Abstract: In the process of coalbed methane drainage, the coal fines in the reservoir will be migrated significantly. This phenomenon is likely to cause the effective circulation space of pores and throats to be blocked, resulting in a significant reduction in reservoir permeability. It has a very adverse effect on the final production of coalbed methane. In order to explore the influence of coal fines migration on the porosity and permeability of cataclastic coal reservoirs, aiming at the process of coal fines start-up, migration and deposition retention in reservoir channels, the pore size distribution characteristics of cataclastic coal were analyzed by low-field nuclear magnetic resonance and low-temperature liquid nitrogen adsorption, a three-dimensional pore network model was established, and a numerical model of migration and sedimentation of coal fines in pore throat channels was constructed. Combined with the previous mechanical model of coal fines start-up and the probability model of particle deposition and blockage in throat, monte Carlo method is used to simulate the migration and blockage of coal fines in reservoir pores. Based on the python language, a numerical simulation program for the migration of coal fines was written to simulate the migration of coal fines in the pore network of broken coal matrix. The variation law of porosity and permeability of cataclastic coal reservoir in the process of coal fines migration and the influence of coal fines migration on porosity and permeability are discussed. Through in-depth analysis of precise numerical models and advanced program simulation techniques, the internal mechanism of the far-reaching influence of pressure difference and coal fines particle size on coal fines output and model permeability is revealed. These two key factors not only play a significant role alone, but also their interaction is more complicated. Specifically, the particle size of coal fines directly affects its migration, sedimentation and output characteristics under different hydrodynamic conditions. Under the condition of low pressure difference and low flow velocity, large particle size coal fines is like a silent giant, which is difficult to be activated and start migration ; however, under the strong drive of high pressure difference and high flow velocity, these coal fines become active, but their migration is more likely to lead to the blockage of effective pores, resulting in a sharp decline in permeability. In addition, the increase of pressure difference is like a double-edged sword. On the one hand, it promotes the increase of coal fines output, on the other hand, it also accelerates the deterioration of permeability. It is worth noting that the settlement position of coal fines migrates to the outlet end with the increase of displacement pressure difference, and the increase of displacement pressure is accompanied by a significant increase in the proportion of small throats. Under the condition that the particle size of coal fines is constant and generally smaller than the throat radius, there is a subtle equilibrium point of the displacement pressure, that is, the threshold value. On both sides of this threshold value, the relationship between the permeability decline rate and the displacement pressure shows a completely different trend. In the drainage and depressurization stage of actual industrial production, the output characteristics and particle size distribution of coal fines have become important indicators for evaluating production efficiency and reservoir permeability changes. With the gradual increase of drainage intensity, the output intensity of coal fines experienced a process from initial slow growth to subsequent rapid decline. At the same time, the particle size distribution of coal fines also reached a maximum, covering a wide range from small particle size to large particle size. This phenomenon is particularly significant in the initial drainage stage. When the drainage intensity is still at a low level, only some small particle size coal fines migrates and outputs under the drive of fluid dynamics. In order to further explore this phenomenon, we used numerical simulation technology to simulate the actual situation of low drainage intensity by setting low pressure difference conditions. In the numerical simulation program, it can be concluded that in the case of low flow rate, small particle size coal fines can indeed migrate and produce, and at the same time, the decline rate of reservoir permeability is relatively gentle. The simulation results are highly consistent with the output of coal fines under the condition of low drainage intensity in actual production, which verifies the accuracy and reliability of numerical simulation technology in predicting the migration behavior of coal fines. Furthermore, the numerical simulation data are compared with the results of physical simulation experiments on the migration of coal fines. The simulation results of the model show that as the simulated drainage continues, the large-size coal fines will preferentially settle in the pores. The settlement of these large particles is like a channel barrier, which effectively blocks the pores and leads to a significant decrease in permeability. At the same time, the settlement probability of small particle size coal fines also increases rapidly. This makes the migration and production of coal fines more and more difficult, which shows that the number of coal fines produced decreases rapidly with time. In addition, the physical simulation experiment of coal fines migration in cataclastic coal reservoir also provides us with valuable reference information. The experimental results show that the decrease of permeability of coal samples is mainly concentrated in the early stage of water flooding, and the higher the fracture density is, the higher the average permeability is. Pores and fissures with a pore diameter greater than 1 000 nm become the main channels for coal fines migration and blockage. The coal fines further leads to a decrease in permeability by migrating and plugging connected pores with a pore size greater than 10 000 nm. This finding is consistent with the numerical simulation results, which further confirms the consistency between the permeability change and the coal fines production in the coal fines migration of the cataclastic coal reservoir. In the simulation program, the high drainage strength is simulated by setting the pressure difference compared with the high pressure difference. When the large particle size coal fines are added, the settlement position of the coal fines begins to increase compared with the low pressure difference, and the output decreases, concentrated at the outlet end. When the small particle size coal fines are added, the output of coal fines increases significantly, and the decrease of permeability of coal fines is reduced compared with that of large particle size coal fines. On the whole, the simulation results of the model are consistent with the output of coal fines after increasing the drainage intensity in actual production. The numerical simulation results show that the larger the particle size of the coal fines, the more concentrated the position of the settlement and blockage of the coal fines to the inlet end, and the smaller the position range of the deposition. When the pressure difference is low, the coal fines is mainly concentrated in the direction of the inlet end. With the increase of pressure difference, the settlement position of coal fines is close to the outlet end, and the range of settlement position becomes larger. In the visual physical simulation of coal fines migration in fractures, the deposition area of coal fines in the fracture throat gradually decreases from the inlet to the two ends of the vertical main migration direction and the main migration direction. In summary, the experimental results of numerical simulation are consistent with the permeability change of broken coal and the settlement of coal fines in the process of coal fines migration in physical simulation experiments.

Key words: coalbed methane, cataclastic coal, coal fines migration, pore permeability, particle size of coal fines, numerical simulation