A preliminary investigation on water quality of coalbed natural gas produced water for beneficial uses: a case study in the Southern Qinshui Basin, North China

Coalbed natural gas (CBNG) is an important unconventional natural gas resource with large reserves in China and receives much attention these years. The CBNG production is accomplished by extracting large volumes of produced water from the aquifer. The CBNG-produced water is commonly managed by discharging into nearby disposal ponds in the Southern Qinshui Basin (SQB), which provides an opportunity for water source for nearby irrigation, livestock, wildlife, and human drinking water. However, utilization of this nontraditional water source in the SQB is hindered by limited knowledge of water quality, practically oxidation/reduction potential (OPR), electrical conductivity (EC), sodium adsorption ration (SAR), and trace element chemistry data. The objective of this study was to collect CBNG-produced water samples at discharge points in the SQB and investigate their water quality principally, including physicochemical parameters, major ions parameters, and trace element parameters. Discharge points were sampled from five main CBNG blocks in the SQB including SZ, ZZ, FZ, CZ, and PZ blocks from July 31, 2013 to August 11, 2014. A composite geochemical data was created with the test results from 145 produced water samples, resulting in information on 40 constituents/parameters. The resulting constituents/parameters were compared to common water use criteria of China to determine possible beneficial uses. Results suggest that the CBNG-produced waters from SQB are characterized by dominated Na-HCO₃ type waters, with approximately 12% Na-SO₄ and Na-Cl type waters. The observed TDS ranges from 615 to 4376 mg/L with 91% are less than 3000 mg/L, and Na⁺ and HCO₃⁻ are the dominating determinants of the TDS content. The EC values vary from 930 to 3844 μS/cm, ranging from class 3 to class 5 based on the suitability for irrigation. The CBNG-produced waters in SQB generally exhibit higher SAR values (avg. 41.98). Among the 25 detected trace elements in CBNG-produced waters from SQB, elements of environmental concerns include aluminum, iron, selenium, barium, manganese, nickel, and plumbum because their concentrations exceed the related Chinese regulatory standards for human drinking. The commonly constituents exceeding standards for human drinking water, livestock water, poultry water, and irrigation water include fluorinion, pH, and TDS. Besides, targeted reduction of SAR and EC also needs to be treated for most of the produced waters if used for irrigation. In contrast, the CBNG-produced waters in SQB are the most suitable for livestock water, because approximately 88% of the CBNG-produced waters are suitable for livestock drinking if the fluorinion is reduced.