Nuclear Magnetic Resonance Relaxation in Porous Media

Abstract

Abstract With the recent introduction of the new generation Magnetic Resonance Imaging Logging (MRIL) tool, it is now possible to make down hole measurements of the proton spin-spin relaxation times, T2, and the spin-lattice relaxation times, T1. It has been previously demonstrated in laboratory measurements that the surface-to-volume ratio and fluid permeability can be estimated from T1 measurements. Permeability values derived from T1 measurements are sensitive to the magnetic impurities and magnetic heterogeneities that determine the surface relaxation velocities. The analysis of T1 measurements is usually complicated by the combination of random internal magnetic field gradients and fluid diffusion. Alternatively, diffusion effects can be quantified using the CPMG pulse train spin-echo experiment with variable pulse spacings. The internal gradients are determined by the pore space geometry and therefore the transverse relaxation of the fluid spins in the presence of these gradients can lead to an estimate of characteristic length scales of the pore space and of the fluid flow permeability. Our experimental results indicate that this permeability estimate is less sensitive to the surface relaxation velocity than are permeability estimates based on T\ measurements. Permeability estimates based on the results CPMG T1 experiments on several sandstone cores are presented.