NMR petrophysical predictions on digitized core images

Abstract

NMR is a popular logging technique used to estimate pore size information, formation permeability, wettability and irreducible water saturation. Quantitative interpretation of NMR data is based on a set of fundamental assumptions, including pore isolation and fast diffusion. These assumptions establish the quantitative link between NMR response and petrophysical predictions. While there is a need to test these assumptions directly, to date no quantitative study on reservoir core material has been undertaken. The ability to digitally image reservoir rock in 3D and calculate petrophysical properties directly from the images coupled with a comprehensive simulation tool to numerically generate a range of NMR response data may help to address this need. In this paper we image a large set of reservoir cores including sandstones and carbonates at the pore scale using high-resolution micro-CT. A set of petrophysical properties are measured directly on the cores, including surface-to-volume, permeability and pore size distribution. The permeabilities of the cores range from 10 mD to several Darcies. Realistic multiphase fluid distributions are derived by simulation of drainage. We then simulate the NMR responses on the same core images using a comprehensive NMR simulator. The internal magnetic field is derived numerically from applied magnetic fields and susceptibility distributions and the phase evolution of the magnetic spins calculated with a random walk method. NMR responses currently include inversion recovery (T1) and CPMG (T2), and the longitudinal and transversal signals are monitored simultaneously. The interpretation of the signals acquired is done by standard 1D Laplace inversion to calculate the pore size distribution from T2 responses, and with a 2D inverse Laplace transform for fluid typing. In this study we compare predictions of petrophysical properties from the interpretation of the NMR response to direct calculations of the same properties on the images. Partitioning of the pore space is also undertaken to directly test the fundamental assumption of pore isolation. This allows us to calculate the coupling constants and magnetization exchange between pores, or between macro- and micro-porous regions. Additionally, the sensitivity of the responses to variations in relaxivities or the presence of magnetic impurities is studied. Fluid typing is performed on a shaly sandstone sample.