Visualization analysis of coal fine migration and settlement under different types of pore-fracture

The production of solid particles mainly composed of "coal fine" runs through the entire process of coalbed methane development, and the resulting reservoir damage is one of the important factors leading to production capacity decline and increased production costs. At present, the laws and mechanisms of coal fine suspension, migration, and settlement under differential fluid action and different types of pore-fracture constraints are not yet clear, which restricts the continuous and stable discharge and efficient development of coalbed methane. Based on this, this paper took the Baode block, where coal fine production has significantly affected well productivity, as the research object. The millimeter sized model samples of different pore types and their combinations was established using 3D printing technology. With the help of independently developed experimental devices for evaluating coal fine transport and settlement, the mechanism of the influence of differential fluid action on coal fine transport under different types of pores and their combination constraints was revealed. The results indicated that under the same fluid action conditions, the order of coal fine migration from easy to difficult is parallel plate shaped pore-fracture, cylindrical pore-fracture, and thin neck bottle pore-fracture. The combination of cylindrical pore-fracture (inlet end), thin necked bottle pore-fracture, and parallel plate shaped pore-fracture (outlet section) has a better ability to resist the deterioration of reservoir properties caused by coal fine deposition than the combination of cylindrical pore-fracture (inlet end), parallel plate shaped pore-fracture, and thin necked bottle pore-fracture (outlet section). The larger the pore size, the more favorable the coal fine is for transportation and production, but not for sedimentation. The higher the mineralization degree of NaHCO3 solution, the anion OH- generated by hydrolysis increases the negative charge on the surface of coal fine particles, which can alleviate the aggregation effect and facilitate coal fine production. The mechanical and chemical effects of fluid together determine the damage degree of coal reservoir induced by coal fine migration.