含夹层致密砂岩储层裂缝发育规律研究

Control Action Research on Fracture Development in Tight Sandstone by the Lithologic Interfaces

摘要: 为了探讨空间上的地层岩性界面对构造裂缝发育的影响，以库车坳陷白垩系和古近系致密砂岩储层为例，结合岩心和野外观察，配合力学实验、应力应变关系推导及破裂准则选取，研究软硬相间岩层在不同受力条件下的变形破坏特征，最终采用弹塑性有限元法模拟三维应力分布和裂缝发育规律。结果表明：砂泥岩组合方式与裂缝类型及发育规律关系密切，岩性界面处裂缝可划分为：界面滑移缝、界面终止缝、界面贯穿缝和界面转换缝四种基本类型；厚层均质砂岩中中尺度共轭缝发育，薄层中层间杂乱缝发育，50 cm以上夹层可挡住大尺度缝的延伸；水平受力条件下，泥质夹层厚度和泥/砂厚比控制着裂缝的发育规律，当泥/砂厚比值位于0.1~0.35时，泥岩内裂缝密度达最高值；围压条件下，随应力的增加裂缝密度迅速增大，在300 MPa时达最大值；同等应力条件下，砂岩层越薄越容易产生裂缝，泥岩厚度和应力强度对砂岩裂缝在泥岩中延伸的长度影响很小，即不管泥岩上下砂岩层的厚度如何，泥岩恰好不被穿透的厚度都为3 m左右。

Abstract: Aiming at a discussion of lithologic interfaces controlling on structural fractures, the paper takes the Cretaceous and Paleogene tight sandstone in Kuqa depression as a case study, combing field outcrops, drilling cores, mechanical tests, formula derivation and failure criterion selection etc. The deformation of interbed formations under various stresses and fracture development law was studied through elastic-plastic finite element simulation method. The results show that the frac-tures near lithologic interface can be divided into four types: slip fractures, termination fractures, penetrating fractures and transformed fractures. Generally, the conjugate fractures develop in thick homogenous sandstones, the scattered and mussy fractures develop in thin sandstones among thick mudstones, the more than 50 cm thick mudstones can restrict the large-scale fracture development. Under a certain stress filed, the thickness of interlayer and sand-mud ratio directly control development law of fractures; when the ratio lies between 0.1a and 0.35, the fracture intensity reaches peak. Under certain confining pressure, the fractures rapidly increase along with persistent loading, until 300 MPa, the fracture intensity reaches the peak. When the sandstone thickness reaches 3 m, the thickness variation of sandstone has little influence on fractures development and penetration degree among mudstones.