Damage mechanism of CBM reservoirs and double-layered screen pipes in the horizontal well completion

Coal reservoirs have the characteristics of high adsorption, low permeability, easy compression and easy breakage. At the drilling and completing stage of horizontal coalbed methane wells, coal reservoir damage often occurs and leads to a significant reduction in coal seam permeability, thereby resulting in single well production. Through experimental analysis, the solid phase particles in drilling and completion fluids for horizontal well and the solid phase components induced by fluid flow are found to block the micro-fractures in the coal reservoir, and the damage mechanism for coal reservoir is elucidated to the drilling and completion reservoir of the horizontal well of coal-bed methane. On this basis, the completion technology for horizontal well is proposed with double-layered screen tube to analyze the mechanism of mitigating the coal reservoir damage, the mud cake of horizontal well wall drilling fluids is eliminated by the hydraulic impact of the flushing pipe, the micro-fissure pulverized coal near the well wall is removed, and the stress and permeation effect is generated in the near well zone. Compared with the conventional multi-branch horizontal wells and segmented fracturing horizontal wells of coalbed methane, the stable daily production and stable production period have been greatly improved with the double-layered screen pipe in the completion of horizontal wells. The application of this technology in some areas of the southern Qinshui Basin shows that the average daily output of No.3 coalbed methane well has increased to 20 000 m³. Moreover, a significant breakthrough has been made in the development of No.15 coalbed methane with the average daily output increasing to more than 10 000 m³. The research results have important theoretical and practical significance for reducing reservoir damage during the drilling and completion of coal-bed methane horizontal wells and enhancing the production of coal-bed methane wells.