Identification and Quantification of Microorganisms involved in Downhole MIC in a Dan Oil Producing Well

In order to investigate the condition of the horizontal Dan West Flank multizone well MFF-25A a caliper survey was performed early 2009. The results showed that the tubing of the well was severely corroded and several fully penetrating holes in the tubing existed. This study investigated whether the accelerated corrosion was caused by microorganisms.

 During the caliper survey, a sample of downhole deposit was collected using a junk basket. The number and nature of the microorganisms was determined using standardized and targeted quantitative polymerase chain reaction (qPCR) protocols for quantification of  sulfate-reducing Bacteria (SRB), sulfate-reducing Archaea (SRA) and methanogens known to promote microbiologically influenced corrosion (MIC). In addition, X-ray fluorescence was conducted for elemental composition analysis of corrosion products.

 Produced water samples were also collected from MFF-25A and analyzed using qPCR. The comparison of sessile and planktonic microorganisms is relevant as most MIC monitoring in the oil field is conducted on planktonic microorganisms.

 The results showed that both SRB, SRA and methanogens were involved in MIC in the well. In the solid sample, the SRA were as abundant as the SRB constituting 50% of the sulfate-reducing prokaryotes (SRP). In the produced water samples the SRA were the most abundant SRP constituting 98%. Thermophilic methanogens were particularly abundant in the sessile sample corresponding to 65% of all prokaryotes. In the planktonic sample the methanogens constituted 4% of all prokaryotes.

 The study demonstrates that SRA and methanogens within Archaea should also be included in future MIC monitoring programs and corrosion risk assessment models together with the SRB. For well surveillance, the evaluation of planktonic samples needs to take into account that the planktonic sample represents the microbiological population over the entire length of the well. In addition, the proportions of the various microbiological groups are different to the local sessile populations.