|
Case Studies |
1 Fossil Fuels and Environmental Geochemistry Postgraduate Institute: NRG, Drummond Building, The University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK
2 BP Exploration, Chertsey Road, Sunbury-on-Thames, Middlesex, TW16 7LN, UK
3 Postgraduate Research Institute for Sedimentology, University of Reading, Whiteknights, Reading, RG6 2AB, UK
Co-produced waters and gaseous H2S were sampled at four wells at Wytch Farm oil field, UK, over a one year period. The chemical and stable isotopic (H, O) compositions of the waters were determined, plus the sulphur isotopic composition of dissolved sulphate and gaseous sulphide. Formation waters contain about 70 000 mg 1–1 total dissolved solids (TDS) and are evolved meteoric waters; there is a 10% fieldwide variation in TDS. H2S was formed by (bacterial?) reduction of formation-water sulphate. A high but variable fraction of the sulphide has been subsequently lost, perhaps by reaction with abundant Fe-bearing minerals. At least some, and possibly a significant fraction, of the sulphide in the reservoir is dissolved in formation water. Simple mass-balance models show that when reservoir sulphide is pre-dominantly within formation water, the concentration of H2S in produced gas will increase dramatically with increasing water cuts. In this case, increasing concentrations of H2S in produced gas do not necessarily imply that souring has been caused by production practices. Seawater breakthrough is recognized and quantified in one well using chemical and stable isotopic data. K, Mg and SO4 are all lost from injected seawater during its passage through the reservoir. Geochemical modelling and isotopic data fail to identify the fate of the missing sulphate, although the isotopic data tentatively suggest that some may have been converted to sulphide. However, the levels of sulphide produced from the breakthrough well are no higher than in other wells, suggesting that reduction of seawater sulphate contributes a small percentage of fieldwide H2S production.
