Integrated geomechanical, geohydraulic and geological (3G) study of coal fines generation and migration

Abstract

Coal fines are generated during both the dewatering and Coal Seam Gas (CSG) production stages. This is due to the interaction between the fluid flow and the coal solids affixed to coal cleat surfaces. The generated coal fines may plug gas flow paths and cause the reduction of coal permeability. They may also deposit at the bottom of a well and bury the downhole pumps. Despite the detrimental effects of coal fines behaviours, limited research has been conducted into the generation and migration of coal fines for different flow regimes, not to mention the permeability variations corresponding to the coal fines behaviours.In this work, both numerical and experiment investigations are conducted to obtain fundamental understandings of coal fines generation and migration in coal seams, and the related permeability changes. A mathematical model is also developed to predict the permeability alteration by coal fines erosion and deposition for bore-scale simulations.The simulation results quantify the coal fines generation under both single-phase and two-phase flows. Key parameters that affects coal fines generation are identified. It is noted that exposed microstructures, cleat elbow regions and micro-fracture tips are more likely to generate coal fines. Furthermore, more coal fines are generated in two-phase flow compared to single-phase flow. Based on the dimensional analysis, two dimensionless numbers, namely Capillary number (Ca) and Euler number (Eu) are introduced to define the coal fines generation process. A new criterion that could be used to evaluate coal fines generation is established.Measurements of coal fines production and its impact on the permeability of two coals from the Bowen Basin, Australia, are performed at different flow conditions (i.e. single-phase water or gas, two-phase water and gas) and pressure conditions. The sizes of coal fines collected from each coal sample range from 1 μm to 14 μm. For both coal samples, during the first 50 hours, the permeability decreases from 0.005 mD and 0.048 mD to 0.0019 mD and 0.0072 mD (i.e. 60.9% and 85%), respectively, followed by gradual decline but with fluctuations. This phenomenon is attributed to the counteraction between formation damage (cleats plugging and coal fines settlement) and breakthrough of coal fines from the samples (widened cleats). It is found that coal fines volumetric production is proportional to the third power of flow velocity once the flow paths for coal fines are established. The critical flow velocities of coal fines production for both samples are also obtained. For hydrophobic coal, water-drive-gas two-phase flow introduces abrupt permeability loss due to coal fines generation and migration. Furthermore, pauses (well shut-in) in the experiments cause slight permeability drops. A comparison between the two samples indicates that narrower and less connected cleating system results in more frequent coal fines generation and migration, resulting in significant permeability fluctuations with general decreasing trend. Tortuosity of the cleats can enhance the deterioration in permeability by coal fines behaviours.The establishment of the mathematical model successfully describes the permeability changes due to coal fines erosion and deposition in coal cleats. The model was implemented in a bore-scale simulation, and the coal permeability decreases dramatically despite the erosion of coal fines, which is in a comparable level with the experimental results. The evolution of coal fines concentration in coal seams was also evaluated and discussed in detail. Sensitivity study on the coal permeability variation due to coal fines generation and migration was conducted, and it was found that the coal fines deposition rate coefficient has the most significant effect on permeability.This study, considering the Geomechanical, Geohydraulic and Geological (3G) aspects, delivers fundamental understandings of coal fines generation and migration during the CSG production processes, and useful guidelines are suggested to be implemented in the field to minimize production loss induced by coal fines behaviours.