Increasing Safety in an Underground Coal Mine through Degasification by Vertical Wells—Influence of the Relationship between the Permeability of Carbon and the Filter Cake of the Bentonite Suspension

The Hullera Vasco Leonesa (HVL) underground coal mine in northern Spain is subject to violent methane (CH 4 ) outbursts. Vertical wells are used to extract CH 4 from coal layers to improve mine safety. Bentonite suspensions are used as drilling fluids in this degasification system. The relationship between the soil and filter cake permeabilities, the filter cake thickness, and the filtrate loss significantly affects the fluid’s rheological properties. Fann mud balance, marsh funnel viscometer, and Fann 300 press filter tests have been carried out to determine the rheological properties of the bentonite suspension. A drilling fluid study was carried out for three drilling zones (across which the rheological properties of the drilling fluid vary for the reasons mentioned above): Zone 1, the lower zone, wherein drilling cuts through the coalbed; Zone 2, the upper zone, wherein drilling cuts through layers of sand; and Zone 3, an intermediate zone consisting mainly of rock. When drilling cuts through the coalbed, the release of methane, which improves the safety of underground operations, depends on the relationship between the permeability of the coal and the permeability of the filter cake of the drilling fluid. The effect of sand contamination increases the filtrate loss, and therefore also increases the permeability of the filter cake. The filtrate reducer decreases filtrate loss by recovering the permeability.