Dr. Greg Thoma, IPEC QA officer, made a laboratory visit on June 6 and performed a quality assurance audit. All facets of the Quality Assurance Project Plan were discussed, and the laboratory and data gathering practices in place were reviewed and found to be satisfactory. We received Award Notification on August 9, 1999. On September 28, 1999 a sampling trip to Bebee-Konawa Oil Field, OK was made to collect samples from the Lake site. The sampling site includes two oil wells and an oil-water separator.
Chemical analysis showed that the produced waters at the selected site were limited in both oxygen and nitrate. They contained sulfate, dissolved sulfide and chloride. Iron was also not detected (Table 1).
Table 1 Geochemical characteristics of the studied site.
|
Sample |
pH |
Temp ( oC) |
Sulfide (mM) |
Sulfate (mM) |
Chloride (mM) |
Iron (ppm) |
|
Well 1 |
7.85 |
25 |
3.6 |
0.38 |
137 |
b.d.l. |
|
Well 2 |
8.0 |
27 |
3.6 |
0.52 |
95 |
b.d.l. |
|
Oil/water separator |
7.87 |
23 |
3.9 |
0.35 |
108 |
b.d.l. |
In the Bebee-Konawa oil field, sulfide concentrations were relatively high (from 3.6 to 3.9 mM), which corresponded well to the low level of sulfate (0.32 to 0.55 mM). Such water chemistry, high concentrations of sulfide and depletion in sulfate, suggests a high probability of active sulfate reduction. Indeed, in Bebee-Konawa field we detected SRA in oil well heads and in the oil-water separator. As it is possible to see from the data summarized in the Table 2, the rates of sulfate reduction in above-ground facilities were greater than the rate in the reservoir, indicating that the majority of the sulfide production occurs after oil is pumped out of the wells. In Bebee –Konawa oil field the rates in the reservoir varied from 0.05 to 0.16 µM S/day, whereas in oil-water separator, where produced waters were collected, SRA was one order of magnitude higher (1.8 µM S/day). These data indicate that major sulfide production occurred in the above-ground facilities, rather than in the reservoir. Potential nitrate reduction measured in laboratory incubations with the addition of nitrate was also more active in the samplers taken from oil-water separators than from oil wells (Table 2).
Table 2 Biogeochemical characteristics of the studied sites
|
Site |
Sulfate Reducing Activity (µM S/day) |
Potential Nitrate Reducing Activity (mmol/L/day) |
SRB (cell/ml) |
DNB (cell/ml) |
|
Well 1 |
0.16 ± 0.04 |
0.0165 ± 0.004 |
2.5 x 103 |
2.5 x 102 |
|
Well 2 |
0.05 ± 0.016 |
0.0115± 0.0005 |
4.5 x 102 |
2.5 x 105 |
|
Oil-water separator |
1.8 ± 0.27 |
0.06 ± 0.02 |
2.5 x 103 |
15 |
Bacterial enumeration showed that both SRB and DNB were present in the studied reservoirs. However, in the Bebee-Konawa field SRB numbers in the oil-water separator were two orders of magnitude higher than that of DNB, and this ratio stayed constant from sampling to sampling. So far, microbial processes were more active in the above-ground facilities than in the reservoirs and sulfate reduction was the predominant terminal electron accepting process.
These findings strongly suggest that the above-ground facilities rather than reservoir should be the target for the future treatment. To study interaction of nitrate reduction with other microbial processes and to find out working concentration of nitrate, laboratory incubations with the produced water from oil-water separator as a possible target for the treatment have been started.