A New NMR-Based Height Saturation Model of a Low Permeability Carbonate Reservoir

Abstract

Abstract Formation evaluation studies suggest that high in the hydrocarbon column, the resistivity logs can precisely quantify fluid saturation due to the large contrast in the resistivities of hydrocarbon-bearing and water-bearing formations. However, in the transition zone where water and oil reside in more or less equal volumes, the determination of hydrocarbon saturation by resistivity value becomes challenging. Some of these intervals exhibit low resistivity pay (LRP) characteristics where resistivity-based log analysis predicts high water saturation, yet they can produce little or no water-cut. Conventional log-based saturation and rock quality evaluation in a low permeability carbonate reservoir is difficult due to the lack of the input measurement's sensitivity to pore size and the amount of pore-filling fluids. Pore size information provided by Nuclear Magnetic Resonance (NMR) logs from this LRP provides good sensitivity, but it needs to be calibrated for quantitative use. The objective of this study is to determine a height-based NMR Bulk Volume Irreducible (HBVI) cutoff to distinguish and quantify the amounts of reservoir fluids across a wellbore using NMR logs. The procedure consists of two-part workflow. The first part describes the acquisition of a data base that includes high-quality laboratory NMR and capillary pressure measurements to determine the pore aspect ratio and the effect of temperature on the formation's NMR properties using core samples from the target reservoir. These measurements are then used to underpin the mathematical description of the HBVI cutoff as a function of displacement pressure that is translated to height above the free water level (HAFWL). The second part of the workflow is a well-log processing scheme where the new formula is implemented to calculate a continuous fluid saturation profile across the well using NMR logs. The laboratory measurements suggest a good agreement between the capillary pressure and NMR T2 measurements. Both data sets indicate a well sorted pore size distribution. The T2 relaxation time increases with temperature, which is then considered in the downhole implementation of the HBVI model. The NMR-based saturation log is consistent with wireline formation testing (WFT) observations and mercury injection capillary pressure (MICP)-based saturation height modeling results in a low resistivity pay reservoir. The results of this study suggest that the laboratory calibration and NMR log processing workflows described herein provide a viable alternative for the calculation of fluid saturations in complex reservoirs where the conventional log-based saturation evaluation faces uncertainties.