EXPERIMENTAL INVESTIGATION OF CATION EXCHANGE BETWEEN FORMATION WATERS AND SANDSTONES AND SHALES, PHALEN COLLIERY, CAPE BRETON

Abstract

The Phalen Colliery mines coal from the Phalen seam in the Upper Carboniferous Sydney Mines Formation. Flooding events in the colliery since 1992 warrant an investigation of the origin and chemical history of the inflow water. Formation waters, sourced from brines and with salinities up to five times seawater, drip into the mine from overlying roof rocks. Formation water chemistry shows a trend of increasing Na/Ca ratio with decreasing salinity which may result from Na-Ca cation exchange with pore-filling clays. This experiment was designed to test the exchange hypothesis. Kaolinite, illite, and chlorite group clays fill or rim sample pore spaces. Clay fractions from six sandstone and three shale samples across the colliery were equilibrated with Na-Ca-Cl solutions (concentration= 4000 to 5400 mmol/L, [Na]/f[Ca] = 70 to 120, pH= 5.6) modelled after Phalen formation water chemistry. Solution cations exchanged with clay cations. A 0.25M Ba(N03)2 solution extracted exchangeable Na and Ca from the clay and cation exchange capacity (CEC) was measured. Measured CEC's range from 0.07 to 0.93 meq/100g (0.21 meq/100g mean). Direct relationships between sample clay percent and 1) cation exchange capacity, 2) exchangeable sodium, and 3) exchangeable calcium, are evidence of cation exchange. CEC depends more on pore clay-filling than sample porosity alone. This experiment suggests that Na-Ca cation exchange between formation waters and in situ clays is a feasible mechanism to explain the formation water Na/Ca ratio trend. Formation water dilution causes an enrichment in Na upon exchange with clays. Literature CEC's are higher than CEC's reported here for the same clay species. CEC's from this study may be underestimated due to experimental constraints, or may have changed over time due to clay burial diagenesis.

Key Words: Phalen Colliery, clay, formation water, brine, sodium, calcium, cation exchange, cation exchange capacity, salinity