71 / 2021-09-15 22:25:09

An approach to assess the recoverable methane gas of Belgian abandoned coalmines

Abandoned mine methane,Langmuir isotherm,Residual gob volume,Residual porosity

Belgium has a long history of coal mining, especially in the Hainaut province. During coal extraction, between 26 and 115 m³ of methane were produced per ton of mined-out coal (Vandeloise, 1971). Despite gobs re-compaction, methane from the distressed seam and from overlying and underlying strata is still trapped in the residual voids. This methane can be valued as Abandoned Mine Methane (AMM). However, one major issue to a profitable valorization of AMM is to estimate the gas reserves still in place long time after the abandonment of the mines.

To address this issue, we built a conceptual model based on a set of assumptions. First, the mining-induced gobs for a longwall panel have a parallelepipedic shape (Zhang rt al., 2016) and their height is 4 times the coal seam thickness. Then, due to caving and re-compaction, the residual vertical stress profile induces a variation of residual porosity throughout the gobs. Four zones are identified, with porosities ranging between 2 and 20% (van Tongeren and Dussar, 2004). As a third assumption, the abandoned underground reservoirs are considered dry and constituted of the virgin reservoir, the transition zone of substantial free gas accumulation and the gob. During coal exploitation gas was drained or pumped and the gob depleted of gas. We must go further to find the initial state (or virgin reservoir). The extent of the transition zone depends on what has been exploited and also how long it takes for the gas to desord into the gob owing to the pressure and temperature gradients between he virgin reservoir and the gob.

Historical production data show that the volume of methane produced per ton of coal mined-out is higher than that can be deduced from Langmuir isotherm. This suggests that methane produced originated not only from the mined-out seam but also from the distresed seam and from the overlying and underlying strata. We consider then, long time after the closure of the mine, that the partial pressure of methane in gob balances the pressure of methane in the virgin reservoir and the transition zone.

This study focusses on the Monceau-Fontaine coal mine (Charleroi district) in which a target area [40.5km²] was selected in its southeastern part. A huge amount of data is available from mining archives, particularly plan views per seam. A mining modelling tool is used to build a geometrical model from those data, to assess mined-out coal volumes and hence abandoned underground reservoirs potential in terms of gob volume. This volume is divided into four rings based on our assumptions, and the related residual void volumes are determined. Detailed computations are applied to a particular coal seam, named "11 Paumes", that was extensively mined out between 1877 and 1968. From the literature, a gas pressure of 22 bar and a temperature of 42°C are assumed in the virgin reservoir/transition zone (Pashin, 2008; Couillet, 1998). The volume of contained gas in the gobs is then assessed, using the ideal gas law (Pagés-Diaz et al., 2018).