基于均匀化理论的页岩微观多孔黏土强度特性

STUDY ON STRENGTH CHARACTERISTICS OF MICROPOROUS CLAY IN SHALE BASED ON HOMOGENIZATION THEORY

页岩强度是页岩油气开发所必需的基础技术参数之一,对页岩强度的研究贯穿于钻完井、压裂工艺施工的全过程.常规宏观室内实验存在试样获取困难、耗时较长,受井下工矿制约,地球物理方法获取资料品质欠佳且增加了井下设备卡、埋风险.因此,提出基于均匀化理论评价页岩微观多孔黏土强度的方法,进行多孔黏土组成与力学分析.基于耗散能原理和Drucker-Prager准则,开展了微观多孔黏土的强度与π函数的应变求解分析;讨论黏土颗粒与粒间孔隙的力学特性,建立多孔黏土的均匀化应变能;采用强度均匀化理论构建微观非线性函数模型,建立与多孔黏土组成、摩擦系数、内聚系数等参数相关的均匀化函数模型;基于纳米力学实验、量纲分析和有限元模拟,分析多孔黏土硬度、强度与组成的内在关系.研究结果表明,页岩微观多孔黏土的弹性模量和硬度与黏土堆积密度正相关,当黏土堆积密度一定时,硬度与内聚系数的比值受摩擦系数影响较大,为非线性递增;通过量纲分析和有限元模拟,求解页岩微观多孔黏土关于硬度–强度–堆积密度的π函数,揭示页岩微观黏土矿物的组成与力学性质的关系,为进一步深入研究页岩细观强度参数和宏观强度预测奠定基础.

As one of the basic parameters necessary for shale oil and development,the analysis of shale strength is carried out in the whole process of drilling and hydraulic fracturing.Macroscopic experiments have problems such as sample preparation and time consuming.Limited by downhole conditions,not only the quality of data obtained by geophysical method is not good enough for mechanical analysis,but also it increases the risk of equipment stuck and buried in downhole.In this paper,the strength evaluation method of microporous clay in shale was proposed based on the homogenization theory.The composition and mechanical analysis of porous clay was carried out.Based on dissipative energy principle and Drucker-Prager criterion,the strength evaluation of porous clay was transformed into a solution to the strain of the microscopicπfunction.The mechanical properties of the intergranular pores of clay were discussed and the homogenization strain energy of porous clay was established.The microscopic nonlinear function was constructed based on the strength homogenization theory.A homogenizationπfunction was established in relation to parameters such as the composition of porous clay,coefficient of friction and cohesion.Based on nanomechanical experiments,dimensional analysis and finite element simulation,the intrinsic relationship between hardness,strength and composition of porous clay was evaluated.The results show that the elastic modulus and hardness of microporous clay in shale are positively correlated with the packing density of shale.The ratio of hardness to cohesion coefficient exhibits a nonlinear increase with increasing friction coefficient when the clay packing density is constant.Theπfunction of porous clay with respect to hardness,strength and clay packing density is solved by dimensional analysis and finite element simulation.The composition and mechanical relationship of shale microporous clay are described.It lays a foundation for further research on shale meso-strength parameters and macro-strength prediction.