砂岩双轴压缩应力状态下的拉伸强度的实验研究

Experimental study on tensile strength of sandstone under biaxial compressive stress

岩石材料的拉伸强度对于井壁稳定校核计算是一个比较重要的基础参数。一般的,对于金属材料,可以通过拉伸试验机来直接获得抗拉强度;但是对于岩土类材料,这种方法从第一步制样上可行性就较差。在实验室内获得岩石抗拉强度最常用的方法是巴西劈裂法,这是一种间接获得岩石单轴抗拉强度的方法,目前被ISRM(国际岩石力学协会)推荐并得到了广泛的应用。但这种方法也存在一定的局限性:首先,由于无法还原岩石的初始压实状态,因此在校核超深层岩石的拉伸强度时会有偏差;此外,这种方法的前提假设是岩石在线弹性阶段过后直接发生脆性破坏,这与砂岩实际变形规律不符。基于以上问题,本文从屈服理论出发,通过双轴压缩实验的方法确定了砂岩在地下的真实拉伸屈服强度以及极限拉伸强度,实验表明砂岩的屈服强度随围压增大而增大,符合岩石材料越致密强度越高的普遍认识。通过拟合外推可以得到等效的砂岩单轴拉伸屈服强度,与巴西实验数据结果非常接近,验证了这种方法获得带围压下砂岩拉伸屈服强度的可靠性。通过研究双轴压缩应力状态下砂岩的拉伸强度,可以为卸载工况下砂岩发生拉伸破坏提供更准确的校核依据,为砂岩井壁稳定提供理论与实验支撑。

The tensile strength of rock material is an important and basic parameter when it comes to wellbore stability.In general,for metal,we can obtain the tensile strength in a direct way–the Tensile Test.However,for geotechnical materials,this method is impractical from the first step of sample preparation.Generally,the most commonly used method to obtain the tensile strength of rocks in the laboratory is using the Brazilian Test,which is an indirect approach.Now,this method is recommended by ISRM and has been extensively accepted by the whole community.However,this method has certain limitations.First,since it cannot restore the initial compacted state of the rock,there may be deviations when determining the tensile strength of ultra-deep rock formation.Additionally,the underlying assumption of this method is that the rock undergoes brittle failure directly after the linear elastic stage,which does not align with the actual deformation behavior of ultra-deep sandstone.Based on the problems mentioned above,in this paper,the real tensile yield strength and ultimate tensile strength of sandstone are determined through Biaxial Compression Tests.The real tensile yield strength and ultimate tensile strength of underground sandstone can be determined by a Biaxial Compression Test.The results show that they both increase along with the confining pressure,which accords with the common understanding that the denser the rock material,the higher the strength.The equivalent uniaxial tensile yield strength of sandstone can be obtained by fitting-extrapolation,which is very close to the results of Brazilian Tests,verifying the reliability of this method.By studying the tensile strength of sandstone under biaxial compressive stress,it can provide a more accurate basis for checking tensile failure under unloading conditions,as well as the theoretical and experimental support for the stability of sandstone wellbores.