MODELING OF NUCLEAR MAGNETIC RESONANCE SURFACE RELAXATION IN POROUS MEDIA WITH IMPROVED CALCULATIONS OF INTERFACIAL ABSORPTION PROBABILITY AND POROUS SURFACE AREA

摘要

In digital rock physics, surface relaxation, a dominant nuclear magnetic resonance (NMR) decay effect in porous media, is often simulated by random-walk methods, where a large ensemble of Brownian walkers representing the magnetized nuclei are absorbed into solid surface controlled by an interfacial absorption probability. Previous works either neglected the pi vel effect of digital pore space, which can easily result in overestimation of the surface area of the porous space, particularly on some unconventional reservoir rocks with large amounts of microscopic pores, or involved arbitrary tuning of surface relaxivity strength (SRS) to match the simulation results with the experimental measurements, which to some extent violates the fact that SRS is a fixed physical parameter of rock type. In this study we propose a new model of interfacial absorption probability for surface relaxation that not only honors the physical principles but also reduces the uncertainty arising from determination of SRS. Effective implementation of this absorption probability into a simulation program requires accurate evaluation of the surface area of pore spaces, which is assured by a widely used method, the marching cubes algorithm. The updates proposed are verified by performing NMR simulation tests on both standard geometries and realistic 3D heterogeneous porous media.