Characterization of Flowback Waters

 

 

 

With all the positive economic upside to the development of this resource, detractors to the industry question the potential environmental impact of the process. Good water management is essential to the economic sustainability of the industry. Unfortunately, the public literature is replete with anecdotal evidence. There remains a dearth of sound scientific publications on which rational decisions may be made. The purpose of this report is help address water management issues by elucidating the chemical nature of flowback waters. Hopefully, this knowledge may be used to help develop sustainable water management plans.

This report covers recent sampling of flowback water from 19 sites in the Marcellus region of Pennsylvania and West Virginia and 5 sites from the Barnett region of northern Texas. The database includes flowback rates, volumes, and chemical analyses of the injected water and the recovered flowback water. Chemical analyses include general chemical information typically used for drinking water analyses. Additionally, the waters were tested for 70 volatile organics, 116 semi-volatile organics, 22 pesticides, PCB’s, and 27 metals. The list of chemicals evaluated for this project was co-developed by industrial partners, and state environmental agency representatives.

The data are of interest for a couple of discernable trends, such as increased salts (total dissolved solids) and decreased flow with time. The main salt composition appears to be sodium chloride. Chloride accounts for more than 93% of the available anionic content. Some sulfate (500-1000 ppm range) is present in the Barnett samples, but is very low in the Marcellus samples (<100 ppm). Carbonate alkalinity, the main pH buffering system in natural waters is quite low (<150 ppm). The commonly observed cations are sodium, calcium, barium, strontium, and iron. The low alkalinity and sulfate concentrations are keys to understanding the relative abundance of the various cation species. They are also keys to understanding the range and capability of treatment/reuse options for the prevention of scale build-up. Heavy metals (other than iron) are generally either not detected, or in very trace concentrations, and generally far less concentrated than levels present in municipal wastewater sludge.

Other components, such as the aromatics such as benzene, toluene, ethyl benzene, and toluene (BTEX) and other organic compounds, such as naphthalene, that are commonly associated with petroleum sites tend to be present only in low to trace concentrations (ug/l to ppm) without a discernable pattern with time. Heavier polynuclear aromatic hydrocarbon compounds (PAH’s) --- such as commonly seen in petroleum wastewater --- are generally not present in the flowback water, and occur occasionally in only very trace concentrations. These compounds are considered by the wastewater treatment industry to be highly treatable. Several standard treatment methods are available to handle these types of compounds at these low levels.

Compounds that can originate from human sources, such as pyridine and the general class of phthalate esters, are found in less than 50% of the samples. Methylated phenols are more commonly encountered, but only in concentrations between 10 to 100 ppb. Halogenated compounds are rarely encountered. One pesticide was detected (<0.05 ppb) in one sample (out of 79) and was likely an error. PCB’s were not detected in any samples.

Most of the organic chemical analyses performed resulted in “non-detects.” These non-observations are vital to begin the discussion of what “not to look for” when performing analyses on shale gas waters. It is important to focus engineering and planning resources on the chemistry that is present, and to avoid effort in fruitless areas.