21 / 2020-02-28 21:49:43

An approach to assess the residual methane gas potential of abandoned underground coalmines: A case study from Charleroi coal basin, Belgium

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, an equivalent of one cubic meter of methane was produced per tonne of coal mined-out. This gas was either drained and utilized or vented to the surface for safety reasons. Despite gobs re-compaction (gob volume decrease), there still residual volume that can contain methane from the distressed seam and from the gas-bearing strata overlying and underlying the seam. 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. Modern software tools are under development since few decades to assess as accurately as possible geological resources, but also underground abandoned reservoirs potential in terms of residual gob volume and porosity. Such assessment requires high quality data for the geology (e.g., borehole logs, cross-sections, geological maps, etc.).
This study focused on the Monceau-Fontaine coalmine (Charleroi District) in which a target area [16.4_km2] was selected in its southeast part. Because the colliery was one of the last active mines in the area, mining data (e.g., N-S vertical cross-sections, plan views per seam) were archived. Amount of these useful data were gathered.
We built a conceptual model based on a set of assumptions: (1) the mining-induced gobs for a longwall panel are of parallelepiped-like shape and have heights 4 times the coal seam thickness, (2) due to caving and compaction model, the residual porosity of gobs ranges between 2 and 18% and is subdivided in four zones based on works by other researchers, and (3) the abandoned gobs are piles of fragments of shales and coal rocks with methane as free gas trapped within. Collected data, particularly plan views per seam were digitized to construct the volume of coal mined-out and create initial volume of gobs using the GEOVIA GEMSTM modelling tool. This volume was divided into four rings based on the assumption (2) and the volume of each was determined.
Historical production data show that the volume of methane drained per tonne of coal mined-out is higher than that can be deduced from Langmuir isotherm. This suggests that methane drained originated not only from the mined-out seam but also from the distressed seam and from the methane-bearing strata overlying and underlying the mined seam. We consider then, after the closure of the mine, there will be a balance between the partial pressure of methane in the gobs and in the surrounding residual coal seams and rocks. This work first calculates the residual volume of abandoned gobs associated to a seam mined-out between 1960-1970 over which mining the coal seam was continuous (according to our assumption 1 about the conceptual gob). Second, depending on depth (then pressure), it assesses the volume of free gas contained in the gobs, this gives the pumping capacity for a new recovery trial.