Impact of Water Film Evaporation on Gas Transport Property in Fractured Wet Coal Seams
Production data of coalbed methane have shown that coalbed may be wet for a long time after the completion of water flow and water gas two phase flow stages. In this period, water flows out in moisture vapor, but the water in matrix does not change so much. The moisture loss is mainly from the water film in fracture network. Experiments also observed that such a moisture loss has a profound impact on the storage and transport of coalbed methane. However, this impact has not been investigated so far. This study investigates this impact through following works: firstly, a new conceptual permeability model is proposed based on water film adhered to the surface of fractures in a dual porosity porous medium. The effect of water film is further described in gas flow equation by a non Darcy law with threshold pressure gradient. Thirdly, a coupled multi physical model is established to consider the interactions among coal deformation, gas flow, gas sorption and moisture loss. This model is validated by the gas production data of a coal seam in the Fruitland formation of San Juan basin. Finally, four scenarios are computed to comprehensively study the impact of moisture loss. These simulations show that the proposed model can well fit the history of gas production data. Non Darcy flow has different velocity profile from Darcy flow. For the non Darcy flow, the gas flow velocity increases quickly, then slowly, and finally decreases once gas starts to flow at a point. Moisture evaporation with gas flow mainly occurs in the zone near wellbore. This loss has a delay to the gas flow velocity. It also reveals that this moisture loss in coal seams can significantly improve coal permeability and thus enhance gas production. Therefore, the change of water film has significant impacts on gas production.
Transport in Porous Media