# Numerical Simulation Analysis of Wellbore Stability in Weak Interlayer of Igneous Rock
## 1 Introduction  
Igneous rocks, or magmatic rocks, originate from the cooling and solidification of magma that either erupts from the Earth's surface or intrudes into the crust. During on-site drilling operations, igneous rocks are susceptible to issues such as block dropping, pipe sticking, and wellbore collapse, thus impacting the safety of drilling operations.  
Current research on wellbore stability in igneous formation has focused on three main aspects: drilling operations, stress distribution in the surrounding rock, and the performance of drilling fluids. [PERSON] et al. [1] utilized real-time logging data and drill cuttings characteristics to explain the factors contributing to complex stresses in mixed igneous and basaltic formation. [PERSON] et al. [2] proposed a method to determine stress levels based on real-time drilling data, deriving in situ stress and pore pressure from drilling fluid logs and conventional logging inversions. They further calculated wellbore collapse and rupture pressure. [PERSON] et al. [3] conducted thermal expansion experiments to understand the impact of temperature on the mechanical and physical properties of igneous rocks. [PERSON] et al. [4] enhanced the formation of the PEM drilling fluid to mitigate the risk of wellbore instability, considering the mechanical properties, hydration characteristics, and mineral composition of igneous rock cuttings. [PERSON] et al. [5] analyzed the causes of wellbore instability resulting from igneous intrusions using well history data and laboratory experiments and proposed an anti-collapse drilling fluid system basedon their findings. Addressing the limitations of traditional wellbore stability model that only considers isotropy, [PERSON] et al. [6] developed a new model that accounts for the effect of weak interlayers. [PERSON] et al. [7] developed a new theoretical scheme for the stress field near the wellbore in coal seam wells, considering the cutting and splitting characteristics of the coal near the wellbore and their impact on distribution. [PERSON] et al. [8] identified the intrusion of high-density drilling fluids into crevices within igneous rocks and the subsequent hydraulic splitting action as causes of wellbore instability. Currently, there are fewer studies investigating the mechanism of wellbore instability in weak interlayer igneous formation, and there is a lack of analysis regarding the specific mechanism of wellbore instability [9; 10; 11; 12; 13].  
The presence of a weak interlayer within the rock mass, exhibiting physical and mechanical properties significantly different from those of igneous rocks, is a characteristic commonly observed in igneous formations. In contrast to existing research on wellbore stability in igneous formations, we used a numerical simulation model that incorporates the weak interlayer and accurately represents its impact on wellbore stability. To study the mechanical properties of rocks affected by immersion in drilling fluid, uniaxial and triaxial compression experiments were carried out to investigate the effect of drilling fluid immersion on the mechanical properties of the rock. Therefore, a stability analysis model for drilling weak interlayer wells in igneous formations was developed and numerical simulations were used for investigating the effects of various factors (such as strength, permeability, horizontal in situ stress anisotropy and abnormal pore pressure) on wellbore stability.