Study On The Migration Law Of Coal Powder After Fracturing Of Coalbed Methane Wells

As ranking third around the world, coal-bed methane (CBM) resources in China are very abundant. At present, the coalbed methane is expected to have a broad prospects for development in the development of unconventional natural gas resources field especially in 13th Five-Year Period. It is difficult to be exploited because of its unique state and unconventional reservoir physical properties. Hydraulic fracturing is the key to ensure CBM to be exploited effectively. But, in the development practice, the low efficiency and high cost caused by pulverized coal damage in fracture and pump cannot be ignored.Therefore, the law of migration and damage of pulverized coal in the support fracture will be discussed in this paper. Based on the field practice, the software FLUENT is used to simulate the damage of the coal migration in the full size support fracture and part of the cross fracture during the drainage stage, so as to guide the practice of the field.(1) In this article, based on the physical features of coal seam in south of Qinshui basin, the characteristics of porosity, permeability, mechanics and the development of cleats, such as irregular features and the mechanism of pulverized coal production have been analyzed. Pulverized coal is the result of the interaction of geological and engineering practice, and has a wide particle size distribution. Pulverized coal exist the whole process of coal bed methane development, but there are the characteristics of intermittent coal production. Pulverized coal is mainly composed of inorganic minerals, mainly kaolinite and illite.(2) According to the field monitoring data, the characteristics of support fractures and the damage mechanism of pulverized coal to the support fracture are analyzed. The shape of branching networks with asymmetric distribution is presented in the coal well fracturing. Meanwhile, the larger size of the proppants, the wider range of coal particle size can be discharged. After entering the support fracture, the pulverized coal will be adsorption gathering and bridging plug, which is the main reason of propped fracture damages.(3) In this paper, ANSYS ICEM software is used to establish the model of full-size support fracture model and the partial branch crack model. The FLUENT software is also used to calculate the damage of pulverized coal under different conditions, according to the field constraint conditions. Some understandings can be obtained by analyzing the simulation results:① The cross section of the support crack is divided into migration area, deposition area and vacant area. The smaller the particle size is, the smaller the area of the deposition area and the vacant area are. Moreover, the volume fraction of pulverized coal increases along the migration direction; (2) The pulverized coal damage of main fracture is greater than branch cracks o Meanwhile, when a large number of coal remain in supported fractures, the damage of pulverized coal can be reduced by fluid, to a limited extent. There is a longer time required for the discharge of pulverized coal, as the affected areas increased more slowly because of the smaller proppant size, the bigger size of pulverized coal and the higher concentration of pulverized coal at the entry. ③The filling rate and permeability damage rate increase with the increase of crack branching number, with the increase of the concentration of pulverized coal at the entry, with the decrease of proppant size, while decrease with the decrease size of coal (from 80 mesh reduced to 140 mesh). Among the factors, the "branch number" has great impact on filling rate and the proppant size dose the same to the permeability damage.(4) According to the pulverized coal damage characteristics of the different fractures it is suggested that the pulverized coal dispersing agent is added in the fracturing or flowback process to prevent the small coal from gathering. At the same time, the main proppants are advised to use the 20/40 meshes proppants and a certain amount of small diameter proppants can be pumped in the front of fracturing, so as to prevent the pulverized coals in deep coal seam from entering the propped fractures and avoid the connection of soft coal seam in the fracturing which would reduce the damage of pulverized coal to propped fractures, with better fracturing effect.