ABSTRACT
At a time when countries are trying to limit their carbon emissions in an effort to combat global warming trends, natural gas has emerged as a transitional fuel between coal-based energy and next generation, renewable sources. Domestic natural gas used in conventional power generation offers a potential environmental advantage over coal for power generation in terms of global warming potential.(1-3) The Marcellus Shale in the northern Appalachian basin (Figure 1) is an unconventional shale gas reservoir with vast development potential but also a source of concern for the potential environmental challenges that its exploitation may create.(4) In 2002, the undiscovered resources were estimated to be only 53.8 billion
m3.(5) Engelder estimated that there is 50% probability that the Marcellus Shale will ultimately yield 13.8 trillion m3 of natural gas.(6) This increase is mainly due to the emergence of horizontal drilling and hydraulic fracturing for well stimulation. In tight formations such as the Marcellus Shale, the density and dimensions (length and aperture) of natural fractures alone is not sufficient to achieve economical gas production. However, a combination of horizontal drilling and hydraulic fracturing greatly improves well productivity and economics of natural gas recovery from tight shales. Fracturing fluid is injected into the horizontal wellbore under high pressure (480-680 bar) to open and prop new and existing fractures in the formation. Hydraulic fracturing of a horizontal well creates a contact area that is thousands of times greater than that of a typical vertical well.(7) Fracturing fluid chemistry is designed according to the geological characteristics of each site and the chemical characteristics of the water supply used. The mix usually contains proppants, friction reducers, scale inhibitors, biocides, gelling agents and gel breakers, and an inorganic acid.(8, 9)
Upon the completion of hydraulic fracturing, the fl uid is allowed to fl ow back to the surface to relieve the downhole pressure and allow gas migration to the surface. The term “fl owback water” typically refers to the fracturing fl uid mixed with formation brine fl owing at high fl ow rate immediately following hydraulic fracturing and before the well is placed into production. “Produced water” then refers to the fl uid that continues to be coproduced with the gas once the well is placed into production and may be present over the lifetime of the well. Chemical and physical characteristics of produced water from conventional and unconventional oil and gas reservoirs worldwide (including shale gas, conventional natural gas, conventional oil, coal-bed methane, and tight gas sands) and the potential treatment options for these waters have been extensively reported.(10-15) Recent study on Marcellus Shale fl owback/produced water defi nes geochemical parameters that characterize the brine with the goal of tracing the water in case of leakage into other water bodies.(16)
Currently, most of the fl owback water from Marcellus Shale in Pennsylvania is stored in surface impoundments or tanks and is then treated (e.g., fi ltration and/or precipitation for metal removal) to enable its reuse as a fracturing fl uid. Flowback water reuse for hydraulic fracturing is an option chosen by an increasing number of oil and gas companies as it
reduces water needs and wastewater management costs. High concentrations of calcium, barium, and strontium are an issue that can limit reuse because of the high scaling potential of these ions if the water were to be reused for hydraulic fracturing.(17) The objective of this study is to report the inorganic chemistry of fl owback/produced water from the Marcellus Shale in the Appalachian basin and correlate it with spatial and temporal information. Data analyzed in this report are from water samples collected in this study as well as those that are available in the literature. Major cations, including barium, calcium, and strontium as the constituents of concern for water reuse, were analyzed and the fl owback/produced water quality from Marcellus Shale was compared with other brines from adjacent formations. The evolution of salinity with time and location across Pennsylvania is also analyzed to provide additional insight into the origin and nature of salinity as it has a major impact on potential management strategies for this wastewater.
