Impact of Water Film Evaporation on Gas Transport Property in Fractured Wet Coal Seams

Abstract

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.

The effect of rank, lithotype and roughness on contact angle measurements in coal cleats

“…The presence of water films on porous mineral coated rocks has been previously reported in the literature (Nakagawa et al, 2000;Nishiyama and Yokoyama, 2013;Nishiyama et al, 2012;Teng et al, 2016;Tokunaga, 2011). In general, for hydrophobic surfaces a film of liquid should be swept away by the air flow and should not be retained.…”

The Effect of Water Occlusion on Gas Production in Coal

“…As water-wet surfaces are more likely to trap water in smaller pores (both micropores and micro-cleats), the result is that sections of the cleat network may not allow Darcy flow, as well as capillary effects that hinder gas diffusion from the matrix until the pressure difference has sufficiently decreased (Saghafi et al, 2014;Sakurovs and Lavrencic, 2011). Teng et al (2016) identified these restrictions in the cleats as a water film threshold pressure gradient (S) that could be ruptured if the differential gas pressure was sufficiently large. If the pressure was insufficient then the section of cleat becomes blocked and Darcy flow does not occur.…”

“…Blockage B demonstrates a scenario where the gas pressure is insufficient to rupture S. Thus preferential flow pathways are created in the cleat network. Figure taken from Teng et al (2016 The focus on the micro-cleat, which can range from 6-260 μm (Dawson and Esterle, 2010;Kumar et al, 2011;Laubach et al, 1998;Mazumder et al, 2006;Solano-Acosta et al, 2007), was selected in this thesis for several reasons. 1) Surface wettability which may induce a residual liquid film will have a greater influence on the absolute flow permeability of micro-cleats compared to larger fractures.…”

“…Gas-liquid movement in narrow channels may develop as either (A) annular film flow in a hydrophilic network where water clings to the coal surface and gas flows through the central cavity, or (B) discontinuous globular or rivulets of two-phase flow in a hydrophobic network, which can leave a dry surface where the gas is in contact with the cleat wall illustrated in Figure 2.32 (Lee and Lee, 2008;Triplett et al, 1999;Wang et al, 2012). In coal, the presence of a liquid film has the potential to block cleats, creating bottlenecks in the network unless sufficient gas pressure can overcome the restrictive liquid droplet (Teng et al, 2016). While coal lithotype (dull and bright) have been shown to possess both varied functional groups , as well as surface roughness (Gamson et al, 1996), no study has reported the collective impact these characteristics have on flow regime in the cleats.…”

“…The wettability of the coal surface plays an important role in determining relative permeability, as water-wet surfaces can trap water in small openings, potentially isolating sections of the cleat network and preventing gas flow through these regions until they are cleared out by a sufficient pressure difference across them (Saghafi et al, 2014;Sakurovs and Lavrencic, 2011;Teng et al, 2016). The ability to alter the coal surface to create a known homogenous wetting state has the potential to mitigate capillary effects and improve fluid flow.…”