2019 AAPG Annual Convention and Exhibition:

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New Opportunities in Old Fields, A Case Study From Dahomey Basin, Offshore West Africa


In 2016, oil discoveries fell to a sixty-year low prompting oil and gas companies to innovate strategies for reserves evaluation and arrest fast declining production in mature fields for a sustainable petroleum business in a prolonged fluctuating oil price regime. And as hydrocarbon exploration and development moves into more challenging environments, a strong geological understanding of the subsurface and the deployment of fit for purpose technologies and evaluation techniques to aid extension of mature fields’ life cycles are of paramount importance. The presented case study is an intelligent logging assessment of reservoir units in a development well drilled through a complex shaly sand sequence offshore Gulf of Guinea; where the novel dielectric dispersion measurement is combined with high resolution magnetic resonance for improved formation evaluation and reservoir management. This integrated data analysis workflow was designed for fast determination of pay zones and movable fluids in the reservoir zone. The study area lies within a complex Turonian formation with varying lithology, cementation, lamination, fresh formation water salinity, heavy oil and compartmentalized reservoirs. Clay content and fresh to very fresh water salinities affect any conventional evaluation and resistivity-based logging methods. Based on the experience gained in offset wells, the measured resistivity in water and hydrocarbon could be similar or even present reverse profile. The deployed dielectric dispersion measurement provides dielectric permittivity and conductivity at multiple depths of investigation through the use of multiple frequencies, receiver spacing, and polarizations that are adequately fit into an innovative mandrel design. Robust inversion of all measurements enables solving for salinity, invasion profile, water fraction, shallow zone resistivity, and saturation even when it is hard to distinguish oil from fresh water. The workflow drastically reduces evaluation uncertainties of conventional approaches and anticipate well productivity. The actual well clean-up results after completion confidently depict a well characterized reservoir and accurate oil-water-contact (WOC) estimation which enabled us to isolate the bottom flushed zone during perforation and helped in resolving early water production in the field.