GEOCHEMICAL VARIABILITY WITHIN THE MARCELLUS FORMATION OF THE NORTHERN APPALACHIAN BASIN AND ITS IMPLICATIONS FOR PRODUCED WATER COMPOSITION OF UNCONVENTIONAL NATURAL GAS WELLS

The geochemical properties of distinct microfacies in the Marcellus shale exposed to hydraulic fracturing fluid (HFF) should be of interest to petroleum geologists and engineers who must anticipate the influence of cation exchange capacity (CEC) and trace metal mobility of produced water on the precipitation of secondary minerals. The interaction of rocks with two types of well completion fluids are of concern. The first fluid is an acid pre-flush, used to displace formation brines containing K, Na, and Ca ions away from the wellbore and to dissolve calcareous and siliceous materials; the second is the HFF. We present experimental results focused on two objectives: a) characterization of the heterogeneity of the reactivity of a clay- and organic-rich laminated mudstone facies of the low Marcellus Subgroup to an acid pre-flush; b) investigation of the influence of variable properties of distinct microfacies within productive strata of the upper Marcellus shale, such as carbonate abundance and clay abundance, on the apparent dissolution and solubilization of trace metals in a simulated HFF. We characterize the variability in concentration of selected trace metals through several Marcellus horizons and we react those horizons with simulated HFF.

The experiment in objective a) was conducted using quarried rock from Seneca Falls, NY, and run in triplicate to assess variability within a microfacies. Crushed Union Springs Formation samples of known composition were subjected to one of three concentrations of HCl. Trends in concentration over time of major and minor cations were analyzed.

The experiment in objective b) utilized samples from a south-central New York core. Trends in concentration over time are analyzed for two groups of materials in the produced water: 1) trace metals such as Al, Cd, Co, Cr, Cu, Ni, Sc, and Zn, the trends of which are influenced by CEC, and have shown to indicate formation of secondary minerals or sorption; 2) radiogenic isotopes such as barium, thorium, and uranium. These naturally occurring radiogenic materials (NORM) of interest tend to become concentrated in well equipment during barite precipitation (TENORM) and may require unique disposal methods.