Estimating porosity and permeability in a karstic aquifer using core plugs, well tests, and outcrop measurements

The statistical and spatial distribution of matrix and conduit/fracture porosity and permeability is important for addressing water resource and contaminant transport issues in karstic aquifers. We used quantitative resource assessment methods, well performance tests, geostatistics, and fractal scaling to develop an integrated understanding of porosity and permeability in the Edwards Group in South-Central Texas. Using porosity and permeability determined with plugs collected from core and outcrop, wireline log interpretation, and three-dimensional geocellular modeling software used in hydrocarbon resource quantification, we estimated 1. the three-dimensional distribution of matrix porosity and permeability in the aquifer, 2. the porosity due to dissolution features, and 3. the volumetries of the aquifer. Matrix porosity measured in core plugs ranged from 0.7 to 49.8% with a mean of 14.4%. Vuggy porosity (dissolution features) estimated from core plugs is about 2%. Hydraulic conductivity of the matrix determined from core plugs ranged from 8.4 × 10 −6 to 8.3 m d −1 with a geometric mean of 1.3 × 10 −3 m d −l . Estimated total volume of water-filled pore space in the Edwards aquifer is 2.13 × 10 11 m 3 . Transmissivity of the aquifer due to matrix, fractures, and dissolution features was estimated using more than 1000 well tests and was found to be log-normally distributed, vary over eight orders of magnitude, have a geometric mean of 540 m 2 d −l , and be spatially correlated. We estimated the porosity of fractures and dissolution features at roadcut exposures using scanlines, image analysis, and fractal scaling. We found that secondary porosity in the outcrops due to fractures is 0.2 to 0.7% and secondary porosity due to dissolution features is 2.5 to 10%.