Fines migration has
posed a great challenge to gas and water production
in CBM reservoirs, resulting not only in dramatic permeability reduction
but also in excessive wear on equipment. The objective of this study
was to investigate critical flow conditions for massive fines detachment
in the dewatering phase, for the purpose of yielding an improved understanding
of fines detachment mechanisms and their effective control in the
field. First, fines migration experiments under saturated conditions,
including effluent concentration and permeability measurements, were
conducted at elevated pressure gradients on fractured coal samples
with various apertures. Experimental results indicate the existence
of a critical pressure gradient (CPG) for massive fines detachment.
Second, a mathematical model was developed to describe single particle
detachment in the fracture, accounting for the coupling effects of
hydrodynamic and extended-DLVO forces. Effects of fines size and fracture
aperture on fines detachment were analyzed, and CPGs were determined
from the proposed model. Modeling results revealed that the pressure
gradient required for fines detachment first decreased with increasing
fines size, reached a minimum value, and then increased; these minimum
values are defined as CPGs, which exhibit a strong negative correlation
with fracture aperture. CPGs obtained from modeling were slightly
smaller than those determined from experiments, due to the assumptions
of homogeneous surfaces and spherical particles in the model. Finally,
the implications of this research on field-scale fines control in
coal were thoroughly discussed.