Low Resistivity Pay has for some time been recognized as one of the challenges in Carbonate Petrophysics. Carbonate rocks are characterized by their complex and heterogeneous pore structures and the associated difficulties in determining accurate formation hydrocarbon saturations. The Archie-equation - in spite of Gus Archie's doubts about the validity of his equation in carbonates - continue to be used to evaluate these reservoirs, with many attempts to modify the equation's parameters; a, m and n to derive a saturation closer to core saturation data.
Although carbonate reservoirs are heterogeneous on almost every scale, there is also organization: parasequences, cycles and sub cycles are seen in outcrops, on cores and borehole imaging logs. Such hierarchies of stratigraphic cyclicity may occur on different scales and are the result of the combined effects of several orders of sea-level oscillations during deposition. If these cycles result in layers with different properties, subsequent diagenesis and hydrocarbon charge may create a sequence of intercalations of capillary bound water in micro-porous layers and movable oil in layers with larger pores. Such thin beds may be below the vertical resolution of most logging tools, resulting in average values being measured. Resistivity tools are particularly sensitive to this challenge, the low resistivity of the water bearing layers effectively short-circuiting the horizontal resistivity measurement while the vertical resistivity is less affected, when the layers are at right angles to the borehole. Most conventional resistivity tools measure horizontal resistivity only, resulting in too high water saturation using the Archie-equation, increasing the risk of overlooking significant volumes of producible oil.
New tensor resistivity tools that measure both horizontal and vertical resistivities allow the conclusive identification of resistivity anisotropy, providing an effective warning that such a Low Resistivity Pay system may be present. The resistivity information from these tools can be used to provide more accurate water saturation using a new saturation model.
This model was put to the test in an appraisal well on land in Abu Dhabi where an extended data set was acquired. The improved carbonate interpretation model will be presented and compared with water saturation data from conventional resistivity tools, borehole imaging logs, core data and production test information. The integration of data from these various sources adds confidence to the estimation of water-saturation in such carbonates.
