A Study of the Collapse of Fractured Sandstone Strata Characterized by High In-Situ Stress Differences in Deep Formations

ABSTRACT: Under the condition of high in-situ stress differences, the issue of wellbore instability in fractured sandstone formations is prominent. Through rock mechanics experiments combined with the theory of weak planes, a quantitative study on the mechanical characteristics of fractured sandstone and the collapsing area around the wellbore was conducted. The results indicate that the matrix strength of tight sandstone is high, but the presence of fractures increases the likelihood of failure in fractured sandstone, resulting in a reduction of its overall strength to 14% of the matrix strength. Under vertical well conditions, compared to uniform horizontal in-situ stress conditions, the risk of fracture instability caused by high horizontal stress differences increases. However, when the fracture orientation aligns with the direction of maximum principal stress, the risk of fracture instability is the lowest. When formations develop fractures with low to moderate dip angles, drilling in the direction of maximum horizontal stress can lead to a larger collapse area for wells with moderate inclinations; if drilling in the direction of minimum principal stress, the maximum collapse area decreases as the fracture angle increases. The study results have certain guidance significance for the safe and efficient development of deep oil and gas resources. 1. INTRODUCTION The deepest oil and gas wells today have reached depths of up to 9000 meters. The successful exploitation of ultra-deep oil and gas resources critically depends on advancements in petroleum engineering and rock mechanics (Xie et al., 2021). However, the geological conditions of deep formations are complex, and the three high characteristics (high temperature, high pressure, and high H2S content) of formations lead to prominent wellbore instability issues (Han et al., 2021). For instance, due to the influence of natural fractures, the safe drilling density window is narrow, and situations where wellbore collapse and leakage coexist may occur. Therefore, wellbore instability in fractured sandstone formations under high in-situ stress differences is one of the key scientific issues that deep drilling engineering needs to consider.