Toluene Depletion in Produced Oil Contributes to Souring Control in a Field Subjected to Nitrate Injection

Souring in the Enermark field is caused by the presence of 0.8 mM sulfate in the injection water. Because of its shallow location, it has a low bottom-hole temperature (30oC) allowing microbial growth throughout. The injected sulfate is reduced by sulfate-reducing bacteria (SRB) to sulfide, which appears in the produced water at an average concentration 0.13 mM. Inclusion of 2 mM nitrate in the injection water results in reduction of the injected nitrate and sulfate along the injection water flow path. From injector to producer zones of nitrate-reducing, sulfate-reducing and methanogenic activity are expected, depending on depletion of the injected electron acceptors. Microbial community analysis by pyrosequencing indicated that produced waters were dominated by methanogens, in agreement with the fact that nitrate and sulfate were absent from most of these. Units in the field with appreciable nitrate concentration had elevated levels of the nitrate-reducing betaproteobacterium Thauera. Following 219 weeks of nitrate injection, breakthrough was observed in production well 2-PW, which is a major producer of water and sulfide at Enermark. The sulfide concentrations produced by 3-PW, 5-PW and 16-PW also reached zero. As a result, the average concentration of sulfide for 14 producing wells has decreased significantly since week 192. GC-MS analysis indicated that oil produced by 2-PW is depleted of toluene, the preferred electron donor for nitrate reduction. Oil production by water injection typically leads to a declining fraction of oil over oil plus water with time. At Enermark this decline has been approximately two-fold over the 219 week period. Because 2-PW produced on averaged 5.1% oil over the 219 weeks, each volume of oil was contacted on average by 19 volumes of produced water. Given that Enermark oil has approximately 5 mM of toluene and that 2 mM nitrate was present in the water it follows that a 7.6-fold excess of nitrate over toluene was present, which exceeds the stoichiometric ratio of 7.2 for nitrate-mediated oxidation of toluene to CO2 and N2. Nitrate breakthrough at 2-PW is thus caused by the fact that injected electron acceptor exceeds available electron donor. Several other wells showing souring control (3-PW, 5-PW and 16-PW) produced a similarly low fraction of oil indicating that nitrate breakthrough may be expected in the future.