井筒内目标层段原位燃爆,实现强化压裂储层的新技术。其中明确甲烷爆燃压裂中储层岩石动态破坏规律,是确保压裂效果和井身安全工艺设计的基础。基于此,为明确甲烷原位燃爆压裂过程中,聚能流体泄入性冲击破岩规律及其主控因素敏感性,借助岩石动态冲击损伤模拟实验装置,针对龙马溪组海相页岩露头,开展拟三轴、真三轴条件下的冲击压裂破岩实验共24组,通过对冲击破岩过程中模拟井眼压力的高频跟踪、压后岩样破裂形态及裂缝面特征观测,分别考察了120~285 MPa理论峰值压力、20.9~131.7 MPa/ms理论加载速率、储层应力与层理方位等对冲击破岩的影响,用铜箔对模拟井眼孔壁密封考察了应力冲击与泄入冲击对岩样破坏形态的影响。结果显示:聚能流体泄入性冲击可形成楔裂效应,降低页岩动态破裂强度,页岩动态强度与升压速率的正相关性弱于纯应力冲击;平行层理页岩、较低的地应力水平,均可减小页岩动态破裂强度;纯应力冲击更易形成复杂缝网,裂缝表面凹凸性强;聚能流体泄入冲击致裂后的裂缝走向平直,且随着升压速率增大,其裂缝条数及复杂程度增加、裂缝面剥离碎裂现象增强;模拟井眼轴线与页岩层理面平行时,压后裂缝面平整、粗糙度小、起伏小;模拟井眼轴线与页岩层理面垂直时,压后裂缝走向曲折、表面起伏大、裂缝网更复杂;页岩水平挤压应力较小,压后裂缝面粗糙,伴随有大量脱落物;随着岩样挤压应力增大,流体楔入基质实现二次冲击的阈值升高,压后页岩脱落物粒径逐渐减小。基于不同参数组合下页岩冲击破坏模式的对比分析表明:甲烷原位燃爆冲击过程是应力冲击与流体泄入性冲击的复合作用,随着冲击升压速率的提升,聚能流体泄入性冲击作用程度逐步加大,压后页岩逐步呈现出应力致裂-流体协同扩缝(27 MPa/ms)、应力致裂-流体冲刷破坏(52 MPa/ms)以及纯应力致裂-流体二次冲击破缝(116 MPa/ms)3种破坏模式;基于实验结果建立的3种破坏模式与加载速率、储层应力的关系图版,用于不同应力条件下甲烷原位燃爆冲击参数的优化设计。

The methane in-situ combustion fracturing is a technology to realize in-situ combustion and fracturing on the target layer section in the shale gas wellbore.The technology can realize the enhanced fractured reservoir modification by means of the combustion-supporting agents injected from the ground and the methane gas released from the formation,which has the advantages of low cost,high safety,and convenient repeatability.Among them,the understanding on the dynamic damage law of reservoir rock during the fracturing is the basis for ensuring desirable fracturing results and wellbore safety.Based on this,this study conducted the quasi triaxial and true triaxial rock dynamic impact damage simulation experiments to analyze the dynamic failure response mechanism of shale rocks under the condition of explosive impact.And the dynamic fracture strength and fracture morphology of shale were evaluated quantitatively,which are influenced by the impact mode,loading rate,reservoir stress,and shale bedding.The results show that the high energy fluid penetrating impact reduces the dynamic fracture strength of shale due to the “wedge fracture effect” of the fluid.And the positive correlation with the pressurization rate is weaker than that of pure stress impact.Parallel-layer shale and lower in-situ stress level can reduce the dynamic fracture strength of shale.Single stress impact is more likely to generate more complex fracture network,and the fracture surface is highly concave and convex.Comparatively,the high energy fluid penetrating impact is more inclined to produce straight radial fractures.Besides,the number and complexity of multiple fractures as well as the peeling and fragmentation of fractures surface are also enhanced as the loading rate increase.More specifically,the postfracturing shale samples show three damage modes gradually including the collaborative fracturing by stress tension and fracturing-fluid wedging(27 MPa/ms),the further fracturing by the fluid erosion of crack surface after stress induced cracking(52 MPa/ms),and the fluid shock cracking after stress induced fracturing(116 MPa/ms).Based on the experimental results,the relationship between the three damage modes and the pressure loading rate and reservoir stress was plotted,which can provide certain theoretical support for the design of methane combustion fracturing.