The CO2 storage in coal seams at the influence of coal fines migration

Abstract: the pressure of the coal seam decays to a certain value due to the production of CH₄, the production wells are switched to CO₂ injection wells. The injection of CO₂ can improve the CH₄ recovery and realize the CO₂ geological storage.The reverse migration of coal fines produced in the CH₄ development stage can be caused by CO₂ injection, which blocks the pore-thorats and fractures in coal seams and increases the difficulty of CO₂ injection. We carried out experiments on coal fine migration and CO₂ injection and storage at reservoir conditions on the simulated coal seam, which was a composite core composed of different types of coal. We focus on the migration of coal fine in simulated coal seam and the impact on CO₂ storage. The experiment results show that, the permeability of the combined core, which is composed of proppant fractured coal, fractured coal and matrix coal in turn, decreases by 40.6% after being injected with 300ml of coal fine suspension with a concentration of 1g/1L. This is due to the deposition or capture of coal fines during the suspension injection, resulting in surface adsorption, bridging blockage, and direct blockage in the pore space, which seriously damaged the connectivity of the coal pore space. The proppant fractured coal can filter 77.1% of the coal fines in the suspension, and the fractured coal rock can filter the remaining 23.9% of the coal fines. The average CO₂ storage capacity and CO₂ storage efficiency of the composite core increased by 4.47 cm³·g^-1 and 10.8%, respectively after subsequent CO₂ injection into the composite core. The corresponding injection pressure difference also increased by 32.5%, and a CH₄ recovery improvement of 13.6% is obtained.The migration and balockage of coal fines lead to the most significant improvement of CO₂ storage in fractured coal (14.4%), followed by proppant fractured coal (10.3%), and the worst improvement of CO₂ storage in matrix coal (3.4%). The migration of coal fines improves the CO₂ storage effect in fractured coal seams to a certain extent, but increases the difficulty of CO₂ injection, which is not conducive to the CO₂ storage of the reservoir.

Keywords: CO₂ storage, coal seams, coal fines migration, proppant fracture