The impact of multiscale cleat geometry on coal's petrophysical properties in the Lorraine basin, NE France: Implications for coalbed methane (CBM) production and CO2 storage

The effectiveness of Coalbed Methane (CBM) production relies on the presence of connected, permeable cleat networks through which the desorbed gas could flow to the producing wells. The geometrical and dynamic (flow) properties of cleats and their spatial variability are essential for predictive economical scenarios, and reference databases are needed for such predictions. This paper contributes to this end by studying cleat networks in a series of coal beds and natural fracturing in the adjacent rock formations, in cores from three wells of the Lorraine basin.

Cleats occur at three scales in non-shaly coal: multi-centimeter- to decimeter-scale master cleats, multi-millimeter- to multi-centimeter-scale primary cleats and sub-millimeter- to millimeter-scale secondary cleats. The last two types are vertically controlled by the distribution of bed interfaces and local heterogeneities such as shale or ash lenses. Only master and primary cleats form connected permeable systems. The cleat geometrical parameters (intersection angle between face and butt cleats, cleat spacing, length and height) are consistent between the studied seams and all exhibit multifractal characteristics. This dataset is used to build representative Discrete Fracture Network (DFN) models of the cleat networks, calibrated to the permeability of the coal beds estimated by two pressure transient well tests. The built models show the key impact of multiscale cleat properties on coal's petrophysical properties and provide a reference source for robust flow simulations, that could be used in first approximation in any CBM context.

The good connectivity of cleat networks expected in the coal beds at a pluri-kilometer scale, the favorable gas content and saturation of the seams and the negligible fracturing detected in adjacent formations suggest a high CBM production potential in the studied area of the Lorraine basin. This area is also particularly favorable for Enhanced CBM (ECBM), where injected CO₂ replaces the desorbed methane, allowing at the same time the production of methane and lasting storage of CO₂.