GEOCHEMICAL FACTORS CONTROLLING TRACE METAL AND BARIUM RELEASE IN DRILL CUTTINGS FROM MARCELLUS SHALE ENERGY DEVELOPMENT

Million tons of drill cuttings generated from Marcellus shale gas development are destined to landfill disposal, in-situ burial, and potential reuse as road fill. These drill cuttings consist of residual drilling mud containing high barite (BaSO₄) and fine-grained black shale rock cuttings, rich in organic matter and pyrite. Secondary release of trace metals or barium from drill cuttings may be of environmental concern under different disposal scenarios. This study aims at investigating geochemical factors (e.g., mineral composition of drill cuttings, different leaching environments, pH, wet vs. dry conditions) controlling trace metal and Ba release from Marcellus shale drill cuttings.

Drill cutting samples with different mineralogical traits (e.g., high in pyrite and carbon, or high in calcite) from the lateral portion of a Marcellus shale well, portions of Marcellus shale core drilled as part of the Marcellus Shale Energy and Environment Laboratory (MSEEL) in WV, along with the operating drilling mud were obtained and studied. All samples were analyzed for elemental and mineral composition. Elemental Distribution were mapped by micro-X-ray florescence at Stanford Synchrotron Light Source and elemental associations with different solid factions were further evaluated by Community Bureau of Reference (BCR) sequential extraction. Leaching tests were also performed to evaluate elemental leachability under various environmental conditions (e.g., rainfall, landfill, different pHs).

Preliminary results indicate that barium (~ 5 wt%) primarily formed coatings on mineral grains in drill cuttings and was >99% non-extractable via BCR extraction. The mobile barium (100-400 kg/mg) was mainly associated with exchangeable and carbonate phases, sensitive to acidic leaching conditions. Although drill cuttings passed the regulatory short-term leaching tests, long-term release of Ba in all samples and As, Ni, Cd and Sb in high pyrite and low calcite samples were possible under simulated rainfall conditions. Selected trace metals (e.g., Ni, Zn, Sb) and Ba release doubled when the waste was first air-dried and subsequently underwent rainfall leaching. Continued efforts are focused on conducting long-term wet/dry cycle effects on leaching contaminants from drill cuttings under different disposal scenarios.