Application study of double hydrophobic film-forming organic salt water-based drilling fluid systems in deep coalbed methane wells: A case study of the Jiatan XX well in Southern Sichuan

China possesses abundant deep coalbed methane (CBM) resources; however, such reservoirs are characterized by highly developed cleat–fracture systems, pronounced matrix heterogeneity, and highly complex pore architectures. These intrinsic geological characteristics tend to induce multiple engineering challenges during drilling operations, including excessive frictional resistance, wellbore instability, and severe reservoir damage. Collectively, these issues significantly hinder the safe, efficient, and large-scale development of deep CBM resources and pose substantial technical challenges for drilling fluid system design.To address these challenges, this study focuses on system construction and engineering adaptability, and develops a dual-hydrophobic film-forming organic brine-based drilling fluid through systematic optimization of key functional additives. The system is designed based on the synergistic regulation mechanisms of dual-hydrophobic lubrication and interfacial film formation, enabling coordinated control over interfacial interactions and fluid microstructure. A comprehensive series of laboratory experiments were conducted to evaluate the rheological properties, lubrication performance, inhibition capacity, as well as film-forming and plugging characteristics of the system. Furthermore, microscopic characterization techniques, including scanning electron microscopy and interfacial analysis, were employed to elucidate the underlying mechanisms. Particular emphasis was placed on revealing the synergistic effects and multi-scale interaction pathways governing friction reduction, wellbore stabilization, and reservoir protection, thereby establishing a mechanistic linkage between interfacial chemistry and macroscopic engineering performance.The results demonstrate that, compared with conventional drilling fluid systems, the developed system exhibits outstanding overall performance, with inhibition performance improved by 73.33%, lubrication efficiency enhanced by 79.62%, and reservoir protection capacity increased by 69.06%. Notably, the system maintains excellent rheological stability and structural integrity under conditions of high mineralization, high hardness, and complex ionic environments, indicating strong resistance to salt and calcium contamination as well as sustained functional reliability.Field application results from Well Jiatan XX demonstrate that the system successfully enabled safe and efficient drilling of a 1125 m horizontal section, with smooth drilling operations and intact wellbore conditions throughout the process. The open-hole section remained stable after 60 days of soaking, with no evident collapse or enlargement observed. Additionally, the system exhibited excellent operational compatibility and process stability in field applications, significantly reducing the risk of drilling complications and enhancing overall construction efficiency.Overall, the proposed system demonstrates strong engineering adaptability and long-term service capability under complex CBM reservoir conditions. By integrating interfacial regulation with structural optimization, the system achieves multi-functional synergistic enhancement. This work provides a novel technical pathway for optimizing drilling fluid systems in deep CBM wells and offers significant potential for reducing drilling costs and improving resource development efficiency in challenging geological settings.