储层砂岩破坏特征与脆性指数相关性影响的试验及数值研究

储层砂岩破坏特征及其脆性评价对油气田开采工程具有重要意义,针对胜利油田某油井隔层和油层砂岩岩芯,开展压缩条件下的物理试验及数值模拟。研究结果表明:储层岩芯在三轴压缩条件下的破坏模式以剪切破坏为主,同种围压的隔层岩芯脆性高于油层岩芯,脆性指数B较好地描述了岩芯脆性的变化趋势;分析了储层岩芯主要力学参数与脆性指数B的相关性,即在围压压缩条件下,岩芯峰值强度、扩容点、弹性模量、泊松比、残余强度及残余强度系数随脆性指数B的增加而降低,且用线性关系拟合各参数与脆性指数B的相关性较好;油层砂岩黏土矿物含量大于隔层砂岩是造成其脆性较高的内因,单轴压缩条件下的岩芯断面CT扫描图从细观上反映了2种岩芯脆性破坏特征的不同;在单轴压缩条件下,数值岩芯脆性随弹性模量、压拉比、内摩擦角的增加而增加,随泊松比、残余强度系数的增加而降低,与峰值强度之间的关系不大;在声发射模式上,储层砂岩以前震–主震–后震型和群震型为主,根据物理试验和数值计算结果,总结了脆性指数B与岩芯破裂声发射模式之间的内在关系,即在相同加载速率条件下,低脆性区、中脆性区和高脆性区岩芯声发射模式分别为群震型、前震–主震–后震型及主震型,通过对灰岩试样和不同种类煤体声发射模式的对比,验证了机制讨论的正确性。

Experiment and numerical research on failure characteristic and brittleness index for reservoir sandstone

Failure characteristics and brittle evaluation of reservoir sandstone are of great significance in oil and gas drilling engineering. The sandstone cores from the well of Shengli Oil Field were studied with the developed physical tests under condition of triaxial compression and numerical simulation. The results show that the failure mode of interlayer and reservoir cores under the effect of confining pressure is mainly the shear failure. Under the same level of confining pressure，the interlayer cores are more brittle than reservoir cores. The brittleness tendency can be evaluated by the brittleness index B effectively. Besides，the relevance between the main mechanical parameters of sandstone and the brittleness index B is analyzed as well. Under the condition of the same confining pressure，the peak strength，expansion point，elastic modulus，Poisson?s ratio，residual strength and residual strength coefficients of cores decrease with the increase of brittleness index B. The relationship between index B and parameters is linearly fitted well. The intrinsic factor of low brittleness is the high clay mineral content of the oil layer rock. The microscopic brittle characteristics of two kinds of sandstone are shown in the CT scans under uniaxial compression. In the case that other parameters are fixed，the numerical cores are more brittle with the increase of the elastic modulus，ratio of compressive strength to tensile strength and internal friction angle. However，the brittleness of cores reduces with the increase of Poisson's ratio，residual strength and residual strength coefficient. In addition，there is little relationship between the brittleness of cores and its strength. The modes of acoustic emission are dominant by the pre-main-after shocks and group shocks. The essential relationship between the index B and the acoustic emission modes is summarized based on the results of physical experiment and numerical calculation. Under the condition of same loading rate，the acoustic emission modes in the low brittleness zone，middle brittleness zone and high brittleness zone are swarm shocks，pre-main-after shocks and mass shock. Lastly，the mechanism of acoustic emission modes was verified with those of limestone and different types of coals.