Clay mineralogy of the Marcellus and Utica Shales: Implications for fluid development via cation exchange

The clay mineralogy of the Marcellus and Utica Shales is described for its potential impact on the development of extremely saline fluids produced during hydraulic fracturing of these units. Cation exchange is a likely mechanism contributing to the interaction between the formation and introduced fluids. The bulk mineralogy of 20 Marcellus Shale and 18 Utica Shale core and outcrop samples was determined quantitatively by powder X-ray diffraction. Clay minerals constitute approximately 40% of the mass on average, and consist primarily of illite with minor chlorite. Non-clay minerals are predominantly quartz and carbonates with minor feldspars and pyrite. The clay-sized fraction was physically separated from 21 Marcellus Shale core and outcrop, and 16 Utica Shale core samples and analyzed by X-ray diffraction. A small degree of mixed layer illite/smectite with 0-5% expandable layers in the Marcellus and 10-15% expandable layers in the Utica Shale was identified from oriented preparations. The cation exchange capacity (CEC) of the bulk sample was measured in 28 Marcellus Shale and 18 Utica Shale samples. The CEC ranges from 0.3 to 9.1 mEq/100g with an average of 5.9 mEq/100g (SD= 1.36) in the Marcellus Shale and 3.5 mEq/100g (SD= 1.08) in the Utica Shale. The relationships between CEC and lithogeochemistry, TOC, and clay mineralogy are explored and the potential contribution to flowback is estimated. Using conservative well measurements, cation exchange is estimated to be capable of exchanging approximately 10% of the total charges in a typical well.