Textural and wettability variations are two main conundrums in carbonate reservoirs. Adding heavy oil presents an incremental challenge to the reservoir description. Ineffectual analysis often leads to poor completion and high water production. Integrating multifrequency dielectric and nuclear magnetic resonance (NMR) logs leads to improved carbonate reservoir evaluation.
Dielectric and NMR logs have similar depths of investigation, both probing the flushed zone. It is difficult to distinguish bound-water from heavy oil in rocks from NMR tools alone. The addition of accurate water-filled-porosity (ϕW) measurements from the dielectric log allows us to differentiate the fluids. Using ϕW as a reference and accounting for restricted molecular diffusion in small pores, NMR analysis based on diffusion relaxation provides more accurate oil viscosity and fluid volumes.
The computation of viscosity from NMR assumes that the oil relaxation is dominated by bulk relaxation; oil-wetting of pore surfaces causes viscosity to be overestimated. Conversely, the wetting state can be inferred if the viscosity is known. Dielectric dispersion with frequency provides a measure of the water-phase tortuosity MN, which in turn is affected by texture and wettability. We compared the in-situ NMR signal of oil to the surface NMR measurement of a bulk oil sample. We also identified a correlation between MN and the Archie cementation exponent, mainly driven from NMR. This leads to interesting conclusions about texture and wettability, which are found to be coherent with the existing reservoir data.
Together with the viscosity profile, the comparison between the pore fluid volumes in the flushed zone and the bulk volume of water from deep resistivity is used to identify the zones of movable oil and to estimate residual oil saturation. The method was applied to datasets from three wells in a shallow carbonate reservoir. The estimated viscosity was validated by pressure/volume/temperature (PVT) analysis. In two wells, the prediction of movable oil and water zones was confirmed by downhole test results. In third well, the free-water zone was confirmed by production logging.