NMR Petrophysics in Thin Sand-Shale Laminations

Abstract

Abstract We use nuclear magnetic resonance (NMR) logging to help with the petrophysical evaluation of thin sand-shale laminations. NMR helps to 1) detect thin beds, 2) determine fluid type, and if hydrocarbon is present, 3) establish the hydrocarbon type and volume, and finally 4) determine the permeability of the sands (as opposed to that of the sand-shale system). Formation evaluation in thin sand-shale laminations starts with their detection. NMR vertical resolution is mainly controlled by the antenna aperture, that is, in the case of a high-resolution antenna, 6 in. or 15 cm. Within that distance NMR tools will cumulatively measure all layers of shales and all layers of sands regardless of their individual thicknesses. Because NMR relaxation time in shales is much faster than in the productive sands, thin sand-shale laminations appear on NMR logs with the characteristic bimodal relaxation distribution. The thin laminations are often below the resolution of conventional logs that have a typical vertical resolution of 6 to 12 in. or 15 to 30 cm. This makes fluid typing in the centimeter-thick sands problematic from conventional logs. Also, formation pressure or sampling tools could hit-and-miss the thin sands. In contrast, since gas, oil, and water have different properties, fluid typing techniques that exploit all NMR relaxation times (T1 and T2) and diffusion (D) offer new ways to determine the fluid type in thin layer sands. From the bimodal relaxation distribution of the laminated sand-shale system, it is often possible to determine a cutoff to separate the two components. Porosity in the sand component can then be estimated separately and with it the hydrocarbon pore volume. Conventional high-resolution permeability from NMR is limited to one antenna aperture. If the sand layer thickness is less than that distance, the determined permeability of the sand-shale system will underestimate the true permeability of the sands. Using a fluid flow model, we show that the permeability of the sand component can be estimated separately. Experiments were conducted to verify the characteristic NMR bimodal relaxation distribution in thin beds, and to investigate whether the fraction of sand/shale and the sand porosity could be determined from NMR logs. The results confirmed observations on logs, of which we show case examples of thin sand-shale laminations that are water-bearing, oil-bearing, and gas-bearing respectively. In each case the NMR detection was verified against imaging logs, and the fluid type in the sands was determined from multi-dimensional NMR analysis. The derived hydrocarbon volume was then compared with the results estimated from a full triaxial (3D) induction tool. Permeability of the sand layers was also computed and compared to that of nearby thick sands. Core data in one well was used to validate NMR detection, porosity, permeability and net sand thickness.