Study on the Mechanism of Wellbore Instability in Complex Shale Formations Considering Fault Influence: A Case Study of Fushan Oilfield

ABSTRACT: Fault is the main controlling factor of wellbore stability in multi-layer complex fault block reservoirs. For a long time, the industry has not clearly understood the mechanism, and the wellbore stability design has not fully considered the fault factors, and the effect of the anti-collapse design of the ground is not expected. Taking Fushan Oilfield as an example, this paper studies the mineral composition, microstructure, physical and chemical properties and rock mechanical properties of the strata in the fault area through experiments. Combined with geological, drilling and logging data, the mechanism of wellbore instability near the fault is clarified, and a four-pressure profile prediction method suitable for heterogeneous strata in the fault development zone is established. The example calculation and analysis can provide reference for drilling and anti-collapse of similar fault fracture zones. The results show that drilling wells parallel to the fault are more likely to lose stability than drilling wells through the fault. The larger the fault distance is, the larger the dip angle is, and the stronger the disturbance is. The bedding plane slip of quicksand port group is the main factor of wellbore instability. 1. INTRODUCTION The Fushan Oilfield is located in the southern part of the Beibuwan Basin and belongs to a typical multi-layered complex fault-block reservoir, with the Quicksand Port Group serving as the main productive reservoir (Zhu et al., 2022). As exploration and development progress, the drilling difficulty of high-angle wells and large-displacement wells has been increasing year by year. Particularly, the Quicksand Port Group frequently encounters downhole complexities such as borehole enlargement, wellbore collapse, and sticking, significantly raising the safety risks and operational costs of drilling into the Quicksand Port Group (Xiong et al., 2022). Therefore, in-depth research into the mechanism of wellbore instability is crucial for the efficient development of oil and gas resources in the Fushan Oilfield. Since the mid-19th century, domestic and foreign scholars have mainly focused on two core aspects: formation mechanics and physical chemistry (Cheatham, 1984; Yun Souley et al., 2015). In the initial stages, emphasis was placed on pure mechanical research, starting from the perspective of the mechanical balance of rock masses, exploring the stress distribution around the wellbore (Chenevert Huang et al., 1993; Germanovich et al., 1965), the three major pressures in the formations (collapse pressure, fracture pressure, and pore pressure) (Ong Jin et al., 1999), rock strength, deformation and failure criteria, as well as constitutive relationships (Zhang Sayers, 2008). The core issue lies in the inability of the original strength of rocks to withstand damaging stresses, leading to instability (Ghazvinian Lu et al., 2013).