GEOCHEMICAL CHARACTERISTICS OF DRILL CUTTINGS FROM MARCELLUS SHALE ENERGY DEVELOPMENT

Drilling operations for Marcellus shale gas development generate tons of drill cuttings per well. These drill cuttings are destined for landfill disposal, in-situ burial, and potential reuse as road fill. The drill cuttings resulting from the horizontal drilling into the Marcellus formation in particular contain significant amounts of fine-grained black shale rock cuttings, rich in organic matter and trace metal content.  Leaching tests were performed in order to evaluate the leachability of metals from drill cuttings under various environmental conditions, particularly from the Marcellus horizon. Understanding the geochemical characteristics of the Marcellus shale drill cuttings will assist in making waste management decisions.

 

Twenty-two drill cutting samples were obtained from the Marcellus Shale Energy and Environment Laboratory (MSEEL) in WV. Cuttings were collected along 320 ft intervals during horizontal drilling of a well into the Marcellus formation. These drill cuttings were analyzed for elemental composition by inductively coupled plasma-mass spectroscopy (ICP-MS), total carbon and sulfur via CHNS/O elemental analysis, and mineral phases by X-ray diffraction (XRD). Trace metal associations were mapped by micro X-ray fluorescence (µ-XRF) at Stanford Synchrotron Light Source (SSRL). Preliminary results show that cutting samples containing trace metals such as: As, U, Zn, and Cu, tend to be samples that have high organic carbon (up to 10.3 wt%) and pyrite content. Micro-XRF mapping shows 50-100µm size U, As and Cu hotspots, mainly co-localized with small Fe grains (50-200 µm), or distributed on larger Fe grains (0.5-2 mm). Barium content is elevated throughout all samples (~ 5 wt%) and can in part be attributed to residual drill mud.  Micro-XRF mapping reveals Ba coatings on mineral grains, which supports Ba being primarily from residual drilling muds in these samples.  Leaching tests further identify various leaching conditions influencing trace metal mobility, such as different leachate composition (simulating rainfall or landfill conditions), pH and liquid: solid ratio. Tests are designed to mimic short-term and long-term leaching tests under different conditions in order to evaluate key elements to monitor in Marcellus shale drill cutting disposal scenarios.