Using Strontium Isotopes to Test Stratigraphic Isolation of Injected and Formation Waters During Hydraulic Fracturing
Kolesar, Courtney A.; Capo, Rosemary C.; Wall, Andrew J.; Stewart, Brian W.; Schroder, Karl; Hammack, Richard W.
High volume slick water hydraulic fracture stimulation is necessary for unconventional shale gas production, leading to concerns about communication between the target formation and overlying units. We will present pre- and post-hydraulic fracturing strontium (Sr) isotope data from a well-characterized Marcellus Shale horizontal drilling site in Greene County, Pennsylvania, in which vertical gas wells tapping the overlying Upper Devonian sands are in close proximity. The Upper Devonian gas-producing formations are >3,000 ft. above the Marcellus Formation and ~2,000 ft. below fresh water aquifers. This provides an opportunity to test the assumption that hydraulic fracturing is confined to the target formation, and that injected and formation waters do not migrate to overlying units. In order for Sr isotopes to be an effective tracer for fluid migration and mixing, there must be a large enough difference between the 87Sr/86Sr ratio of the end-member fluids. Samples collected at gas-water separators and/or stock tanks from four wells tapping Upper Devonian sands at this site yield 87Sr/86Sr values from 0.7199 to 0.7209 (typical analytical uncertainty ±0.00002). This is significantly different from produced waters from horizontal wells in the Marcellus Formation from the same site, which range from 0.7113 to 0.7116, well within the range of values previously determined for Marcellus produced waters across Pennsylvania (0.7101-0.7121; Chapman et al., 2012, Env. Sci. Technol. 46, 3545-3553). Because Marcellus Formation produced waters can have Sr concentrations that are an order of magnitude greater than those of the Upper Devonian waters in this region, 87Sr/86Sr ratios of the latter are sensitive to small (>1-2%) intrusions of the former.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013