Ku-Maloob-Zaap (KMZ) is the most productive oil field asset in Mexico located offshore in the Bay of Campeche with an oil production of more than 850,000 barrels per day. The primary producing reservoir is a highly fractured Cretaceous carbonate reservoir with a wide range of petrophysical properties.
Nitrogen has been injected into the crest of KMZ as part of the pressure maintenance and recovery program. However, with even the most effective gas-displacing-oil sweep efficiency, it is still unlikely to reduce the oil saturation to the true residual oil saturation (Sor) throughout the field in the initial displacement process. Movable oil and connate water remain behind in the gas cap to eventually be captured through gravity drainage. Residual oil (Sor) and movable remaining oil saturations (ROS) are important petrophysical properties that have a significant impact on reservoir simulations, reserves calculations and reservoir management.
The determination of the remaining oil saturation by using logging measurements in this highly fractured reservoir has resulted in overcoming multiple challenges. Massive drilling fluid losses are a crucial issue that had to be taken into account. Massive fluid losses affect even the deepest reading of resistivity logs in some intervals and increase the uncertainty of other logging tools with shallow depth of investigation (DOI). It has also been found that azimuthally oriented, pad-type logging measurements can underestimate the total porosity in this highly heterogeneous reservoir with large solution features. Finally, perhaps the most difficult challenge has been the ability to sample the reservoir fluids at multiple intervals to confirm if the interval is at Sor or if there is a movable oil component in the gas swept regions using a wireline formation tester (WFT).
This paper presents a methodology that was used to evaluate ROS in this highly fractured reservoir. A comparison of independent log measurements to validate the ROS results is presented. Techniques ranged from the direct measurement of the bulk volume water from the new generation dielectric tools to the enhanced diffusion method from NMR tools. In addition, an integrated formation evaluation technique to determine the best locations for pressure measurements and reservoir fluid sampling is proposed. This integrated approach illustrates the use of the borehole images, the mobility information from acoustic data and NMR logs prior to sampling. The corresponding limitation of these measurements is also discussed.
