基于Cohesive单元法的弃置井水泥塞-套管界面胶结失效数值模拟

Numerical Simulation of Failed Bonding between Cement Plug and Casing String in Abandoned Wells Based on Cohesive Element Method

水泥塞-套管界面胶结失效对弃置井井筒完整性提出了严峻挑战。考虑地下流体与水泥塞的流固耦合作用,基于cohesive单元方法,建立水泥塞-套管-水泥环-地层系统三维有限元模型,模拟垂直井水泥塞-套管界面裂缝剥离过程,研究地应力对界面裂缝损伤演化的影响,并分析水平地应力、水泥塞力学参数及界面性质对裂缝剥离高度的影响。结果显示:水平地应力均匀时剥离裂缝沿着整个界面圆周延伸且高度相等,水平地应力非均匀时剥离裂缝倾向于沿着界面某一圆周角扩展且在最大水平地应力方向具有较大的高度;水泥塞弹性模量从30 GPa减小到1 GPa,裂缝剥离高度降低9.3 m,临界法向强度从0.25 MPa增大到2.0 MPa,裂缝剥离高度降低6.5 m,表明较低的弹性模量及较大的临界法向强度有利于减小水泥塞-套管界面胶结失效的风险;水泥塞泊松比从0.35减小到0.10,裂缝剥离高度仅降低2.0 m,临界剪切强度从0.5 MPa增加到4.0 MPa,裂缝剥离高度仅降低3.3 m,表明泊松比和临界剪切强度对界面胶结失效影响较小。建立的模型能够为水泥浆配方优选和井筒弃置工艺优化提供指导。

Failure of bonding between cement plug and casing string in abandoned wells imposes a rigorous challenge to the borehole integrity.A 3D finite element model of cement plug,casing string,cement sheath and formation was established to simulate the stripping process of cement plug-surface of casing in vertical wells,to study the effects of geostress on the evolution of failure of the interface between cement plug and casing string,and to analyze the effects of horizontal geostresses,mechanical parameters of cement plug and the property of the interface on the height of the stripping fractures.The establishment of the 3D finite model takes into account the fluid-solid coupling effect between underground fluids and cement plug and is based on Cohesive element method.It was found in the study that when the horizontal geostresses are uniform,the stripping fractures extend along the entire circumference of the interface and have the same height.When the elastic modulus of the cement plug is reduced from 30 GPa to 1 GPa,the height of the stripping fracture is decreased by 9.3 m.When the critical normal strength is increased from 0.25 MPa to 2.0 MPa,the height of the stripping fracture is reduced by 6.5 m.These data indicate that low elastic modulus and high critical normal strength are beneficial to the mitigation of failure of bonding between cement plug and casing string.When the Poisson’s ratio of the cement plug is reduced from 0.35 to 0.10,the height of stripping fracture is reduced only by 2.0 m.When the critical shear strength is increased from 0.5 MPa to 4.0 MPa,the height of stripping fracture is reduced only by 3.3 m,indicating that Poisson’s ratio and critical shear strength play a minor role in the failure of the bonding.The model established is believed to be instructive to the optimization of cement slurry composition and techniques for well abandonment.