Numerical Simulation Study of Fines Migration Impacts on an Early Water Drainage Period in Undersaturated Coal Seam Gas Reservoirs

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.

Dewatering Optimization of Coal Seam Gas Production Wells: A Review and Future Perspectives

“…Their results show that neglecting stress-dependent permeability and porosity barely affects the pressure propagation in the early dewatering stage. However, with the further reduction of BHP, the 239 Jia et al, 240 Su et al 137 Add wetting agents to modify wettability Pan et al, 214 Awan et al, 241 Aslannezhad et al 242 Inject small-sized propants and dispersants Bai et al, 228 Zhu et al, 237 Tao et al, 204 Peng et al 238 Control drilling direction and optimize pressure drawdown to minimize the impact of stress change on reservoirs 248 0.379−1.623 Finjord 249 2.82 Jones 250 4 Kutasov and Hejri 251 2.03−2.14 Xu et al 9 0.12 Streltsova 252 2 Van Poolen 245 1.78 a Note that the modified n* is used here for a universal form of eq 5 as influence of stress-dependent permeability and porosity become more notable. Therefore, it turns out to be necessary to consider this phenomenon in the dewatering strategy.…”

Experimental Investigation on the Impact of Coal Fines Migration on Pores and Permeability of Cataclastic Coal

“…The issue of coal fines production has long been severely affecting the stable production of coalbed methane. During the generation and migration process, coal fines can block coal reservoir fractures, causing a decrease in reservoir permeability and blocking the bottom-hole screen pipes and pump inlets, leading to production failures. − The production of coal fines during the coalbed methane production process is controlled by engineering and geological factors, − with different production stages exhibiting varying coal fines production patterns. − The drilling process, including the grinding of coal seams by drilling tools, the erosion of coal reservoirs by water flow during drainage, and the abrasion of coal reservoirs by fracturing fluids or proppants during fracturing, can result in a large amount of coal fines. − In addition, stress changes induced by coalbed methane production can also lead to shear failure in the coal reservoir, resulting in the generation of coal fines . Geological structures and coal quality also affect the production of coal fines; , tectonic coal, which forms under tectonic deformation, has a complex pore structure and features low strength and low permeability, making it prone to damage and the production of large amounts of coal fines during the production process − and causing reservoir damage and seriously affect production.…”

Numerical simulation of coalbed methane under the mining conditions based on dynamic virtual well and variable permeability field

“…Sun et al (2018) developed a multi-factor coupled gas-water phase effective permeability model for CBM production process based on the P-M model and the Kerry relative permeability model, which quantified the effective permeability of the gas/water phase as a function of pressure and saturation. Peng et al (2019) studied in detail the effect of coal dust on the early drainage and development permeability of coalbed methane reservoirs, and established a mathematical model of the four phases of gas-water-fluidsolid-static describing the fluid transport pattern in coal seams, which was solved using finite-difference equations. Danesh et al (2022) studied the dynamic stress distribution in CBM reservoir development and established a mathematical model of CBM with coupled fault stresses and fault interactions, and the effect of matrix shrinkage on reservoir stress generation was considered in their paper.…”

“…The three countries with the largest coalbed methane reserves in the world are Russia, Canada and China. (Qun 2003; Qin 2006; Song et al 2013). The current coalbed methane extraction technology is changing from the old way of underground extraction to a combination of surface development and underground extraction.…”