Research on the debugging of ultra-low density cement slurry foundation formula for CCUS coalbed methane wells

Cementing operations for coalbed methane (CBM) wells differ from those for conventional natural gas wells. The challenges of CBM well cementing are mainly reflected in the following aspects: Under low-temperature and low-pressure conditions, cement slurry exhibits slow strength development; micro-particles in the cement slurry tend to fill and block cleats and fractures in coal seams; wellbore stability is poor; and coal seams are characterized by a low-pressure and multi-pressure system, with poor regularity in the vertical distribution of formation pressure coefficients, making pressure balancing during cementing difficult. To improve the cementing quality of CBM wells, a design concept for a low-density cement slurry system suitable for CBM well cementing was established based on the characteristics of CBM wells. Through the screening of admixtures, an ultra-low-density cement slurry system for CBM well cementing was developed. Indoor experimental evaluations of the formulated system show that it has excellent sedimentation stability with zero free fluid, good rheological properties, and a filtration loss of less than 50 ml. Its thickening time can be adjusted according to actual operational needs, and it generally exhibits right-angle thickening behavior. Under medium- and low-temperature conditions, the early compressive strength of the set cement reaches over 14 MPa within 24 h, with a favorable static gel strength transition time and excellent gas channeling resistance. All properties meet the requirements for field operations, and the basic formula for the anti-corrosion ultra-low density system required for later research on CCUS coalbed methane wells is determined.