基于线性强化弹塑性疏松砂岩破裂模式

Fracture mode of unconsolidated sandstone based on elastic-plastic with linear strain-hardening

渤海油区疏松砂岩储层埋深较浅,地层天然压实作用较弱,在压裂充填过程中表现出明显的非线性塑性特征,分析和研究疏松砂岩地层压裂过程的破裂模式对于破裂压力的预测和压裂充填施工设计具有重要的实际意义。基于线性强化弹塑性岩石本构力学模型,考虑压裂液渗滤效应,建立了疏松砂岩地层弹性和塑性双区井周应力模型,通过理论分析验证了应力模型的正确性。基于岩石的拉伸破坏和剪切破坏准则,获得了疏松砂岩压裂充填过程中井壁破裂压力预测模型和4种破裂模式,并利用现场数据验证了模型的可靠性。研究结果表明:井壁屈服后塑性区呈椭圆型,且长轴在最大主应力方向;塑性模量系数越大,塑性半径越小,破裂压力越小;屈服应力越大,塑性半径越小,破裂模式由塑性拉伸破坏、塑性剪切破坏向弹性拉伸破坏变化;随内摩擦角和内聚力逐渐增加,破裂压力增加到一定值后不再变化,破裂模式由塑性剪切向塑性拉伸变化。在渤海油区常规疏松砂岩物性范围内,破裂模式主要为塑性剪切破坏。

Unconsolidated sandstone reservoirs in Bohai oil region are characterized by shallow burial depths, weak natural compaction, showing obvious non-linear plastic characteristics during frac packing. Analysis and study of the fracture mode for the fracturing process of unconsolidated sandstone strata has important practical significance for prediction of fracture pressure and construction design of frac packing. Based on the constitutive mechanical model of rock for elastic-plastic with linear strain-hardening, this study establishes a stress model around wells in unconsolidated sandstone strata in elastic and plastic zones while considering the percolation threshold effect of fracturing fluid and verifies the correctness of the stress model by theoretical analysis. Based on the tensile failure and shear failure criteria of the rock, the prediction model of the fracture pressure of sidewall and 4 kinds of fracture modes in the frac packing process of unconsolidated sandstones were obtained, and the reliability of the model was verified using field data. The results show that the plastic zone was elliptical at the post-yield sidewall, and the long axis is in the direction of the maximum principal stress;the larger the coefficient of plastic modulus, the smaller the plastic radius, and the smaller the fracture pressure;the larger the yield stress, the smaller the plastic radius. The fracture mode changes from plastic tensile failure, plastic shear failure to elastic tensile failure;as the internal friction angle and cohesion gradually increase, the fracture pressure does not change after increasing to a certain value, and the fracture mode changes from plastic shear to plastic tensile. In terms of the physical properties of conventional unconsolidated sandstone in Bohai oil region, fracture mode is mainly shown as plastic shear failure.