CHARACTERIZING TRACE METAL RELEASE IN DRILL CUTTINGS FROM MARCELLUS SHALE ENERGY DEVELOPMENT

Shale gas drilling operations in PA, WV and OH are estimated to generate millions of tons of drill cuttings from the Marcellus shale formation. The drill cuttings consist of residual drilling mud and fine-grained black shale rock cuttings, rich in organic matter and pyrite. These drill cuttings are destined for landfill disposal, in-situ burial, and potential reuse as road fill. This study aims at evaluating the heavy metal mobility and investigating the source of heavy metal release from drill cuttings under different disposal scenarios. Understanding the heavy metal mobilization from the Marcellus shale drill cuttings will assist in making waste management decisions.

Drill cutting samples (depth: 7438ft), two Marcellus core samples (depth: 7438 and 7440ft) and the operating drilling mud were obtained from the Marcellus Shale Energy and Environment Laboratory (MSEEL) in WV. Cuttings with different mineralogical traits (e.g., high in clay, high in pyrite and carbon, high in calcite, and low in calcite) were selected along the horizontal drilling of a well into the Marcellus formation. All samples were analyzed for elemental and mineral composition. Trace metal associations in selected samples were mapped by micro-X-ray florescence (micro-XRF) at Stanford Synchrotron Light Source. Leaching tests were performed to evaluate the leachability of metals from drill cuttings, Marcellus core and drilling mud under various environmental conditions. Barium is elevated in drill cuttings (~ 5 wt%) and can in large part be attributed to residual drilling mud. Micro-XRF mapping reveals Ba coatings on mineral grains; which supports Ba being primarily from residual drilling muds in these samples. Selected trace metal hotspots (e.g., As, Zn and Cu) were found to co-localize with Ba coatings, in addition to their major association with pyrite or iron grains. Analysis of the leachate further identifies that trace metal mobility was impacted by the mineral composition of drill cuttings (e.g., As release from the pyrite-rich sample) and various leaching conditions, such as different leachate composition (simulating rainfall or landfill conditions), pH and liquid: solid ratio.