Instability mechanism and anti-sloughing drilling fluid technique for deep coal seam of Tarim basin

In view of the complicated problems of drilling fluid leakage and hole collapse during the drilling of deep coalbed in Tarim basin, the microstructure and physical and chemical properties of coal were tested by vitrinite reflectance analysis, X ray diffraction and scanning electron microscope, and the mechanism of coal seam instability was confirmed. The fixed carbon content of the coal exceeds 60%, the vitrinite reflectance is greater than 1.1%, and coal have high degree of coalification, is brittle and easy to break. The total amount of clay mineral is less than 25%, and there is no strong water-sensitive mineral montmorillonite in clay. The cleat of coal is reticulated, whose width is mainly concentrated in 1-10 μm, easy to leak off. The instability type of coal and rocks was judged comprehensively as mechanical instability. Taking the microstructure of coal as the object and combining with the size distribution of drilling fluid materials, the asphaltic blocking agent was optimized and the asphaltic polysulfonate anti-collapse drilling fluid for coal seam was designed. Its performance test shows that the asphaltic polysulfonate anti-collapse drilling fluid has low filtration loss and thin mud cake, can effectively block most of coal cleats, reduce drilling fluid invasion, and prevent coal seam collapse. The research results can provide technical support for safe and efficient drilling in the deep coal seam in Tarim basin.