The 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.

Horizontal wells are increasingly being utilized in the exploration and development of oil and gas resources. However, the high temperature that occurs during drilling processes leads to a number of problems, such as the deterioration of drilling fluid properties and borehole instability. Therefore, the insight into heat transfer behaviors in horizontal wells is certainly advantageous. This study presents an integrated numerical model for predicting the temperature distribution during horizontal wells drilling considering the effects of drill pipe rotations, and hydraulic (i.e., circulating pressure losses) and mechanical frictions. A full implicit finite difference method was applied to solve this model. The results revealed that the mechanical frictions affect more on wellbore temperature variation than the effects of heat transfer intensification and circulating pressure losses; Moreover, the drilling fluid temperature was found higher than the stratum temperature at horizontal section, the temperature difference at the bottom hole reached up to 16 °C if pressure drops, heat transfer strengthened by rotations and mechanical frictions were all taken into account. This research could be utilized as a theoretical reference for predicting temperature distributions and estimating risks in horizontal wells drilling.

The gravity displacement gas kick often occurs due to the fractures developed in fractured carbonate formation. The improper control of the gravity displacement gas kick is prone to blowout accidents. In this study, the gravity displacement gas–liquid two-phase flow law was simulated by CFD software, and the variation of fracture gravity displacement rate under two boundary conditions was obtained. Besides, the simplified gravity displacement models were established for fractures with two boundaries based on the gas–liquid two-phase flow theory. The analysis results of the influence factors related to the gravity displacement gas kick revealed that the gravity replacement rate of a fracture with an inclination angle of 90° was almost three times that with an inclination angle of 30°. When the wellhead backpressure was not applied, the gravity replacement rate was 0.35 m³/h; while, after the wellhead backpressure of 12 MPa was applied, the gravity replacement rate decreased to 0.22 m³/h. When the consistency coefficient was larger than 1.0 Pa sⁿ, the fluidity index was larger than 0.6. In conclusion, the gravity displacement gas kick will inevitably occur under large inclination closure fractures. The larger the inclination and opening of the fracture, the smaller the wellhead backpressure, and the more serious the gravity displacement gas kick. During field operations, the gravity displacement overflow can be alleviated under low density and high viscosity drilling fluid by applying wellhead backpressure.