GEOCHEMICAL BEHAVIOR OF MARCELLUS SHALE DRILL CUTTINGS WITH DIFFERENT TYPES OF DRILLING FLUIDS

The Marcellus shale, like many types of black shale, has been shown to be organic, pyritic and clay-rich by the nature of its depositional environment. Previous studies have identified that trace metals are present in the Marcellus shale in core and outcrop materials. Little work has been done evaluating fine-grained drill cuttings from the Marcellus, and the geochemical signals that might result under environmental conditions at the earth’s surface and under landfill conditions. Leaching studies were performed along with characterization of the solids to help identify potential trace metal point-sources.

Samples of drill cuttings from two different locales in Pennsylvania (Southwestern PA and North Central PA) were evaluated. Drill cuttings represent the fine grained rock pieces that are broken away from the formation by the action of the drill bit and often include remnants of various types of drilling fluids (e.g. water-based, and nonwater based) and additives despite rinsing in the field. The additives may vary greatly between sites. The drill cutting solids were analyzed via X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The SW PA site utilized nonwater based drill cuttings and analysis of the solids show an abundance of barite in addition to the typical shale mineralogy of quartz, clay, calcite and pyrite. The NC PA samples did not contain barite, but contained added salts.

The dried drill cuttings were leached with DI water in different L:S ratios, and under various pH conditions. In addition, the drill cuttings were exposed to both synthetic rain water and natural rain water collected in the Pittsburgh area. TCLP and sequential extractions are underway. The natural pH of the solids from both locales were alkaline (~ 9.0 – 9.4), and both samples show a high buffering capacity. There was a marked color difference between leachates from the nonwater based drill cuttings versus the water based drill cuttings. The potential influence of organic compounds and overall geochemical trends at different pH are being evaluated for both non-water based and water-based drill cuttings.