This paper compares production results and the evaluation of conventional logs versus advanced logs as nuclear magnetic resonance (NMR) in a mature field, discusses vertical reservoir heterogeneity and different flow units in apparent homogeneous reservoirs, and identifies bypass reserves oil resulting from poor sweep efficiency.

A petrophysical evaluation was performed using conventional and advanced logs. Comparing the results with dynamic production logs demonstrated that the NMR evaluation was more accurate than conventional log evaluation in identifying different flow units.

Traditional interpretations with conventional tools did not identify the water entry problems caused by different flow unit systems, which resulted in production sweep effects from nearby wells. NMR identified this problem, the reservoir quality, and the vertical heterogeneity and its effect on water entry. It was determined that a uniform sweep did not exist from the bottom to the top of the reservoir; thus, the aquifer water entry depended exclusively on reservoir quality related to flow unit characterization and capacity, as permeability and seal distributions are locally developed.

Adding to this complex scenario of identifying bypassed zones and the noncorrelation with conventional logs is carbon dioxide (CO2) hiding the presence of the true porous zones, potentially leaving remaining reserves that could be overestimated or underestimated.

Comparing dynamic logs versus static logs resulted in a credible endorsement of NMR, demonstrating its advantages and accuracy over conventional logs and a better technical application for recovering the remaining reserves and optimization of a mature field.

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