A New Approach to Model Saturation Below Free Water Level, A Case Study From a Giant Reservoir in the Middle East

Abstract

The calculation of oil saturation below the free water level is often disregarded, considering it relates to uncertainty in calculations, or non-exploitable isolated residual hydrocarbon. In this giant field of the Middle East exploited from crest wells, a recent campaign of flank appraisal wells revealed significant hydrocarbon saturation that is incompatible with the original free water level and existing saturation height modeling in drainage. A completely new thinking of the saturation height modeling involving an imbibition cycle became one of the reasonable options to model the phenomena. To include imbibition into the saturation height modeling process, two additional steps are required. The first step is the construction of the imbibition saturation function from available core Pc imbibition experiments. A saturation function fitted to the data and combined with end points relationships defines the scanning curves behavior of the imbibition process from any initial water saturation in drainage. The second step is the search of the Paleo-free water level position in each well that ultimately is mapped to implement the saturation modeling in 3D. The log saturation profiles in the new flank appraisal wells, mud logging and core saturations have confirmed that hydrocarbon is present at depth significantly below the initial free water level and only an imbibition cycle post initial accumulation can explain this observation. The fitting of the Pc imbibition core data with a Van Genuchten type of function is appropriate and more practical for the 3D implementation than using other non-linearizable functions. The mapping of the paleo-free water level revealed that the structure at the time of accumulation was similar in shape but flatter than today’s structure suggesting a compression post-migration. The match between the Sw from logs and the Sw from this new saturation modeling approach is very robust leading to a much better in-place estimation and prospectivity. This case study is certainly an eye-opener for fields with similar migration and post-migration history. The methodology honors the principles of capillary pressures to model saturations and can readily be implemented in 3D in the same way saturation height modeling with drainage only is implemented. The process also improves the understanding of the geological post migration structural events that impact reservoir performance. Finally, it opens the opportunity for tapping additional oil in place below initial free water levels.