# Numerical Simulation Analysis of Wellbore Stability in Weak Interlayer of Igneous Rock
## 6 Conclusions  
The immersion of igneous cores with weak interlays in drilling fluid has been found to have significant effects on the mechanical properties and stability of well-bores. The results of this study indicate that soaking the cores leads to an increase in Poisson's ratio, while the modulus of elasticity, cohesion, and internal friction angle all decrease to a certain extent, resulting in an overall reduction in rock strength.  
Furthermore, strengthening the weak interlayer has shown potential for improving the stability of the well's perimeter. As the strength of the weak interlayer increases, there is a reduction in the area of plastic deformation, peak deformation extension distance, and peak strain of the weak interlayer. In terms of collapse risk, there is a de-crease in most areas, but an increase in certain specific areas.  
Figure 16: Variation of plastic strain with extension length under different pore pressures.  
Figure 15: Variation of the maximum and minimum principal stress difference with the well circumference angle.  
The permeability of the weak interlayer also plays a role in wellbore stability. A higher permeability of the weak interlayer leads to a larger area of plastic deformation, while the peak deformation extension distance remains relatively unchanged. Only the peak strain and the difference between maximum and minimum principal stresses increase slightly. This indicates that an increase in permeability reduces the stability of the well's perimeter, but to a lesser extent.  
In terms of the horizontal in-situ stress ratio, a greater ratio leads to a rapid in-crease in the 90\({}^{\circ}\) principal stress difference and a substantial rise in the risk of collapse. The area where plastic deformation occurs in the weak interlayer and the peak deformation extension distance remain largely unaffected, But the increased risk of collapse is primarily concentrated in the direction of maximum in-situ stress. Consequently, an increase in the horizontal in-situ stress ratio reduces the stability of the rock sur-rounding the well to some extent.  
Comparing the weak interlayer to the normal formation, both types of abnormal pore pressure (high and low) cause an increase in the area of plastic deformation, peak deformation extension distance, peak strain, and risk of collapse. However, abnormal high pressure has a more significant negative impact on wellbore stability compared to abnormal low pressure.  
This paper presents a unique characterization of the distribution of weak inter-layer in the investigation of wellbore stability. The current setting is idealized, and future research can explore methods to accurately characterize the content of soft and weak inclusions in igneous rocks. This would enable the determination of wellbore patterns in various types of igneous rocks [25; 26; 27; 28; 29]. Additionally, exploring wellbore stability for fractures in three-dimensional space within igneous rocks could be a potential future research direction.  
Conceptualization, D.S. and Z.C.; methodology, N.L.; software, Z.C.; validation, Z.C.; formal analysis, D.S.; investigation, D.S.; resources, N.L.; data curation, N.L.; writing original draft preparation, D.S.; writing review & editing, N.L.; visualization, N.L.; supervision, D.S.; project administration, X.C. All authors have read and agreed to the published version of the manuscript. This research received no external funding.  
Not applicable.  
The data are not publicly available due to privacy.  
The authors declare no conflict of interest.