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Surfactant adsorption is not just a primary economic impediment to chemical enhanced oil recovery (EOR), it has also been identified as a leading uncertainty in economic forecasting (Anderson et al., 2006). This is somewhat surprising, as the basic phenomena controlling the adsorption of anionic surfactants – at least on the majority minerals present in hydrocarbon reservoirs – are well established (Gaudin and Fuerstenau, 1955; Cases et al., 1982; Zhang and Somasundaran, 2006). Nevertheless, chemical EOR field trials have yielded results that diverge from lab measurements in poorly-preserved cores by a factor of 2 to 6 (Wang, 1993). In this article, we attempt to resolve the discrepancies between lab and reservoir adsorption measurements, continuing along the lines of the investigation by Wang (1993). The oxidation of reduced iron in reservoir materials may be linked to increased adsorption by several mechanisms: • Increased surface charge of iron-containing clays upon oxidation; • High specific surface area of positively charged hydrous ferrous oxide (HFO) colloids; • Higher iso-electric point (IEP) of some oxidized iron minerals with regards to those found in the reservoir. Total’s mineralogical database is queried to determine typical reservoir iron content and mineralogy, and these results add considerable detail to the sparse literature on the subject. Median iron content is around 1.2 wt%, although this is as high as 5 wt% in catchment zones associated with significant volcanism, and below 0.1 wt% in a carbonate formation. No evidence of Fe(III) is found in reservoir cores from oil-bearing zones, including in a shallow, biodegraded region where Fe(III) is present immediately above the oil zone, however the methods used are may not be capable of distinguishing Fe(II) from Fe(III) in clay minerals. Adsorption of surfactants on various minerals found in the lab and reservoir are presented in order to illustrate the points listed above. |
