Hydrogeochemical Characteristics and Water–Rock Interactions of Coalbed-Produced Water Derived from the Dafosi Biogenic Gas Field in the Southern Margin of Ordos Basin, China

Abstract

The hydrogeochemical characteristics of coalbed-produced water can provide insights into the sources of ions and water, the groundwater environments, hydrodynamic conditions, and water-rock interactions of depositional basins. To study the water-rock reaction process and reveal whether there is a microbial activity in the groundwater, a case of the Dafosi biogenic gas field was chosen by testing the ionic concentrations and hydrogen and oxygen isotopic compositions of coalbed-produced water and employing R-type cluster and principal component analyses. The results showed that Na+, Cl − , and HCO3- are the principal ions in the coalbed-produced water, while the water type is mainly a Na–Cl. Due to the hydrolysis of HCO3-, the pH in this region was controlled primarily by HCO3-. As the main cation in water, Na+ contributed substantially to the total dissolved solids. Na+ is also related to the exchange between rock-bound Na+ and Ca2+ and Mg2+ in water or surrounding rocks. The coalbed-produced water’s oxygen isotopes displayed a characteristic 18O drift and enrichment, indicating that the 16O isotope in the water was preferentially exchanged with the coal organic matter. Early evaporation is also contributed to the enrichment of TDS (total dissolved solids) and 18O in the water. The central part of the study area, including the Qijia anticline, was affected by the Yanshanian uplift and denudation and subsequently developed a water-conducting fissure zone and was recharged atmospheric precipitation; these conditions were conducive to the formation of secondary biogenic gas.

Hydrogeochemical characteristics and water-rock interaction mechanism of coalbed-produced water in the Linfen mining area, eastern margin of Ordos Basin, China

“…The content of Ba, Sr, Mn, Li and other elements are relatively high. On the one hand, these trace elements have their own active chemical properties and easily loss or gain electrons with existing in ion form in Relationship between δD(H 2 O) and δ 18 O(H 2 O) of coalbedproduced water in the Linfen mining area (modified from Bao et al, 2021).…”

“…The principle of CBM drainage is to pump the groundwater into the coal seam and its surrounding rocks (Liu et al, 2013), reduce the pressure in the coal reservoir, desorb the gas adsorbed in the coal reservoir and diffuse, percolate and transport it into the wellbore through the pores and fractures, so that it can be exploited (Tao et al, 2011). The coalbed-produced water experienced a long-time contact with the coal seam and surrounding rocks underground, which contained rich geochemical information (Bao et al, 2021). As a direct product of CBM well drainage, coalbed-produced water has an important impact on CBM enrichment mechanism and exploration and development research (Bao et al, 2021).…”

“…The coalbed-produced water experienced a long-time contact with the coal seam and surrounding rocks underground, which contained rich geochemical information (Bao et al, 2021). As a direct product of CBM well drainage, coalbed-produced water has an important impact on CBM enrichment mechanism and exploration and development research (Bao et al, 2021).…”

“…Previous studies have also done a lot of research around the geochemical characteristics of the coalbedproduced water (Bao et al, 2016;Guo et al, 2017;Guo et al, 2020). Since Craig (1961) first proposed the global precipitation line (GMWL), hydrogen and oxygen isotopes have been widely used in hydrogeochemical studies (Zheng et al, 1983;Chen et al, 2011;Bao et al, 2021). Hydrogen and oxygen isotopes have played an important role in determining the source of water and the types and mechanisms of water-rock interactions (Bozau et al, 2017;Hao et al, 2019).…”

“…Water-rock interaction refers to the physicochemical reaction between groundwater and surrounding rock during the geological action of coalbed-produced water, which generally includes cation exchange, rock dissolution and filtration, and evaporation and concentration (Li Q. G. et al, 2016;Li X. et al, 2016;Huang X. J. et al, 2018). Bao et al (2021) investigated the hydrogeochemical characteristics of coalbed-produced water from the Dafosi biogas field on the southern margin of Ordos and found that cation exchange occurs in coalbed-produced water in this area. Qin et al (2014) proposed a method to distinguish between the source resolution of the produced water and the mixed CBM feasibility discrimination based on the trace element concentration in the coalbed-produced water.…”