Experimental study of the effects of fine retention on fracturing proppant permeability in coalbed methane reservoirs

Hydraulic fracturing with proppants is the key technology in developing coalbed methane (CBM) reservoirs. However, coal fine retention in the fracturing proppant will directly affect its permeability and drop the CBM production. In this paper, coal fine retention experiments in fracturing proppants not containing coal fines, along with permeability tests of fracturing proppants containing coal fines, were successively conducted. The results showed that as the suspension concentrations of the coal fine increased, the coal fine retention amounts (W-r) of fracturing proppants increased monotonously, while the permeability decreased monotonously. Furthermore, as the suspension concentrations of the coal fines increased, the coal fine retention rate (R-r = W-r/W-t, where W-t represents the coal fine injected mass) first sharply increased monotonously to a peak level, and then slowly decreased monotonously. In addition, with the increases in the coal fine residue rates (R-cp = W-r/W-p, where W-p represents the mass of the proppants), the fracturing proppants permeability loss rates (eta) were observed to first sharply increase and then slowly increase, indicating that the influences of the W-r on the permeability mainly occurred during the initial stage of the coal fines intrusions into the fracturing proppants. Furthermore, the relationship between the R-cp and eta was also derived in this study as eta = 0.71578R(cp)(0.4575).