Natural Gas Industry B (Feb 2021)
A new coal reservoir permeability model considering the influence of pulverized coal blockage and its application
Abstract
In order to accurately predict the production performance of coalbed methane (CBM) wells and to formulate a reasonable production system, this paper established a coal reservoir permeability model considering the influence of pulverized coal blockage. Then, on the basis of this model, the flow velocity sensitivity (FVS) experimental data of 15 groups of coal samples taken from the Baode Block, Qinshui Basin, Liulin Block, Hancheng Block, and the Huanglong Coalfield were fitted to determine the permeability models for different coal samples. On this basis, this newly established permeability model was incorporated into a previously developed CBM well performance analysis software, and production history matching was carried out on two CBM wells. Finally, the effects of the parameters of pulverized coal blockage on the permeability of coal reservoirs and the production performance of CBM wells were studied by taking the fitting parameters of CBM Well W1 as the reference. And the following research results are obtained. First, this new model considering the influence of pulverized coal blockage can quantitatively describe the variation of coal reservoir permeability with fluid velocity. In addition, this model can be incorporated into a CBM numerical simulation software or a CBM well performance analysis software to apply it in a wide range. Second, the coal reservoir permeability is less affected by pulverized coal blockage in the Baode Block, but this effect shall not be ignored in the Qinshui Basin and the Huanglong Coalfield. Third, the greater the theoretical maximum permeability damage degree (Dmax) and the permeability damage degree index (n) are, the lower the relative flow velocity (v0.5) corresponding to the critical flow velocity of pulverized coal blockage is and the more obvious the effect of pulverized coal blockage on coal reservoir permeability is. Fourth, in order to reduce the adverse effect of pulverized coal blockage on coal reservoir permeability, it is suggested to reduce the production pressure difference appropriately in the process of production, especially in the initial stage of gas production, so as to avoid severe damage to coal reservoir permeability.
