Numerical Simulation of Wellhole Stability in Cretaceous Fractured Water-Sensitive Formation in Tarim Basin

The Cretaceous Baxigai Formation and the Shushanhe Formation in the Tarim Basin are fractured water-sensitive formation. Wellbore instability is frequent during the drilling process. It's urgent to clarify the mechanism of the fractured water-sensitive formation, and to guide drilling operations. The formation occurrence is determined based on the formation micro-scanner imaging (FMI). And the digital wellbore model is established. Numerical simulation of the fluid-solid coupling process in the digital wellbore model is carried out by 3DEC to analyze the effects of drilling fluid density, fracture permeability, number of fractures and drilling fluid immersion time on the wellbore stability. The results show that: (1) increasing density is beneficial to improve the wellbore stability at the early stage; (2) with the increase of the fracture permeability, the wellbore state becomes more serious; (3) with the increase of the number of fractures and the increase of the immersion time of drilling fluid, the risk of the wellbore instability increases. The model in this paper can be used to study the dynamic process of wellbore instability in fractured water-sensitive formations, reveals the mechanism of wellbore instability in the Cretaceous mudstone of the Tarim Basin, and it can provide opinions and guidance for drilling. 1. INTRODUCTION With the increasing demand of energy and the exploration of conventional energy, deep well and ultra-deep well drillings are now more widely performed exploratory (Zhang S et al., 2015). However, several problems such as collapse, jamming and leakage are encountered during the drilling process owing to high temperature, high pressure and complex geological conditions, and these wellbore instability increases costs of ultra-deep drilling (Gholami R et al., 2014). In fractured formation, drilling fluids penetrate into formation leads to the reduction of rock strength, decrease in friction angle, especially in water-sensitive formation (Chen X et al., 2002). A large amount of oil and gas is founded in the Ordovician carbonate formation of the Tarim Basin with buried depth of more than 7200m. Some reservoirs are distributed near the faults and it substantially makes formation fractured (Lai et al., 2021). The upper formation named Cretaceous Baxigai formation and Shushanhe formation is mudstone with a thickness of 150m-300m. Wellbore instability occurs frequently in the progress of drilling mudstone formations. Clarify the mechanism of wellbore instability to ensure safe drilling and reduce drilling costs is prompted recent exploration.