NMR Measurements for Pore Size Mapping at Fine Scale
Authors: Valori Andrea , Ali Farhan , Al-Zoukani Ahmad , Taherian Reza 
Published: March 2015

Abstract
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.

Introduction
Rock pores are important in reservoir production and planning. The pores parameters such as size distribution and volume are used in estimating important reservoir properties. The pore size distribution is usually determined using mercury injection capillary pressure (MICP) by injecting mercury and measuring the volume as a function of applied pore pressure. However, this method is not applicable to in situ measurements; further, there are no methods of estimating pore size distribution in situ. Correlations between MICP and NMR T2 distribution have been used to estimate the pore size distribution, but the results are often not satisfactory. A clear correlation between NMR T2 and pore size is not as yet available. This is partly due to the fact that NMR T2 is related to pore radius while MICP results are mostly controlled by the throat size. More research focused on understanding the relation between these two approaches is needed to advance our ability to relate the two in a quantitative way. In this paper we attempt to do that by measuring the specific NMR T2 peaks for each pore pressure.

Experimental
Core plugs from Carbonate outcrops in Saudi Arabia were used in this study. The plugs were cleaned and saturated with 100kppm NaCl brine. The plugs were dried and evacuated for 8 hours before brine was introduced into the vacuum chamber. To ensure complete saturation, the plugs were subjected to a pressure of 2000 psi for 16 hours. The porosities were calculated from the weight of the plugs before and after water saturation using standard approach. Helium porosity was 17.1% and Nitrogen permeability 27mD.
One of the cores, M7V, was selected for detailed studies; the remaining three were used as counterbalance weights in the centrifuge. Before subjecting the cores to centrifugal force, NMR T2 measurements were performed on fully water saturated M7V plug (SW=1). An Oxford Instrument NMR spectrometer, operating at 2MHz frequency was used for making these measurements. In addition, we measured the 1-D NMR porosity image of the M7V core plug. These measurements require a linear gradient coil which is a built-in feature of Oxford Instrument spectrometer.