致密储层岩石应力各向异性与材料各向异性的实验研究

Experimental study on stress-related and matrix-related anisotropy in tight reservoirs

本文在实验室中对四组来自不同地区的致密砂岩和页岩的波速和波速各向异性开展了研究.结合扫描电镜分析和波速应力敏感性测试,分析了引起致密砂岩和页岩各向异性的主要影响因素及规律.研究结果:(1)层理和微裂隙是引起致密砂岩各向异性的主要影响因素,而页岩的各向异性主要由定向发育的矿物成分导致,本研究中所用的致密砂岩各向异性强于页岩的各向异性.(2)平行地层方向的岩石波速高于垂直地层方向的岩石波速,超声波速随着应力增加而增加(3)波速各向异性随应力的变化分为两阶段,第一阶段随应力增加而减弱的应力各向异性,与定向排列的微裂隙相关.第二阶段随应力不再发生变化的材料各向异性,与基质中的矿物成分相关.(4)利用体积应变获得的裂隙孔隙度,以及矿物成分含量验证了这两类各向异性,应力各向异性与裂隙密度成正比,材料各向异性与黏土矿物含量线性相关.(5)根据波速各向异性随应力变化的两阶段,可以定量区分裂隙和基质(矿物)对岩石各向异性的影响,研究结果对体积压裂的裂缝扩展具有重要的意义.

The wave velocity and wave velocity anisotropy of four groups of tight sandstones and shales from different areas were studied in the laboratory.Combined with SEM analysis and wave velocity stress sensitivity tests,the main influencing factors of anisotropy of tight sandstone and shale were analyzed.Results show that(1)bedding and microcracks are the main factors leading to the anisotropy of tight sandstone,while the anisotropy of shale is mainly caused by the directional development of mineral composition.The anisotropy of tight sandstone used in this study is stronger than that of shale.(2)The wave velocity of the rock parallel to the formation is higher than that of the rock perpendicular to the formation,and the ultrasonic velocity increases with the growing stress.(3)The variation of wave velocity anisotropy with stress can be divided into two stages.One is the stress-related anisotropy weakened with the increase of stress,which is related to the directional arrangement of microcracks.The other is the matrix-related anisotropy does not changing with the stress,which is related to the mineral composition in the matrix.(4)These two kinds of anisotropy are verified by the crack porosity obtained using volume strain and mineral content.The stress-related anisotropy is directly proportional to the crack density,and the matrix-related anisotropy is linearly correlated with the clay mineral content.(5)According to the two stages of wave velocity anisotropy changing with stress,the influence of microcracks and matrix on rock anisotropy can be quantitatively distinguished.These research results are of great significance to understanding the fracture propagation in hydraulic fracturing.