The influence of closed pores on the gas transport and its application in coal mine gas extraction

Abstract

The contribution of closed pores to total porosity in most cases exceeds 60%. It plays a critical role in gas storage and transport in coal seam. However, few studies about gas transport model considering the closed pores were reported. In this paper, we tried to construct a constitutive equation of coal considering the closed pore effect. Then, the model of gas transport in coal seam with closed pores was proposed, including gas diffusion between closed pores and open pores as well as gas seepage in fracture. Then, 4 cases were carried out to investigate the influence of various factors on the gas transport in coal seam. Results show that the total gas production decreased with the rising closed porosity, desorption time and stress, while increased with the initial permeability. Furthermore, the closed porosity and desorption time shows a remarkable effect on the contribution of pore to total gas production, followed by the initial permeability and stress. Then, their influence degree on gas pressure difference shows the same trends with that on gas production contribution. Furthermore, the stage characteristics of various curves were analyzed. Finally, the field data from gas extraction of Yangliu coal mine in Huaibei coalfield was used to validate our model, indicating that our model can match the field data better compared with the model without closed pores effect.