Abstract
Coal fines migration exerts negative impacts on early water drainage of undersaturated coal seam gas (CSG) reservoirs. The complicated migration process results in ineffective and inaccurate forecast of coal fines production. Hence, a robust modelling tool is required to include the mechanisms of fines migration and to predict their impacts on rock and production. In this paper, fines migration in coal is categorized into three stages: generation, migration, and deposition processes. The corresponding models for different stages are established, including (1) a fines generation model, (2) the maximum fines-carrying concentration model and deviation factor of the modified Darcy model, (3) a fines deposition model, and (4) a dynamic permeability and porosity model. The above models are coupled with a water flow model, solved numerically using the finite difference method. Then, two dewatering strategies, including fast and moderate depressurization, are compared using the proposed models to study their effects on coal properties and following production. Finally, the production history of a CSG well in the Qinshui Basin, China, is utilized for history matching in a field case study. The simulation results indicate that new fines will be generated in a fast depressurization process and the water rate decline reduces the cleat permeability significantly. The newly generated fines can enhance the permeability temporarily, but they will block the flow channels and bring serious damage to the permeability when the water rate declines. The moderate depressurization strategy can produce the coal fines in a continuous mode, and the formation damage induced by fines deposition can be reduced to the acceptable level, which is the more reliable way to maintain well productivity. In addition, multiple well shut-in can trigger the irreversible fines deposition, reduce the permeability, and decrease the production rate.
Highlights
The dual porosity model is usually employed to describe the fluid flow in coal rock [1, 2]
In undersaturated coal seam gas (CSG) reservoirs, gas is stored in the matrix and the cleat system is saturated with water [3, 4]
In the undersaturated CSG reservoirs, the fluids and the transport of solid particles occurs in the coal cleats during the early water drainage period
Summary
The dual porosity model is usually employed to describe the fluid flow in coal rock [1, 2]. In the undersaturated CSG reservoirs, the fluids and the transport of solid particles occurs in the coal cleats during the early water drainage period. To focus on the coal fines migration in the early water drainage period, a series of models are proposed in this paper to study the migration of coal fines, including the generation, migration, and deposition processes and its impacts on coal properties. Based on these models and the mass conservation equation, a flow model is established to describe coal fines migration coupled with water flow.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
