Collapse Period of Paleogene Mudstone in W Oilfield Based on Hydration Effect of Mudstone

One of the most important elements affecting the mechanical characteristics of rocks is their water content, and ground water in particular is a constant problem for drilling operations. The strength and stiffness of rocks that have been submerged in water for varying duration of time are frequently measured in laboratory tests to quantify this effect. In this study, the collapse period of Paleogene mudstone formation in W Oilfield was investigated based on hydration effect of mudstone. The results showed that: 1) Under soaking in water-based mud for 96 h, the water content increased by 3.78%, the mudstone cohesion decreased by 0.761 MPa and the internal friction angle decreased by 8.757; 2) After soaking in oil-based drilling fluid, the water content rised by 0.78% and the core water absorption was small; 3) The collapse period of W Oilfield Paleogene mudstone in water-based and oil-based drilling fluid ranged respectively between 4–6 days and 6–8 days. This study presents a comprehensive method to investigate the interaction between the mudstone hydration and wellbore collapse that can be replicated in the future on other field sites to evaluate wellbore instabilities.