Impact of salinity of fracturing fluid on the migration of coal fines in propped fractures and cleats

During the hydraulic fracturing of coalbed methane (CBM) well, the deposition of coal fines in propped fractures and the migration of coal fines in cleats will damage the permeabilities of propped fractures and cleats, consequently affecting the hydraulic fracturing and the subsequent drainage of CBM well. For the purpose of dredging propped fractures and avoiding the clogging of cleats effectively, a novel method for the control of coal fines in propped fractures and cleats during hydraulic fracturing was proposed by optimizing the salinity of fracturing fluid. With the salinity decreasing stepwise, the experiments on the migration of coal fines in propped fractures and cleats were conducted on quartz sand-packed columns and anthracite coal plugs, respectively, to investigate the response characteristics of the migration of coal fines to the change of salinity. Additionally, the migration of coal fines was simulated by using the extended DLVO method, to elucidate the influence mechanisms of salinity on the migration of coal fines. On this basis, the optimal salinity range that takes into account the control of coal fines in propped fractures and cleats was explored. The results indicated that there existed a critical salt concentration (CSC) for the migration of coal fines in both propped fractures and cleats. When the salinity was lower than the CSC, the permeability of propped fractures abruptly increased while that of cleats decreased sharply, accompanied by a large amount of coal fines produced. The value of the CSC for the migration of coal fines in propped fractures was higher than that in cleats, which can be attributed to the fact that the surface electronegativity of proppants was stronger than that of cleats, while the hydrophobicity was weaker than that of cleats. With the gradual decrease of salinity, the electric double layer (EDL) repulsive force between coal fines and channel increased continuously. When the salinity decreased to the CSC, the EDL repulsion started to be greater than the sum of Lifshitz-van der Waals attraction and Lewis acid-base attraction, resulting in the migration of coal fines. Both the values of the predicted CSCs for the migration of coal fines in propped fractures and cleats were consistent with experimental data, indicating the effectiveness of the model. During hydraulic fracturing, the salinity of fracturing fluid can be designed between the CSCs for the migration of coal fines in propped fractures and cleats. In that case, the production of coal fines in propped fractures is promoted while the migration of coal fines is inhibited in cleats, so as to achieve the dual purposes of coal fines control in propped fractures and cleats.