Core Analysis with Two Dimensional Nmr

We developed several novel two-dimensional (2D) NMR core analysis techniques, which not only enhance the analysis of fluid-saturated rock samples, but also open up an unexplored area for petrophysical research. This is different from the conventional one-dimensional (1D) NMR core analysis, in which only the proton population versus relaxation times, i.e., T1,2 distribution, is displayed to characterize the rock properties. In the new techniques, the proton population can be expressed in a 2D correlation map, where one axis is the relaxation time, and the other axis can be internal field gradient, diffusion coefficient, or proton chemical shift. When the diffusion coefficient is used as the axis, the oil and water in the pore space can be separated in a 2D display. Using a Magic Angle Spinning technique, which removes the internal magnetic fields caused by the susceptibility contrast between solid matrix and pore fluids, the proton population can be displayed as a function of high-resolution proton NMR spectrum and T1 or T2 relaxation time in a 2D plot. This T1-MAS 2D NMR technique provides a possible way to study the surface wettability. Several experimental examples are presented to illustrate the two dimensional NMR techniques. INTRODUCTION Nuclear magnetic resonance (NMR) technology has been successfully applied to petrophysical studies of porous media. Among these applications, NMR relaxometry using the CPMG pulse sequence at low fields (600 kHz~2MHz) has been routinely used to estimate porosity, irreducible water saturation, permeability, and pore size distribution of rocks [1]. Combined with NMR diffusion measurements, NMR relaxometry has also been extended to perform fluid typing and oil viscosity estimation [2]. The successful applications of fluid typing techniques depend not only on the properties of the fluids in porous media, but also on the properties of its solid matrix [3]. This is because the relaxation time of hydrogen nuclei of the fluid is related to three factors: bulk T1,2 relaxation, surface relaxation, and molecular diffusion. The bulk relaxation (T2B) characterizes the process of energy transfer of spin nuclei through the coupling with lattice. The relaxation time is related to the dipolar coupling between spins, Larmor frequency, and degree of the random motion. The surface relaxation (T2S) represents the interaction of molecules in the pore fluid with the solid matrix, and is greatly enhanced by the presence of paramagnetic impurities at the pore-solid interface. The surface relaxation time is related to the surface relaxivity and the surface-to-volume ratio of the pore space. The effect of molecular diffusion (T2D) to the spin is related to the diffusion coefficient of the molecule, the magnetic field gradient, and the inter-echo spacing of a CPMG sequence. The experimentally measured apparent relaxation time (T2a) is given by