含纹层陆相页岩力学参数的尺寸效应及其对压裂裂缝的影响

Size effect of mechanical parameters of laminated continentalshale and its impact on hydraulic fractures

济阳坳陷陆相页岩中的含纹层泥质灰岩相页岩具有较好的储集物性,但发育的纹层使储层力学性质具有强非均质性,导致其力学参数的尺寸效应明显。以胜利油田博兴洼陷含纹层泥质灰岩相页岩岩心为例,采用三维有限元数值模拟方法,系统研究岩心力学性质及压裂裂缝的尺寸效应。结果表明:当岩心尺寸小于500 mm时,其单轴抗压强度和弹性模量的尺寸效应显著,不同围压下岩心抗压强度和弹性模量与单元尺寸呈负指数关系,且水力压裂破裂压力随单元尺寸的增大而减小。在博兴洼陷,当单元尺寸为1.5 m时,数值计算得到的破裂压力和改造体积更接近于真实地层,建议在满足计算效率的条件下采用单元尺寸小于1.5 m的单元网格开展模拟设计。

The laminated argillaceous limestone facies shale in the continental shale reservoirs of Jiyang Depression has good physical properties of reservoirs,but the mechanical properties of reservoirs have strong heterogeneity due to the dense laminae,which leads to the significant size effect of its mechanical parameters.The cores of the laminated argillaceous limestone-facies shale in Boxing Sag of Shengli Oilfield were taken as an example for systematic research on the mechanical properties of cores and the size effect of hydraulic fractures by three-dimensional finite-element numerical simulation.The results show that when the core size is less than 500 mm,the uniaxial compressive strength and modulus of elasticity have a significant size effect,and they have a negative exponential relationship with the core size under different confining pressures.The breakdown pressure of hydraulic fracturing decrease with the increase in the unit size.In Boxing Sag,when the unit size is 1.5 m,the breakdown pressure and stimulated reservoir volume(SRV)obtained by numerical calculation are closer to those of real formations.It is suggested that the unit grid with a unit size of less than 1.5 m should be used for simulation design under satisfied computational efficiency.