Agglomeration-dispersion effects of coal fines to control the permeability of propping fractures: A visual experimental study

Coal fines are carried from coal seams during hydraulic fluid return, which affect the permeability of its propping fracture and efficiency of coalbed methane (CBM) extraction. Aggregates are the presentation form of coal fines typically. However, the impact of the two opposing coal fine states, agglomeration and dispersion, on the permeability of propping fractures is not clear. This work employed sodium dodecyl benzene sulfonate (SDBS) which is an anionic surfactant to regulate the condition of coal fines. Sand-packed tube tests with visual feedback were used to analyze the impacts of coal fines' agglomeration-dispersion effects under varying pressures. The findings showed that SDBS efficiently disperses coal fines in hydraulic fluid, increasing the amount of coal fines with tiny particle sizes in the range of 0–20 μm. The zeta potential test revealed that adding SDBS to coal fines enhances their surface's negative charge, strengthening the repulsive force between particles. Moreover, SDBS solution has better wetting properties on coal fines, which aids in the coal fines' dispersion. Consequently, the dispersion impact of SDBS on coal fines causes an increase in filtering loss and a delay in the filling of coal fines in propping fractures. Initially, SDBS improves the dispersion of coal fines that are more likely to pass through proppants, which lessens the damage to the propping fractures' permeability coefficient. In the end, coal fines in the fracture's front end form a denser filter cake, which significantly reduces the permeability coefficient of propping fractures. It was observed that the higher the pressure differentials, the greater damage to the permeability. Finally, field cases from two CBM wells validated that with increasing of hole bottom pressure, coal fines raised in the discharged water. Moreover, fracturing fluids with SDBS could promote the discharge of coal fines. The work can offer insightful information about coal fines control.