Effect of Coal Fine Retention on the Permeability of Hydraulic Propped Fracture

The damage caused by coal fine retention to the permeability of hydraulic propped fractures is one of the key factors restricting coalbed methane (CBM) production. In this paper, permeability experiments of propped fractures under different coal fine retention conditions are carried out, and the retention coefficient of the coal fines is introduced. In addition, the permeability model of propped fractures is established to study the influence of constant rate flow and discontinuous flow on the coal fine retention, along with the influence of coal fine retention coefficient on the permeability of propped fracture in the single-phase flow drainage stage. The study results show that, with the increase of the coal fine flow rate, the retention of the coal fines decreases exponentially, in turn resulting in a logarithmic increase of the porosity and a logarithmic increase of the permeability in the propped fracture. The larger the coal fine particle size is, the likelier it is to remain, and likelier it is to reduce the porosity of the propped fracture, thereby resulting in a decrease in the propped fracture permeability. It is also shown that the retention coefficient of coal fines decreases exponentially with the increase of the flow rate, and increases with the increase of the coal fine particle size. In addition, the retention coefficient ratios of the two flow conditions increase linearly with the increase of the flow rate. The slower the flow rate is, the likelier the coal fines will be deposited in the constant rate flow; and the larger the flow rate is, the likelier the coal fines will be deposited in the discontinuous flow.