Geochemical multiaquifer assessment of the Huntly coalfield, New Zealand, using a novel chloride-bicarbonate-boron ternary diagram

Abstract

This study investigates the chemical signature of produced water from the Huntly coalfield in New Zealand during coalbed methane (CBM) development using a newly proposed chloride-bicarbonate-boron (Cl-HCO3-B) ternary diagram. This ternary diagram is used to identify the source of fluid in the different aquifers (groundwater, CBM and deep basement) and the potential communication between these aquifers. A comparison study of Huntly with other local and international CBM fields (Maramarua, Powder River Basin, San Juan Basin, Raton, Atlantic Rim, Liulin and Alberta) was also carried out to help characterize and verify the origin of water being abstracted. Results from the ternary diagram showed that produced water samples from Huntly CBM wells shows CBM water characteristics, with high chloride (Cl−) concentrations originating from moderate permeability coal seams. The water samples also indicate that there is some communication with shallower aquifers based on significant bicarbonate (HCO3−) concentration. Water samples from the basement well meanwhile, showed deep basement water characteristics, with high boron (B) concentrations and originated from low permeable formations, far from the recharge area. Water samples from the shallow groundwater well displayed a shallow groundwater signature high in bicarbonate (HCO3−), and showed no significant connection with the coal seam or deeper aquifer. When comparing the Huntly CBM water with water samples from other CBM fields, the former showed similar CBM water chemical signatures and comparable patterns, with minor ion concentration differences. The novel ternary diagram proved to be a reliable tool to identify the possible links between deep and shallow aquifers, qualitative formation permeability and origins of water in one diagram. It therefore enhances our understanding of the natural CBM reservoir setting and could improve field development, wastewater management and water quality monitoring in the future.