Rock-fluid Interaction Mechanisms in Deep Coal Rock-parting Formations and Drilling Fluid Countermeasures in the Daji Block

To address wellbore instability in deep coalbed methane horizontal wells in the Daji Block on the eastern margin of the Ordos Basin, systematic analyses were conducted of the core mineral composition, macro- and microstructural characteristics, physicochemical properties, and mechanical properties. The rock-fluid interaction mechanisms in the deep coal rock-parting formations in this block were subsequently identified. The results show that the hydration-induced weakening of the primary cleat-fracture system in the coal rock acts synergistically with hydration-induced microfracture dilation, secondary-fracture development, and clay mineral hydration and swelling in the dark-gray argillaceous partings. The resulting deterioration in the mechanical strength of the coal rock-parting composite formation is the dominant factor controlling wellbore instability in the horizontal sections of coalbed methane wells. Based on the identified rock-fluid interaction mechanisms, a synergistic wellbore-stability strategy integrating “multistage plugging, hydrophobic inhibition, density matching, and precise control” is proposed. A drilling fluid system with strong plugging, inhibition, and supporting capabilities is developed using film-forming and hydrophobic functional additives as the core treatment agents. Laboratory performance evaluations show that all performance parameters meet the engineering requirements. Field application demonstrated that this drilling fluid system enabled the successful completion of Well JS6-3P02. The well had a 2062 m horizontal section, achieved 100% penetration of the coal-seam target interval, and required only 7 d to drill the horizontal section. The horizontal section was drilled to total depth in a single bit run without any downhole complications. It was the first deep coalbed methane horizontal well with a horizontal section exceeding 2,000 m to be completed in the Daji Block. The developed drilling fluid system meets the requirements for drilling coalbed methane wells with long horizontal sections and has good potential for wider application.