WATER MIGRATION IN AN UNSATURATED, HYDROPHOBIC, FRACTURED POROUS MEDIUM: ELEMENTAL S BLOCKS

Elemental sulphur (S⁰) is recovered as a by-product in the production of sweet crude from oil sands and is stored as solid sulphur in above ground blocks. The blocks are constructed by pouring and cooling consecutive two to 12 cm lifts of molten sulphur. The infiltration of precipitation and the ingress of atmospheric oxygen into these blocks leads to oxidation of S⁰, which results in the production of H₂SO₄. The resulting leachate is characterized by elevated SO₄^2- concentrations (> 18 gL^-1) and hyper-acidic pH (pH < 1.0). The pathways for water migration (and the flushing of SO₄^2- and the associated H⁺) through these unsaturated, jointed, hydrophobic blocks was determined by the following: characterization of the jointing patterns, the measurement of porosity and hydraulic conductivity (K) of both the matrix and the bulk media (joints + matrix), and by measurement of the water retention curve and residual water content in a typical S⁰ block (380 m x 170 m x 25 high) located at Syncrude Canada Ltd, Ft. McMurray, Alberta, Canada. Fracture mapping showed polygonal vertical and horizontal joints which yielded an open fracture porosity of 1.4%. Packer testing yielded bulk K values that decreased with depth from 1 x 10^-4 to 2 x 10^-4 ms^-1 near the top of the block to 1 x 10^-5 to 1 x 10^-6 ms^-1 near the bottom. Laboratory testing of core samples of the matrix material (K measurements, specific gravity measurements, water intrusion porosimetry and x-ray computed tomography) provided a mean effective matrix porosity of 5.5%, a total matrix porosity of 9.3%, and a K of about 2 x 10^-6 ms^-1. Water retention curves showed that a pressure head of about 7m is required to introduce water into the matrix. These data were used with the finite element models Hydrogeosphere and SEEP/W to simulate the migration of precipitation through the block. The results to date suggest that water migration through the blocks is rapid and is focused along the jointed network. These results are in keeping with the results of a field tracer test.