Iraqi Kurdistan All in the Timing

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Reservoir Management oil company that reservoir souring due to microbial metabolism was occurring. This worrying conclusion, however, was based on two mistakes. When the data was received it was attributed to separator gas from a specific well, but the individual who collected the sample did not appreciate that knowing the precise location that the sample was collected from was important to the interpretation of results. The second mistake was not realising that sulphur isotope abundance in various materials in nature can vary considerably, so that merely obtaining data for the sulphur isotopic ratio in a given sample will generally not allow conclusions to be made as to whether a given material originated from chemical or from biochemical reactions. For the microbiological conversion of sulphate to hydrogen sulphide, depletion of 34 S in the sulphide has been reported to typically range from -15‰ to -25‰ and this can only be determined by obtaining sulphur isotopic data from the sulphate in the produced water and in the H 2 S in the gas: both samples should be from the same well. Moreover, the precise location of any samples collected is important, as was shown in subsequent testing. The data in the table below illustrate that subsequent, more thorough testing documented that there was essentially no change in the sulphur isotopic ratio when H 2 S samples obtained from primary separator gas samples were compared with values from sulphate in the produced water from these same wells. However, when H 2 S samples were obtained from topside water-handling system locations downstream from the primary separators, then significant depletion of 34 S in the sulphide was observed, which is consistent with microbiological sulphate reduction. Additional testing (data not shown) also demonstrated that organic acids such as acetic acid and propionic acid were present in topside water samples but not in separator water samples. Organic acids are a byproduct of microbial metabolism. Furthermore, microbial growth tests and genetic tests (qPCR or quantitative Sulphur isotopic data from offshore gas and sulphate samples Well Sample Location Biological cause: desulfovibrio vulgaris is the best-studied sulphate-reducing bacteria species; the bar in the upper right is 0.5 micrometre long. δ 34 S ‰ #1, sample set A separator 33.9 32.9 1.0 #1, sample set B topside water system 8.4 32.9 -24.5 #2, sample set A separator 30.8 32.7 -1.9 #2, sample set B topside water system 8.6 32.4 -23.8 Gas polymerase chain reaction) documented the presence of acidproducing bacteria, sulphate-reducing bacteria, and other microorganisms in topside water samples but not in separator water samples. Biological Processes Not Responsible When all of these data are considered, there is no evidence for microbiological activity in the subsurface creating reservoir souring; however, once the produced water is brought topside it is vulnerable to microbial contamination and the subsequent production of H 2 S. The source of the thermogenic (chemically created) H 2 S recently observed in the gas from some wells is unknown, but is presumably due to the presence of a H 2 S-containing gas/oil/water source that is in communication with the oil-producing zones tapped by these oil wells. Knowing that biological processes are not responsible for the observed H 2 S in reservoir gases will allow the oil company to take appropriate steps in producing oil from existing wells, and ‰ Differential it will influence the future Sulphate exploration and production activities at this oil field. As a result of this study biocide has been added to the topside water-handling system at this oil platform, and a programme for the routine monitoring of corrosionassociated microorganisms has been implemented. 62 GEOExPro November 2015
KNOWLEDGE FOR DECISIONS. DAKS TM is a knowledge platform that enables you to apply lessons learned and best practices from over 1,400 rigorously documented reservoirs. Benchmarking and analytical tools, along with the ability to capture knowledge about your own assets for comparison against proven analogs, empowers you to make better E&P decisions. • Validate your project assumptions against facts from proven reservoirs to help reduce subsurface uncertainty • Capture your asset knowledge and benchmark against global analogs to identify potential areas for improvement • Identify reserve growth potential and candidates for acquisition or divestiture to help optimize recovery • Generate and test new ideas for exploration and production CCRESERVOIRS.COM For more information, contact info@ccreservoirs.com
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vol. 12, no. 6 - 2015 GEOSCIENCE &
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Joining up the exploration process
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Market Update The Train Has Left th
Page 8 and 9: Regional Update Markets Behaving Di
Page 10: Update Endgame for the North Sea? T
Page 14 and 15: A Minute to Read… News from aroun
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Page 18 and 19: Licensing Opportunities Africa Lead
Page 20 and 21: AA 0 AA 0 Croatia’s 2016 Licence
Page 22 and 23: migration have been modelled across
Page 24 and 25: Cover Story Iraqi Kurdistan: It’s
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Page 32 and 33: Geology Black &Blue Using blue sky
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Page 36 and 37: GEO Profile Mahmoud Abdulbaqi, reti
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Page 40 and 41: The East Shetland Platform A Cleare
Page 42 and 43: Figure 6. Total subsidence curve be
Page 44 and 45: History of Oil The Petroleum Gulf O
Page 46 and 47: History of Oil Shell International
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