Abstract

Geochemical Evolution Of Water And Solute Sources For Fluids Produced From Shale Gas Reservoirs

Mark A. Engle¹,², Elisabeth L. Rowan¹, Thomas F. Kraemer³, and Anne L. Bates¹
¹U.S. Geological Survey, Reston, Virginia, USA
²University of Texas at El Paso, Dept. of Geological Sciences, El Paso, Texas, USA
³U.S. Geological Survey, Woods Hole, Massachusetts, USA

Numerous efforts are underway to characterize fluids injected in order to hydraulically fracture shales and other continuous hydrocarbon reservoirs. In contrast, few studies have examined the chemical composition or attempted to identify reactions that control the chemistry of the water produced during gas extraction. We have explored time series (up to a year or longer) geochemical analyses of produced water samples from more than twenty Marcellus Shale gas wells in the northern Appalachian Basin to provide information about the dominant reactions and processes occurring in the reservoir. Previous investigations have cited either dissolution of halite [which was observed in portions of the Marcellus Shale by Blauch et al. (2009)] by fracturing fluids or mixing of injected fluids with formation brines, thought to represent evaporated paleoseawater, as the primary source of solutes in associated produced waters. Our results, using a newly developed mathematical approach for Na-Cl-Br systematics (Engle and Rowan, 2013), suggest that both sources contribute Na and Cl to produced fluids. Results from compositional multivariate analyses of geochemical data indicate that SO4 reduction in the reservoir allows for significant concentrations of dissolved Ba and Sr in later-stage produced waters. This is consistent with the presence of SO4-reducing bacteria in batch reactor experiments of produced waters from the Marcellus Shale. The ²²⁸Ra/²²⁶Ra ratio of water produced from Marcellus shale wells (<0.3) is typical of uraniferous shales and suggests that radium originated within the shale, rather than in adjacent sandstone units. Time series geochemical and δ¹⁸O data reflect the compositional transition in the produced water from injected water to formation water. The transition is likely hastened by imbibition of a large fraction of the injected fluid into water-wet zones in the shale, followed by mixing with formation brines. Our findings suggest that reservoir petrophysical/hydraulic properties as well as geochemical reactions control the composition and source of fluids produced from hydraulically-fractured shales.

AAPG Search and Discovery Article #90164©2013 AAPG Southwest Section Meeting, Fredericksburg, Texas, April 6-10, 2013