Simulating mining-induced strata permeability changes

Abstract

Mining processes fracture the surrounding strata and may modify the flow of groundwater by inducing new fractures or changing the permeability of existing defects. The result of mining-induced permeability changes can be disturbance to aquifers or other surface or sub-surface water bodies. Traditional methods for predicting mining-induced fracture connectivity and enhanced permeability based on empirical strain-based criteria may not satisfy modern regulatory demands, nor adequately reflect local geological, geotechnical and hydrogeological conditions. Standard continuum numerical methods may indirectly estimate permeability enhancement from plastic strains however they are not able to track aperture on flow paths or predict fracture connectivity. This paper presents a numerical approach that is demonstrated to be capable of representing longwall mining induced fracturing in sedimentary rock masses. By initiating and propagating fractures, determining connectivity and calculating aperture in a piecewise manner on flow paths, we have estimated permeability enhancement from first principles. Fracture intensity and porosity metrics are calculated and identify the height of the enhanced permeability fractured zone above a longwall goaf. Permeability within the overburden is estimated from the Kozeny-Carman permeability–porosity equation. At a mine site studied in detail in this paper a permeability increase from the in situ state is predicted to range from approximately eight orders-of-magnitude in the caved zone to one to two orders-of-magnitude in the strata above the fractured zone. Realistically simulating cracking, fracturing and crushing of rock strata remains numerically intensive and challenging at the scale of a longwall panel. It is demonstrated in this paper and provides valuable insights into the rockmass response to mining.