Optimization of drilling system for CBM horizontal wells in Zhengzhuang Block of Qinshui Basin

Zhengzhuang block is located in the southwest of Qinshui Basin. The main gas producing horizons are No.3 coal of Shanxi Formation and No.15 coal of Taiyuan formation. The occurrence characteristics and gas bearing properties of coal seams in the area are quite different both vertically and horizontally. In this study, the main fracture extension direction and gas content of No.3 coal and No.15 coal were analyzed respectively. On this basis, the horizontal well drilling system in this formation was optimized. The results showed: The maximum horizontal principal stress direction of No.3 coal seam should be NE 45°−60°, SW 225°−240°. The maximum horizontal principal stress direction of No.15 coal seam should be NE 60°−75°, SW 240°−255°. The difference of maximum horizontal principal stress direction between No.15 coal and No.3 coal is 15°. No.3 coal seam belongs to extra low sulfur - low sulfur anthracite, the thickness of coal is 5−6 m. No.15 coal seam belongs to medium high sulfur high sulfur anthracite, The thickness of coal seam is generally stable horizontally, generally 4−5 m, The structure of coal seam is complex. The gas content of No.3 coal seam and No.15 coal seam increases slightly from the south to the north of the block, there are relatively rich areas with Northeast Distribution and relatively low gas content areas with small area scattered distribution. The optimization of drilling system was mainly implemented from two aspects: drilling design optimization and drilling process optimization. The optimization of drilling design mainly included: the optimization of drilling design concept, well location and drilling trajectory. The optimization of drilling process mainly included: drilling equipment optimization, drilling fluid system optimization and well trajectory control. Well trajectory control includes precise landing without pilot hole, well trajectory control in curved section, well trajectory control during landing and well trajectory control in horizontal section after landing. The optimized drilling system was used in well zh-l36, and the drilling rate of coal seam reached 99% and the gas production after fracturing reached 40000 m³/d. By using the optimized drilling system, the daily gas production increased by 100 m³/d.