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Geochemistry and Effective Management of Produced Water from Natural Gas Extraction
Figure 5: Selenium Concentrations in Coalbed Natural Gas Outfalls and Figure 6: Arsenic Concentrations in Coalbed Natural Gas Outfalls and
Corresponding Disposal Ponds in the Powder River Basin
19
Corresponding Disposal Ponds in the Powder River Basin
19
3
18
16
2.5
14
2
12
µg/l
µg/l
10
1.5
8
1
6
4
0.5
2
0
0
CHR BFR LPR PR TR
CHR BFR LPR PR TR
Outfalls Disposal ponds
Outfalls Disposal ponds
CHR = Cheyenne River; BFR = Belle Fourche River; LPR = Little Powder River; PR = Powder River; CHR = Cheyenne River; BFR = Belle Fourche River; LPR = Little Powder River; PR = Powder River;
TR = Tongue River. TR = Tongue River.
of BFR. High Fe concentrations observed in the disposal ponds are Conclusions
attributed to the high pH and increased solubility of Fe solution species. The pH and TDS concentration of CBNG produced water increases
moving from CHR to TR watersheds within the PRB. Moving north
Mean selenium (Se) and arsenic (As) concentrations in water samples from and west towards deeper coal seams within the PRB also produces
outfalls and from disposal ponds across all watersheds in the PRB are more saline, alkaline and higher-SAR water. Studies indicate that
shown in Figures 5 and 6. Se concentrations were very low (<2.5µg/l) CBNG produced waters are dominated by Na
+
, HCO
–
3
and higher
whether in outfalls or in disposal ponds, but results suggest that the Se dissolved CO
2
, which buffer the pH of outfall water between 7 and
concentrations are higher in the disposal ponds than in the outfalls. 8. When CBNG outfall water is in a disposal pond, CO
2
degasses from
Concentrations of As are also very low in outfalls across all the watersheds the disposal ponds and pH increases to 8–9. In high-pH
(<1µg/l), but increased significantly in all disposal ponds; for example, in environments, the solubility and availability of trace elements (e.g. As,
the LPR watershed As concentrations were up to 10µg/l in disposal ponds. Se) increase in disposal ponds. The high-pH environment in disposal
Investigation of As concentrations in water samples from disposal ponds also enhances the precipitation of CaCO
3
. Subsequently,
ponds has revealed an incremental increase from year to year across all precipitation of CaCO
3
increases the SAR of disposal pond water.
PRB watersheds. Many studies have identified that As solubility is directly Increased SAR in some disposal ponds creates a sediment seal,
related to adsorption and desorption processes of metal oxides and preventing infiltration of trace elements. Treatment options using
hydroxides.
19,20
In semi-arid alkaline environments, mineral oxides clinoptilolite, a locally available zeolite, and/or an EDR process may
and hydroxides tend to have a negative surface charge. In nature, As is an prove suitable for improving the quality of CBNG produced water for
anion and it is thus expected to be soluble and mobile in these watersheds irrigation use in the PRB. Finally, most CBNG produced waters in the
and to increase in concentration over time in disposal ponds. The increase PRB meet the criteria for livestock and wildlife drinking water.
in As concentration is significant because As has been identified as a major Furthermore, produced water in CHR and LPR watersheds also meets
contaminant of groundwater resources and a public health concern. the criteria for irrigation water. n
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EXPLORATION & PRODUCTION – VOLUME 7 ISSUE 2
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