The formation mechanisms and management techniques of carbonate scale in CO2-rich coalbed methane wells

Abstract

Scale deposition during the drainage of coalbed methane (CBM) can significantly damage both coal reservoirs and wellbore utilities, consequently impairing well productivity. The mechanisms underlying scale formation in carbon dioxide (CO₂)-rich CBM wells, however, remain inadequately understood. This study incorporates laboratory experiments with field production data to systemically investigate the formation mechanisms and management techniques for scale in CO₂-rich CBM wells. Initial investigations involved acid dissolution experiments and X-ray diffraction (XRD) analyses of scale samples from two CBM wells to determine their mineral compositions. The results reveal that carbonate scale is the predominant component, with a minor presence of iron oxide scale. Further laboratory experiments were performed to simulate the formation of carbonate scale and examine its controlling factors. The findings demonstrate that the CO₂ partial pressure significantly influences carbonate scale formation. Specifically, higher initial CO₂ partial pressures enhance the dissolution of CO₂ in the water, leading to the generation of more carbonate ions under alkaline conditions. These carbonate ions then react with calcium ions to form CaCO₃ precipitates upon the release of gas pressure. In contrast, lower initial CO₂ partial pressures result in reduced dissolution of CO₂ and a smaller amount of CaCO₃ precipitate following gas pressure release. Field production data from two CBM wells confirm that gas drainage gradually decreases both bottom-hole pressure and CO₂ partial pressure, thus facilitating significant carbonate scale formation. Based on these insights and outcomes of field acid stimulation operations in a CBM well, this study concludes that maintaining higher CO₂ partial pressure during the initial gas drainage stage and utilizing acid treatments are effective methods for preventing and removing carbonate scale.