黔西北地区龙马溪组页岩有机质和脆性矿物的控缝机制

“Sweet spot”effect and mechanism of shale fracture development in Longmaxi Formation,northwestern Guizhou

页岩"甜点"层段中有机质和脆性矿物对裂缝的发育具有极为重要的控制作用,揭示并把握裂缝萌生、发育和演化的机理与规律,对于压裂目标层段的优选以及页岩气增产和稳产具有极为重要的工程实践价值。以黔西北地区龙马溪组页岩为例,通过地化测试、岩石力学实验以及扫描电镜,深入剖析和揭示了"甜点"层段中与有机质和脆性矿物相关的控缝机制,并结合数值模拟实验展现了"甜点"层段中裂缝的形成和演化的完整过程。研究结果表明,有机质和脆性矿物的同步富集为裂缝的发育和演化提供了重要的物质基础,其中裂缝趋向于沿着有机质形成的结构面传播,有机质的孔隙结构对裂缝具有较为明显的诱导机制。在外力作用下,内在积累的能量转化为脆性矿物的变形和变位,脆性矿物含量越高者裂缝发育能力越强,裂缝发育程度越高。数值模拟实验呈现了一种由于有机孔隙和脆性矿物所导致的应力集中从而诱发裂缝萌生和演化的整个过程。该研究成果可为深化认识裂缝的成因机理以及压裂施工过程中相关压裂参数的调整提供一定的借鉴与参考。

Organic matter and brittle minerals in"sweet spot"shale play an important role in controlling the development of fractures as well as revealing and grasping the mechanism and law related to fracture initiation,development and evolution,and hence they have a very important engineering practice value for the optimization of fractured intervals and the stimulation and stable production of shale gas.Taking Longmaxi Formation shale in northwest Guizhou as an example,the authors analyzed the mechanism of fracture control related to organic matter and brittle minerals in the "sweet spot"layer,and demonstrated a complete process of fracture formation and evolution in the "sweet spot"layer by numerical simulation experiment.The results show that the synchronous enrichment of organic matter and brittle minerals provides an important material basis for the development and evolution of fractures.The fracture tends to propagate along the structural plane formed by organic matter,and the pore structure of organic matter has obvious inducing mechanism for the formation of the fracture.Under the action of external force,the internal accumulated energy is transformed into the deformation and displacement of brittle minerals.The higher the content of brittle minerals,the stronger the fracture development capability and the higher the fracture development degree.The numerical simulation shows a whole process of fracture initiation and evolution due to the stress concentration at the end of organic pores and brittle minerals.This paper is expected to provide some references for further understanding of fracture mechanism and adjustment of fracturing parameters in the process of fracturing construction.