One of the most prominent unexplained phenomena observed in the Marcellus and some other shale plays is the concentration of dissolved salts in produced waters after hydraulic stimulation. In this paper, we present both geochemical and lithologic laboratory and field data to address the salt question. Is salt being dissolved from the shale, or are deep saline aquifers being breached during hydraulic fracturing? What evidence do we have to support or refute either theory?
To address these questions, over 100 flowback analyses were collected over 18 months from both the southwestern and northeastern regions of the Marcellus Shale play. These data incorporate both cation and anion water analyses in either a full or partial determination of the cation and anion balance. Detailed inorganic geochemical and mineralogical analyses of shale samples were integrated to help determine the presence or absence of physical evidence of minerals that may be the root cause for high salinity. If present in the shale, is halite dissolution desirable from a formation stimulation perspective? If so, is this gain a worthwhile trade when balanced against the costs of returned load water handling and reuse or disposal? This paper provides interpretations at both the regional and local scales to try to explain basinal variations observed in the data. It discusses implications of the phenomenon of high saline frac flowback fluids, along with methods being used to mitigate environmental problems associated with the post-frac flowback water geochemistry.
Our own and citizen concerns about the potential environmental impacts on surface water purity, municipalities' worries about possible diminishing fresh water supplies, and increased scrutiny by various regulatory agencies are drivers behind efforts by the natural gas industry to reduce both the consumptive use of fresh water by large fracture stimulation jobs and subsequent discharge of post-frac flowback fluids to the environment. This is especially the case in the Marcellus Shale play in Pennsylvania, where rapid development is occurring and deep-well disposal capacity is very scarce. Highly saline produced waters from Marcellus wells are presenting an enormous challenge to both regulators and operators.
Flowback waters from the Marcellus characteristically carry high levels of total dissolved solids (TDS) in the form of soluble chloride salts. The make-up water used to fracture the well is normally fresh. Operating and well-service companies have approached the problem of renewable freshwater supply by separating, filtering, and even distilling produced formation waters and frac-fluid flowback waters for future use or surface discharge (Weatherford Telegram 2007). What to do with the post-frac flowback waters in the light of scarce brine disposal facilities and substantial handling costs is an enormous burden to the economic development of the Marcellus natural gas resource.
In the Commonwealth of Pennsylvania, new regulatory limits have been proposed further limiting discharges. The Pennsylvania Department of Environmental Protection announced on April 15, 2009 that all industrial discharges will be limited to 500 mg/L TDS on January 1, 2011. There are currently no facilities in the state that can treat flowback fluids to this level. The options for an economic solution are few for operators in dealing with these saline flowback fluids. Evaporation/crystallization (EC), the only established technology for treatment of the produced waters that can achieve the newly proposed TDS limit, produces a very highly concentrated brine solution or large volumes of crystalline salt cake that still must be disposed. A 1 million gal/day crystallization plant will generate approximately 400 tons/day of salt waste. Unless some beneficial use for these residues can be found, they will require disposal in a secure solid waste facility. A typical municipal landfill cannot accept large volumes of crystalline salts and suitable facilities can do so only at a premium. Further, an EC plant is very energy intensive and thus has the potential for increased air quality impact and greenhouse gas emissions in addition to its cost of operation. The Marcellus shale gas industry may be left with no economically viable disposal options.