A review of reservoir oil-water transition zone characterization and potential recovery methods

The transition zone (TZ) in an oil reservoir has traditionally been a volume of lesser interest compared to the oil-saturated zone. Researchers have suggested that it can contain commercial hydrocarbon volumes. Therefore, this paper seeks to summarize the characterization methods of TZs for the assessment of oil production opportunities. Another goal is to summarize the potential methods of oil production from TZs. It is conceivable that TZs will produce both water and oil together. However, some surprising instances of dry oil (i.e., 100% oil, with no associated connate water) production, due to the formation of water clusters, have also been observed earlier. Also, oil can possibly be found below the current free water level. Characterizing TZs is more complicated compared to the oil-saturated (irreducible water saturation) zone. TZs can show variable wettability and permeability characteristics due to several complex phenomena related to buoyancy, capillarity, diagenesis, cementation, and reservoir tilting. Careful TZ core characterization followed by reservoir simulation and oil production can increase the overall reserves. Methods for TZ characterization include petrophysical logs (resistivity and NMR), geophysical analysis (AVO and P-wave absorption), and reservoir modeling (saturation height functions and wettability). Analysis of core obtained from TZs using the centrifuge method can reveal the residual oil saturation and relative permeabilities, which can aid the prediction of future oil production. More complicated analyses include structure and stratigraphic geological models and basin modeling for hydrocarbon migration history. Possible oil production methods from TZs include CO2 injection, surfactant flooding, combined carbonated water and surfactant flooding, and smart well placement. We recommend including TZs that span several meters in depth as part of reserves calculation.