During the drilling of a horizontal gas well in the North Shaanxi Gas Field, with the extension of the horizontal section length ( the length of the horizontal section is greater than 1500m) and the heterogeneity of the formation, the probability of encountering mudstone, especially carbonaceous mudstone is higher during the drilling process. After drilling into large sections of carbonaceous mudstone in the horizontal section and highly deviated well section, the borehole lining instability occurs frequently resulting in high sidetracking rate, especially in the carbonaceous mudstone of Shanxi Formation of the Upper Paleozoic, with hard texture and micro-cracks and micro-fractures are developed, which can easily cause larger and more carbonaceous mudstone to fall off, which is difficult to be broken with the movable drilling tool, and difficult to be carried out and the wellbore is hard to be clean. It is easy to cause the drilling tool to get stuck in the process of tripping, or even to get stuck in the casing in the process of tripping, resulting in downhole accidents. In the process of vertical well drilling, in the carbonaceous mudstone section of the Shanxi Formation, the borehole stability problem is generally rare. Therefore, after the horizontal section is drilled into a large section of carbonaceous mudstone, it is still difficult to solve this problem only by strictly controlling the performance of the drilling fluid. If the performance of the drilling fluid is strengthened on the basis of sealing and inhibiting properties, with careful and reasonable engineering measures, it can significantly reduce the degree of resistance and jamming of carbonaceous mudstone well sections, and will play a positive role in drilling through large sections of carbonaceous mudstone in horizontal sections, stabilizing the borehole, cleaning the wellbore, and ensuring downhole safety in drilling operations.