Numerical investigation of low-velocity non-Darcy flow of gas and water in coal seams

Fluid flow in low-permeability coal seams shows characteristics of low-velocity non-Darcy flow. A two-phase mathematical flow model considering the effect of the threshold pressure gradient (TPG) for gas and water transportation and flow in such reservoirs has been developed. The corresponding numerical model has been formulated and solved. From the numerical results, we can conclude that both the gas production rate and cumulative gas production in the case when TPG is considered are always less than those in the cases when TPG is not considered because of the sharply decreased pressure and increased energy consumption. A comparison of the gas production rates obtained from the calculation results and from monitoring data indicates that the gas production rate predicted using the model with TPG is more accurate. Under the calculation conditions, the gas production rates when considering different values of TPG are approximately 35%-70% less than those for cases without TPG. In addition, the gas production rate and total gas production decrease as the bottom hole pressure and TPG increase, but they increase with the fracture half length. However, these factors have little influence on the water production regardless of the incorporation of TPG. The research expands the theoretical basis of gas recovery from the tight coal seams and provides a more accurate method to predict the gas production rate efficiently.