海陆过渡相页岩力学特征及破坏模式——以鄂尔多斯盆地东缘二叠系山西组为例

Mechanical characteristics and failure mode of marine-continent transitional facies shale:Case study of Permian Shanxi Formation in the eastern margin of Ordos Basin,NW China

鄂尔多斯盆地东缘二叠系山西组是目前中国页岩气勘探开发的重要接替层位,研究储层页岩力学破坏特征是页岩地质精细评价和后续压裂施工方案制定的基础内容。在对山西组页岩样品开展矿物组成分析、沉积构造特征描述、岩石力学测试的基础上,明确了主要岩相类型及其力学特征,分析了储层岩石力学破坏特征的影响因素。结果表明:山西组页岩主要矿物为石英和黏土质矿物,可划分为硅质页岩、硅质黏土质页岩和黏土质页岩,储层页岩具有力学强度变化范围大、整体低值的特点。受控于岩相的影响,硅质页岩和硅质黏土质页岩力学强度和弹性模量强于黏土质页岩。纹层、充填裂纹以及生物碎屑等结构和构造,增大了力学非均质性,也容易影响裂缝的传播路径。巴西圆盘劈裂实验表明山西组页岩纹层和泥质条带对裂缝具有极强的捕获作用,容易导致裂缝沿着纹层和泥质条带扩展。围压降低了页岩的脆性,使页岩由劈裂破坏转换为剪切破坏,并伴随着充填裂纹和泥质条带的破坏,呈现出破坏形态由沉积构造和围压双重控制的模式,复杂的破坏模式有利于减弱页岩气开发过程中储层裂缝网络渗透率的应力敏感性。研究成果为后续水平井布井选层和压裂施工方案制定提供了借鉴。

The Permian Shanxi Formation in the eastern margin of the Ordos Basin is currently an important superseding stratum for the exploration and development of shale gas in China.The failure characteristics of rock mechanics are the basic content of the research for the fine evaluation of shale geology and the formulation of subsequent fracturing construction plans.Based on the mineral composition,sedimentary structure and rock mechanics test of shale samples,the main lithofacies types and their mechanical characteristics are clarified,and the factors affecting the rock mechanical properties are analyzed.The results show that the dominant minerals in the Shanxi Formation shale are quartz and clay minerals.Based on this,the shale can be divided into siliceous shale,siliceous clay shale and clay shale,with the characteristics of wide range of mechanical strength and of low value as a whole.Controlled by the lithofacies type,siliceous shale and siliceous clay shale have stronger mechanical strength and elastic modulus than clay shale.Structures such as laminae,filling cracks,and biological debris increase the mechanical heterogeneity and easily affect the propagation path of the cracks.The Brazilian disk splitting experiment shows that the shale lamina and argillaceous bands of Shanxi Formation have a strong trapping effect on fractures,and it is easy to cause the fractures to expand along the lamina and argillaceous bands.Confining pressure reduces the brittleness of shale,which converts shale from splitting failure to shear failure,accompanied by the destruction of filling cracks and argillaceous bands.It presents a mode in which the damage morphology is controlled by both sedimentary structure and confining pressure.The complex failure mode is beneficial to reduce the stress sensitivity of the permeability of the reservoir fracture network during the shale gas development process.The research results provide a reference for subsequent horizontal well layout selection and fracturing construction plan formulation.