The mechanism and countermeasures of inclined well wellbore instability in Dibei deep coal seam

Abstract

AbstractThe wellbore instability of the deep coal seam has always been a difficult problem limiting drilling engineering in Dibei area of Tarim basin, China. Therefore, the mechanisms of wellbore instability and anticollapse countermeasures in the Dibei deep coal seam are investigated in this study. Firstly, the microstructure and physical and chemical properties of coal and rock are studied to analyze instability mechanism. In addition, orthogonal cleat model and 3 Dimension Distinct Element Code (3DEC) model were proposed to calculate the collapse pressure of the coal seam. And we propose a method for optimizing well trajectory based on wellbore stability. Finally, by optimizing plugging material, an anti-collapse drilling fluid for coal seams is designed and applied in the field. The results indicate that there are many cleats and no montmorillonites in the clay minerals present in coal rocks, indicating that the deep coal seam in Dibei is mainly mechanically unstable. 3DEC model is more accurate, and the orthogonal cleat model can reflect the collapse pressure distribution more intuitively. The reliability of these two models is demonstrated by the actual drilling fluid density of the DB-X01. Furthermore, this study confirms that the blocking-material system of the wellbore stabilization strategy in a coal seam is 3% sulfonated asphalt, 3% emulsified asphalt, and 3% calcium carbonate based on microstructure characteristics. The results of rolling recovery rate and rheology experiments show that the new anti-collapse drilling fluid has better sealing and inhibiting properties on coal seams. Field testing in DT2 well demonstrated the reliability of the collapse pressure prediction model and anticollapse countermeasures proposed in this study. The findings of this study can help for better understanding of wellbore instability mechanism and countermeasures in coal seam. The study is useful for the safe and efficient development of the coal seams in the Dibei area.