CORAL Complex Reservoir AnaLysis

CORAL Analysis provides an accurate detailed description of reservoirs and mixed lithology. Its primary use is in the evaluation of formations, which contain shale, sand, limestone, dolomite, anhydrite, and gypsum. Minimum log data consists of a gamma ray or spontaneous potential log; a density, neutron, or sonic porosity log; and an induction or laterolog resistivity log. Improved results are obtained if photoelectric (Pe), shallow resistivity, sonic, dielectric, and spectral gamma ray data are used.

        These logs will provide determination of shale content, cross plot lithology (sandstone-limestone or limestone-dolomite), anhydrite content, porosity, secondary porosity or gypsum (if an acoustic log is available), permeability and water saturation. If it is known that there is no limestone in a formation, the combination of sandstone-dolomite can be evaluated.
        Shale content can be computed from a number of shale indicators: Gamma Ray, SP, Deep resistivity curve, Thorium, Potassium, Gamma ray Thorium+Potasium, Neutron, Alternative Indicator, External Shale Volume, Density-Neutron cross plot, and Density-Acoustic cross plot. Several of these shale indicators can be used simultaneously, and then the minimum shale content of several shale volumes will be computed.
        Matrix densities of major minerals (sand, lime, dolomite, and anhydrite) can be changed within certain limits if necessary when solving density-neutron cross plot for lithology and porosity. Correction for gas can be done if density of gas is known. Salt and coal contents can be identified by setting the appropriate flags. There are options to compute porosity from single porosity logs (Density, Neutron or Acoustic) Several equations are included to compute water saturation: Archie. Simandoux, Indonesia, Waxman-Smits, and Dielectric analysis models. Permeability can be computed from one of two equations: Timur or Wyllie-Rose. Effective permeabilities for hydrocarbons and water can be computed from one of the three equations: Park Jones, Boatman, or Pirson.

This program has been used popularly in L&TD for sedimentary rock evaluation.

Tool response equations available are SP, GR, Rt, Rxo, density, neutron and sonic.

Figure 1 shows the result of a CORAL processing for Shaly Sand-Lime-Dolomite with coal. The unknowns are porosity, clay volume, sand volume, water saturation, water saturation in flushed zone, hydrocarbon density, lime volume, dolomite volume, coal volume, and permeability index.