NMR Measurements for Pore Size Mapping at Fine Scale

As fluids move through a rock their flow path is controlled by the capillary forces from the local pore size distribution. The pore structure causes the fluid not to follow a simple path which is a familiar challenge in reservoir production and recovery. In this paper we examine this effect at a small, more manageable scale of a core plug in laboratory. Fully water saturated plugs were centrifuged in air using small capillary pressure steps. At each step the T2 distribution of the core was measured. The capillary pressure steps were incremented at one psi steps for careful mapping of the evolution of fluid distribution. In this experiment the water was replaced by air which has no NMR signal, thus the results clearly showed gradual removal of free water from the larger pores with no reduction of bound water signal. Comparing T2 distributions from different capillary pressure steps, we were able to pinpoint the pores contributing to fluid displacement at each pressure. These results, for the first time, reveal more detailed pore information that is apparent from normal T2 distribution alone. The new approach enables deeper understanding of rock pore structure and how the fluid distribution is influenced by the pore sizes involved in conducting the fluid. These results, once up-scaled to reservoir level, will help optimize and improve oil recovery. The technical contribution of this paper includes pore size study at a finer scale by NMR than previously reported.