PRELIMINARY ANALYSIS OF THE WEATHERING POTENTIAL OF MARCELLUS SHALE DRILL CUTTINGS

Black shale is a natural material, and is common in outcrops, where it has been oxidized, weathered and leached over geological time. Black shale cuttings from deep horizontal boreholes are different. When cuttings are brought to the surface, the reduced minerals in them are exposed to air and fresh water for the first time. Oxides of many elements are more water-soluble and mobile than the metallic or reduced forms. The concerns are magnified by the large quantities of drill cuttings. The mass of black shale removed from a lateral borehole that is 30 cm across and up to 2 km long may exceed several hundred tons.

Samples of drill cuttings, core, and outcrops of the lower, middle and upper parts of the Marcellus Shale were chemically analyzed. Carbon, sulfur and hydrogen were assayed by Controlled Atmosphere Programmed Temperature Oxidation (CAPTO). Scanning electron microscopy and elemental analyses were run on select samples. A leaching procedure followed by Inductively Coupled Plasma (ICP) analyses was used to detect “toxic” metals such as arsenic, lead, mercury, and selenium. An ICP analysis of a composite sample was performed to determine bulk rock elemental composition. Radioactivity was measured using a low background radiation analyzer.

The CAPTO analyses showed organic and inorganic carbon in the samples, hydrogen as free moisture, hydrocarbons and bound water on clays, and sulfur in both sulfides and sulfates. The ICP analysis of the composite sample showed the major components of the rock to be Al, Ca, Fe, K, Mg, Na, S, Si, and Ti, which is not unexpected for pyritic, clay-rich, black shale. Minor elements detected by the ICP at levels above the minimum reporting limit included As, B, Ba, Cd, Co, Cr, Cu, Li, Mn, Mo, Ni, P, Pb, Sn, Sr, V, Zn, and Zr. The alpha radiation counts on the bulk rock samples ranged from background levels to about 8 times above background. Analysis on the leachate prepared for the ICP tests showed alpha counts at background levels.

The preliminary results suggest that black shales do contain small but detectable amounts of heavy metals and other elements that can be detrimental to the environment if mobilized and concentrated. Additional analyses are needed to better define the fate and transport of leachate from black shale cuttings, and identify potential environmental hazards.