Optimization and application of sand bailing technology for coalbed methane horizontal wells

Objective and Methods Coalbed methane (CBM) horizontal wells face challenges such as sand accumulation in completion strings, pipe wall scaling, and pump sticking, leading to declined CBM production or shutdown of these wells. This severely restricts continuous, stable CBM production. Removing pulverized coals and sands from wellbores through mechanical sand bailing is recognized as an effective technique to restore and improve the CBM production of horizontal wells. To address challenges such as poor string sealing performance, low efficiency, and sand consolidation in mechanical sand bailing, this study created optimized tools including claw-shaped scraper tips, sand-breaking drill bits, eccentric flaps, and mechanical rotating tools. By introducing hydraulic additional axial force into the mechanical model of the sand bailing string based on the working principle of pumps for mechanical sand bailing in horizontal wells, this study established a string-hydraulic coupling mechanical model. Accordingly, this study proposed a method for optimizing sand bailing operation parameters. Through engineering application, this study analyzed the impacts of the single-stroke reciprocating cycle of the sand bailing pump on the fluid flow velocity in sand storage pipes and pulverized coal migration, as well as the impact of the opening degree of the flap’s suction port on hydraulic additional axial force. Additionally, this study compared and analyzed the axial forces calculated using the string- hydraulic coupling model and a conventional mechanical model. Results and ConclusionsThe results indicate that claw-shaped scraper tips improve sand-breaking efficiency at the horizontal well bottom through structural optimization. The mechanical rotating tool, combined with sand-breaking drill bits and clip springs, effectively addresses the challenge of consolidated sand removal. Furthermore, the eccentric flap and ball-type check valve enhance the area of solid-liquid phases flowing through the flap and the string sealing performance. Under a single-stroke reciprocating cycle of 60 s, the ø110-mm sand bailing pump can transport coal debris with particles diameters ≤ 2.5 mm and a small quantity of coal debris with particle sizes ranging from 3 mm to 12 mm, thus effectively transporting pulverized coals accumulating within wellbores into sand storage pipes. Compared to the conventional tripping, the axial forces of the sand bailing string increased by 906.94 N and decreased by 37.41 N, respectively under upward/downward conditions of the sand bailing pump, suggesting improved calculation accuracy of forces applied to the string. Field applications to 10 horizontal wells in the Qinshui Basin demonstrated that optimized tools and technologies increased single-well and per-operation sand bailing volumes by 1.5 times and 0.9 times, respectively, with the gas and water production rising by 1 483.7 m³/d and 2.18 m³/d, respectively. The optimized tools and technologies provide technical support for stable high-yield CBM horizontal wells. They apply to horizontal and highly deviated wells with screen or perforated casing completions in coal seams at medium-to-shallow depths, holding great practical significance for efficient CBM exploitation.