## 4 Finite Element Numerical Simulation of Casing Damage Well
### Von Mises Stress Analysis of Casing under Mudstone Creep
#### 4.2.2 Finite Element Analysis of Casing under Mudstone Creep  
1. Establishment of finite element model  
Select one oil production well and one water injection well in the G155 block, with a model length of 300 m, width of 100 m, and height of 100 m. Considering the influence of structural morphology and water injection pressure on the casing, study the variation characteristics of the casing bearing stress under different injection production ratios. The finite element model established using ABAQUS is shown in Figure 11, and the mechanical parameters are shown in Tables 2 and 3. The model consists of 28,365 elements and 18,933 nodes, including 15,749 octahedral elements with eight nodes. Normal stress analysis  
In the process of oilfield water injection development, an increase in the injection production ratio means that the formation absorbs more water while the produced liquid is less, resulting in an imbalance between injection and production. At the same time, a large amount of injected water infiltrates the mudstone layer, causing it to creep and further exacerbating the damage to the casing. In oilfield development, the normal stress experienced by the casing represents the horizontal tension or compression of the casing. Due to the presence of non-uniform ground stress in underground rock layers, the normal stress around the casing exhibits a non-uniform elliptical distribution, which can be approximated by a cosine function. As the stress gradually increases and exceeds the yield limit of the casing, it will cause deformation and damage to the casing. The variation pattern of the normal stress contour under different injection production ratios is shown in Figure 12. From the graph, it can be seen that S11 is the normal stress in the \(X\)-axis direction, and S22 is the normal stress in the \(Y\)-axis direction. The normal stress in the S11 direction is significantly greater than that in the S22 direction. This is because the casing has undergone necking deformation, and the \(X\)-axis direction is the short axis of the ellipse, with a large amount of deformation and resulting stress. Comparing the stress cloud trends of oil wells and water wells, it can be seen that the normal stress of the oil well casing is greater than that of the water well casing. Therefore, the oil well needs to break before the water well. At the same time, as the injection production ratio increases, the normal stress on the casing gradually increases. Shear stress analysis  
Due to the injection of water entering the mudstone layer along the perforation section or corresponding layer fractures, mudstone creep occurs, while the strength changes in the non-mudstone layer after encountering water are different from that of the mudstone layer. Therefore, a stress difference is formed between the mudstone layer and the non-mudstone layer, resulting in shear stress. We simulated the changes in shear stress borne by the casing under different injection production ratios, as shown in Figure 13. It can be seen that S12 is the shear stress in the \(X\)-axis direction, and S13 is the shear stress in the \(Y\)-axis. As the injection production ratio increases, the water content of the reservoir increases, and the shear stress generated by the creep force on the casing increases. When the injection production ratio is greater than 1.3, the shear force on the oil well casing exceeds the yield limit of the casing, causing the casing to gradually undergo shear deformation. When the injection production ratio is 1.9, shear stress also begins to appear near the water well, exceeding the yield limit of the casing and causing deformation. Vertical stress analysis  
Vertical stress indicates that when water infiltrates the mudstone layer, it not only undergoes creep, but also expands, causing the formation to rise and causing damage to the casing due to the stretching of the rock layer.