Abstract:

During coalbed methane (CBM) production, coal fines within the reservoir can migrate, potentially blocking pore throats and resulting in a significant reduction in reservoir permeability. This process adversely affects the final CBM yield. To investigate the influence of coal fines migration on the porosity and permeability of cataclastic coal reservoirs, this study focuses on the processes of fines initiation, migration, and deposition within reservoir channels. The pore size distribution characteristics of cataclastic coal were analyzed using low-field nuclear magnetic resonance and low-temperature liquid nitrogen adsorption experiments. Subsequently, a three-dimensional pore network model was constructed, and a numerical model for coal fines migration and deposition in pore throat channels was developed. By integrating with an existing mechanical model for coal fines initiation and a probabilistic model for particle deposition and throat blockage, the Monte Carlo method was used to simulate the migration and blockage of coal fines within reservoir pores. A numerical simulation program written in Python was developed to simulate coal fines migration within the pore network of the cataclastic coal matrix. The variations in porosity and permeability of the reservoir during coal fines migration, as well as the influence of this migration, were discussed. The analysis revealed the internal mechanisms by which pressure difference and coal fines particle size influenced coal fines output and model permeability. Both factors were significant, and their interactions were complicated. Specifically, the particle size of coal fines directly affected their migration, deposition, and output characteristics under different hydrodynamic conditions. Under low pressure difference and low flow velocity, large coal fines particles were difficult to mobilize and initiate migration. In contrast, under high pressure difference and high flow velocity, these particles became mobile but were more likely to block effective pores, resulting in a sharp decline in permeability. In addition, an increase in pressure difference had dual effects. It promoted coal fines output but also accelerated permeability decline rate. As the displacement pressure difference increased, the deposition location of coal fines shifted toward the outlet end, accompanied by a higher proportion of small throats. When the coal fines particle size was constant and smaller than the throat radius, a displacement pressure threshold was observed. On either side of this threshold, the relationship between the permeability decline rate and the displacement pressure showed distinct trends. During the drainage and depressurization stages of actual CBM production, the output characteristics and particle size distribution of coal fines served as important indicators for evaluating production efficiency and reservoir permeability changes. As drainage intensity gradually increased, the output intensity of coal fines experienced an initial slow growth followed by a rapid decline. Simultaneously, the particle size distribution of the produced coal fines reached its widest range, encompassing sizes from small to large, particularly during the initial drainage stage. When the drainage intensity was low, only small coal fines particles were mobilized and produced by fluid flow. To further investigate this, numerical simulations were conducted to replicate low drainage intensity conditions by setting a low pressure difference. The simulation results indicated that under low flow rates, small coal fines could indeed migrate and be produced, accompanied by a relatively gentle decline in reservoir permeability. These results were consistent with field observations under low drainage intensity, confirming the accuracy and reliability of the numerical simulation in predicting coal fines migration behavior. Furthermore, the numerical simulation results were compared with those from physical simulation experiments on coal fines migration. The model simulation results showed that as drainage continued, large coal fines particles settled preferentially within the pores. The deposition of these large particles formed channel barriers, blocking pore throats and significantly reducing permeability. Simultaneously, the deposition probability of small coal fines also increased rapidly. This made subsequent migration and production of coal fines increasingly difficult, resulting in a rapid decrease in coal fines production over time. In addition, physical simulation experiments of coal fines migration in cataclastic coal reservoirs provided valuable reference data. The experimental results showed that the permeability decrease of coal samples primarily occurred during the early stage of water flooding, and a higher fracture density corresponded to a higher average permeability. Pores and fissures with diameters greater than 1 000 nm served as the main channels for coal fines migration and blockage. The migrating coal fines further reduced permeability by plugging connected pores larger than 10 000 nm. These findings were consistent with the numerical simulation results, further confirming the alignment between permeability evolution and coal fines production during migration in cataclastic coal reservoirs. In simulations, high drainage intensity was simulated by setting a high pressure difference. When large coal fines were introduced, their deposition locations shifted compared with those under low pressure differences. Also, fines output decreased, and deposition became concentrated near the outlet end. When small coal fines were introduced, fines output increased significantly, and the permeability reduction was less severe than that caused by large coal fines. Overall, the model simulation results were consistent with coal fines output observed in actual production under increased drainage intensity. The numerical simulation results indicated that larger coal fines particles led to more concentrated deposition and blockage near the inlet end, with a smaller overall deposition range. Under low pressure difference, coal fines deposition was mainly concentrated near the inlet end. As the pressure difference increased, the deposition locations of coal fines shifted closer to the outlet end, and the deposition range expanded. In visual physical simulations of coal fines migration within fractures, the deposition area gradually decreased from the inlet toward both ends along and perpendicular to the main migration direction. In summary, the numerical simulation results were consistent with the permeability changes and coal fines deposition patterns observed during coal fines migration in physical simulation experiments.

Key words: coalbed methane, cataclastic coal, coal fines migration, porosity and permeability, coal fines particle size, numerical simulation