Several mechanisms have been proposed to account for the increased oil recovery observed as a result of Microbial Improved Oil Recovery (MIOR) processes, both in laboratory experiments and field cases, but few of these mechanisms are verified. One of the reasons may be that these systems include living bacteria that ideally should be kept in a growing state during the investigations. This paper presents experimental results and a literature review where mechanisms are discussed, with main focus on changes in interfacial tension (IFT), wettability and permeability reduction during MIOR experiments. Special emphasis is put on wettability changes in a laboratory study of an MIOR core. The wetting conditions are studied by using an environmental scanning electron microscope (ESEM) that has been improved by modifying the cryogenic vacuum system for sample preparation. The ESEM equipment enables qualitative and semi-quantitative investigations of wettability changes after a core flood. The saturation profile in an MIOR core should show where the incremental oil is mobilized. An alternative technique for saturation determination is therefore presented, which is based on image analysis from electron images acquired in the same process as the wettability investigations. The number of cross sections with saturation estimations in this study is too few to make a reliable oil saturation profile (e.g., compared to a computer tomography scan). However, the average saturation from the images is close to the saturation calculated from the flooding process. The results of the study indicate that only small parts of the pore space have changed from their original strongly water-wet condition to a more oil-wet state.
MIOR has shown significant potential for improving the recovery factor and should therefore be considered for implementation in producing oil fields and also in reservoir development plans. The technology is very cost efficient in a field case due to several reasons: When a water injection system is in use, the MIOR process can be initiated without major changes, thus the investment costs are low. Further, the use of biocides to limit the growth of bacteria producing H2S, and thereby causing reservoir souring, can be reduced since the MIOR process stimulates the growth of other bacteria which suppresses the growth of the H2S producing strains.