基于粒径分类的致密砂岩力学特性及破坏特征研究——以鄂尔多斯盆地二叠系下石盒子组为例

鄂尔多斯盆地二叠系下石盒子组致密砂岩具有物性差、地层压力异常、隔夹层发育等工程地质特征，储层压裂改造力学参数的选取及其破坏特征对油气田实现效益开采具有重要意义。针对下石盒子组致密砂岩，以平行试验为设计思路，开展抗张强度测试及模拟原地状态下的高温高压三轴力学试验，在粒径分类基础上，探索致密砂岩力学特性和变形破坏特征。该区下石盒子组致密砂岩可分为中—细粒岩屑砂岩、粗粒岩屑砂岩以及巨—粗粒岩屑砂岩，粒径越粗则物性越好，其石英含量越高则黏土含量越低；随着粒径由细至粗，力学强度和弹性参数逐渐减小，岩石抵抗变形以及保持结构完整性的能力降低，力学试验变形破坏呈脆性向基质混合型过渡的特征。在相同的地应力及施工作业条件下，岩石类型不同则储层压裂难易及改造效果均有差别，需根据地层实际情况选用不同的工程力学参数。

Mechanical properties and fracturing characteristics of tight sandstones based on granularity classification: a case study of Permian Lower Shihezi Formation,Ordos Basin

Tight sandstone reservoirs of the Lower Shihezi Formation in the Ordos Basin are characterized by poor physical properties, abnormal formation pressure, and the development of high-strength interlayers, etc. Thus, the selection of mechanical parameters and the fracturing characteristics of the reservoirs are significant for the effective development of oil and gas fields. For the tight sandstone samples from the Lower Shihezi Formation in the Hangjinqi area of the Ordos Basin, we carried out parallel experiments to test tensile strength, and to make triaxial mechanical experiments with high temperature and high pressure of in-situ conditions. Based on granularity classification, the mechanical strength properties and fracturing characteristics of tight sandstones were discussed. The tight sandstones in the Lower Shihezi Formation can be classified as medium- to fine-grained lithic sandstones, coarse-grained lithic sandstones, and giant- to coarse-grained lithic sandstones. The coarser the particle size, the better the physical properties, and the higher the quartz content, the lower the clay content. As particle size increases from fine to coarse, the mechanical strength and elastic parameters gradually decrease, and the ability of rock to resist deformation and maintain structural integrity decreases. The fracturing of the samples show the characteristics of brittleness to matrix mixing. Under the same stress and construction conditions, different rock types will result in various reservoir fracturing difficulty and stimulation effects. Different engineering mechanic parameters should be selected according to actual formation conditions.