Research on the mechanism of wellbore instability and countermeasures of deep coal seam horizontal wells in the Ordos Basin

In response to the complex accidents such as wellbore collapse that are prone to frequently occur during the drilling process of deep coal seam horizontal wells in the Ordos Basin, which constrains the drilling speed and increases the difficulty in drilling operations. Based on the case studies of collapsed wells, the analysis of coal cleats, fractures, and mechanical properties, etc. was carried out, which reveals that the cleats and fractures development of coal rock reservoirs are small but strongly connected, enabling the drilling fluids invasion. The difference in mechanical properties between coal rock and coal seam gangue is significant, and the coal fragmentation dramatically reduces support for partings, intensifying their collapse. Thus, the wall-strengthening and plugging capability of drilling fluid are crucial for coal seam collapse prevention, which has been supported by the comparison of logging interpretations from two typical wells. A deep coal seam anti-collapse drilling fluid system was developed by optimizing the nano polymer sealing agent with a "nuclear membrane-like" structure as the anti collapse agent, based on the core sealing concept of "rigid temporary plugging + flexible sealing". Laboratory experiments have shown that after adding 4% nano anti-collapse plugging agent to the base slurry, the medium filtration loss decreased from 27 mL to 12 mL, and the compressive strength of the rock core soaked in deep coal seam anti-collapse drilling fluid increased by 55.8% compared to the base slurry. The field trials in test wells was successfully completed without any incidents such as pipe sticking. This research provides technical support for the efficient and safe development of coalbed methane in the Ordos Basin.