Experimental study on deep CBM well CO2 fracturing scaling:From microscale SEM observation to macroscale scale Inhibition—Ion polarization, thermal activation, and EDTA formulation

Abstract

In the process of carbon dioxide dry fracturing of deep coalbed methane wells, serious scaling phenomenon occurred in the wellbore, but the wellbore scaling pattern are not clear, the scaling process was simulated indoors by adding saturated NaCO₃ and NaHCO₃ solutions into brines with different mineralisation levels and studied the chelating agent on the scaling process of the role of the law. The influence of formation water ion type, ion concentration, formation temperature, chelating agent EDTA on the amount of scaling and scale inhibition effect was investigated, and the change rule of scale crystals in the process of precipitation of carbonate ions with calcium and magnesium ions and the mode of action of scale inhibition were observed from the microscopic point of view. The results show that: ① the strength of cation polarity will change the scale crystal shape, Ca²⁺ polarization weak tend to form block CaCO₃, Mg²⁺ polarization strong easy to generate needle MgCO₃, both types of crystal deposits tend to aggregate.; ② the ion concentration in the formation is proportional to the amount of scaling. Mineralisation of 100000 mg/L, adding excess saturated Na₂CO₃ solution, the amount of scaling reached 1.084 g, mineralisation of 250000 mg/L, the amount of scaling reached 1.549 g, compared with the former increased by 42.90 %, the mineralisation increases through the solubility product breakthroughs and ionic strength effect significantly increases the amount of scaling, formation water mineralisation increased by 1 %, the amount of scaling increased by 0.286 % on average; ③ Temperature increases alter the state of ions by changing the activation energy. When the temperature reaches 80 °C, the shape of scale crystals changes from irregular spherical (calcium bicarbonate) to regular blocky (calcium carbonate). Compared with magnesium ions, calcium ions preferentially combine with anions to form scale. Under higher temperature conditions, the scale formed at the bottom of the well is mainly calcium carbonate. ④ Chelating agent EDTA can significantly reduce the scale formation by complexing Ca²⁺. In a system with a calcium ion concentration of 15,000 mg/L, the minimum scale inhibition rate is achieved by adding 20 mL of 0.1 mol/L EDTA. However, further increasing the dosage of the chelating agent will cause colloid flocculation and increase precipitation, leading to an increase in the scale formation.