Paper No. 9
Presentation Time: 10:20 AM
GEOCHEMISTRY AND ISOTOPIC COMPOSITION OF PRODUCED WATER FROM MARCELLUS SHALE WELLS IN SOUTHWEST AND NORTH-CENTRAL PENNSYLVANIA
In order to characterize the major, trace element, and isotopic geochemistry of water produced from hydraulically fractured Marcellus Shale gas producing wells, four sets of time-series samples were collected at wells in southwestern Pennsylvania, and twelve grab samples were collected from wells in north central PA. The Na-Cl-Br system and isotopes of radium (228
Ra), strontium (87
Sr), oxygen, and hydrogen were analyzed to investigate regional and temporal trends in Marcellus Shale produced water and to shed light on the origin of its dissolved solids. At the southwestern Pennsylvania site, where the injected fluid had a total dissolved solids (TDS) value of ~50,000 mg/L, produced water salinity increased rapidly during the first week of gas and water production, then more slowly over the next three to six months to a level near 165,000 mg/L TDS. A corresponding shift in δO and δH occurred during the first week, from the meteoric water signature of the injected water toward isotopically heavier ratios characteristic of deep formation water. This shift, as well as a strong enrichment in bromine relative to seawater, is evidence that the increase in produced water salinity is not due to simple mineral (e.g., evaporite) dissolution, but is consistent with fluids originating from a highly evaporated seawater source. Increases in Ra activities, Sr concentration and 87
Sr ratios approximately parallel the trend in TDS.
Results from the two sites suggest possible systematic regional variations in the geochemical and isotopic composition of Marcellus Shale produced water. However, the range of Ra and Sr isotopic compositions of Marcellus produced waters remains distinct from reservoir fluids from other formations in the Appalachian basin. The differing chemistries and origins of Ra and Sr in produced waters suggest that, used in combination, their concentrations and isotopic compositions can identify solute provenance and help to interpret the fluid’s migration and diagenetic history.