Cementing and film-forming hydrophobic drilling fluid technology for long horizontal wells in deep coal-rock gas reservoirs

Abstract

Through a systematic analysis of the physical properties of coal and gangue, including microscopic pore structure, surface wettability and mechanical strength, the mechanism of borehole wall collapse in deep coal formations was revealed. Based on this understanding, a wellbore-stabilizing drilling fluid concept was proposed, featuring high-efficiency plugging of medium and large pores and fractures + cementation and film-formation in micro and small pores and fractures + overall surface hydrophobic inhibition. An adaptive plugging agent and a cementing film-forming hydrophobic inhibitor were developed, and a cementing, wall-strengthening, film-forming, and hydrophobic drilling fluid system was established. The adaptive plugging agent consists of organic-inorganic hybrid polymer microspheres, which enables self-adaptive plugging of pores and micro-fractures in coal rock through flexible deformation, effectively preventing direct contact between the drilling fluid and medium-to-large pore-fracture systems in the formation. The cementing film-forming hydrophobic inhibitor contains strong adsorption groups and hydrophobic groups, which provides both cementing reinforcement and dense film-forming functions, significantly enhancing the overall structural strength of coal rock, greatly reducing surface hydrophilicity, and inhibiting hydration swelling of clay minerals. The developed drilling fluid system exhibits favorable rheological behavior, filtration-control performance and lubricity. It can substantially improve the compressive strength of rock samples and markedly reduce their linear expansion rate. Field application results demonstrate that the system delivers excellent anti-collapse, cuttings-carrying and lubrication performance, with outstanding wellbore stabilization effectiveness.