Comparison of Petrophysical Properties of Porous Rocks Using NMR, Micro-CT, and Fluid Flow Simulations

Apoorv Jyoti,Ralf R. Haese

doi:10.3390/geosciences11120500

Abstract

Micro-computed tomography (micro-CT) is increasingly utilized to image the pore network and to derive petrophysical properties in combination with modelling software. The effect of micro-CT image resolution and size on the accuracy of the derived petrophysical properties is addressed in this study using a relatively homogenous sandstone and a heterogenous, highly porous bioclastic limestone. Standard laboratory procedures including NMR (nuclear magnetic resonance) analysis, micro-CT analysis at different image resolutions and sizes and pore-scale flow simulations were used to determine and compare petrophysical properties. NMR-derived pore-size distribution (PSD) was comparable to the micro-CT-derived PSD at a resolution of 7 µm for both the rock types. Porosity was higher using the water saturation method as compared to the NMR method in both rocks. The resolution did not show a significant effect on the porosity of the homogeneous sandstone, but porosity in the heterogeneous limestone varies depending on the location of the sub-sample. The transport regime in the sandstone was derived by simulations and changed with the resolution of the micro-CT image. The transport regime in the sandstone was advection-dominated at higher image resolution and diffusion-dominated when using a lower image resolution. In contrast, advection was the dominant transport regime for the limestone based on simulations using higher and lower image resolutions. Simulation-derived permeability for a 400 Voxel3 image at 7 µm resolution in the Berea sandstone matched laboratory results, although local heterogeneity within the rock plays an integral role in the permeability estimation within the sub-sampled images. The simulation-derived permeability was highly variable in the Mount Gambier limestone depending on the image size and resolution with the closest value to a laboratory result simulated with an image resolution of 2.5 µm and a size of 300 Voxel3. Overall, the study demonstrates the need to decide on micro-CT parameters depending on the type of petrophysical property of interest and the degree of heterogeneity within the rock types.

Highlights

Fluid flow and multiphase flow in porous media play an important role in many natural and engineered subsurface processes including contaminant transport, hydrocarbon production and CO2 injection and migration
This non-destructive analysis is increasingly seen as superior compared to traditional methods such as mercury intrusion capillary pressure (MICP), mainly for two reasons: firstly, MICP only estimates the size of the pore throats in the pore network and, secondly, MICP is destructive for the sample; it cannot be used for further analysis
The core has a bulk volume of 57.004 mL, so the total porosity of the Berea sandstone sample based on the nuclear magnetic resonance (NMR) analysis is calculated as 16.6%

Summary

Introduction

Fluid flow and multiphase flow in porous media play an important role in many natural and engineered subsurface processes including contaminant transport, hydrocarbon production and CO2 injection and migration. The amplitude of the dipole moment is directly proportional to the number of hydrogen atoms present within the sample and can be used to measure the volume of the pores filled with the fluid [1]. This is effectively the water-saturated pore network equivalent to the connected porosity in case of water saturation. NMR-derived porosity included microporosity with pores with a diameter less than 3.6 nm, which was beyond what could be achieved from MICP [6]

Methods

Results

Discussion

Conclusion