Reinvestment in, and the rejuvenation of, aging oil fields are becoming increasingly commonplace. Because the price of oil has stabilized at an equitable value and enhanced oil recovery methods have been proven, previously-depleted oil reservoirs are being revisited to determine their potential for tertiary recovery by means of CO2 flooding. Accurate determinations of unswept oil and permeability distribution within the reservoir are critical elements in understanding and optimizing the CO2 flood. This paper presents a pilot study in utilization of NMR logging in the redevelopment of a field that has been waterflooded since 1953.

These mature reservoirs pose well-known challenges for formation evaluation. Resistivity-based saturation models are inadequate and uncertain because of previous waterfloods and variable formation water salinity (Rw) values. The variations in grain size and the onlapping of sand bodies cause uncertainty with the use of a porosity-permeability transform to estimate permeability.

These challenges must be overcome for optimal well placement, infill drilling, CO2 flood design, and other reservoir management practices. The standard logging suite for newly-drilled wells, triple combo and nuclear magnetic resonance (NMR), has evolved to address many of these challenges.

This paper presents a pilot study that demonstrates the effectiveness of NMR logs for reservoir characterization. These reservoirs were evaluated for estimating formation porosity, bound and moveable fluid volumes, permeability, and remaining oil saturations. The log data were compared and calibrated with X-ray diffraction (XRD), Specialized Core Analysis (SCAL), and Capillary Pressure data from core. Pore-size distribution from NMR has been used to identify facies changes and for geomodeling purposes. Variations in the facies and the distribution of the sand bodies have been correlated to 3D seismic data. This has helped with the accurate mapping of the reservoir units to understand sweep efficiency. Production results are used to validate the log interpretations. A similar method can be used in other aging oil fields and to evaluate the development plan of a CO2 flood.

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