Resolution of Micro-CT Images and Its Impact on e-Core Simulation Results

Abstract

The objective of this study was to acquire micro-CT images of two different types of sandstones (Bentheimer and Berea) and a carbonate. The samples had to be scanned with different resolutions (5, 3 and 1,5 microns), and the CT-images was used for calculating single-phase petrophysical parameters (e.g. absolute permeability and formation resistivity factor) as well as multiphase flow properties (relative permeability and capillary pressure) by applying the e-Core technology. Simulation results were compared with single-phase petrophysical parameters and multiphase flow properties measured at the laboratory in order to check the impact of different scanning resolutions on the simulation results.

Two sample groups of the same rock types were used; one group was used to perform the laboratory measurements and the other group was used to perform scanning, image processing and simulations. This workflow was decided in advance before starting the project due to the time limitation of this work.

Image processing and segmentation is the most important stage in preparing a 3D CT-image model which is imported to e-Core software to perform different calculations. The segmentation (thresholding) defines pore spaces, clay and matrix grains in the images and has a significant impact on the calculated results. There exists no universal thresholding method that can be used on all rock types, and it was necessary to have a fixed thresholding method in this project in order to check the resolution effects on the calculations. A new thresholding method was defined for the first time in this project which is based on analyzing the grey values of the images obtained from using an image filtering method called Susan Filtering in ImageJ software. The method was successfully used with the rock types in this project, and testing of the method gave a slight error in the thresholding values obtained from this method in few cases.

Results from different laboratory measurements and simulations were compared to examine the impact of the different scanning resolutions on the calculations. The scanning resolutions had direct effect on the thresholding values which had a significant impact on the porosities and clay contents in the images and simulation models. Clay content influenced the absolute permeability which also affected the formation resistivity factors of the models. It was also discovered resolution effects on the irreducible water saturation (Swi) which also affected the curvature of the capillary pressure curves. The resolution impact on relative permeability curves was mainly on the endpoints of these curves not on the shape or curvature of these curves.