Quantitative Characterisation of Microporosity in Carbonate Rocks

The application of digital technology to the study of carbonate rocks is far from being mature. Opportunities to expand its domain of applicability are high, particularly in the context where a better understanding of the spatial distribution of "microporosity" is integrated with observations of transport and multi-phase displacement processes. In this context, microporosity is regarded as the fraction pore space where mass exchange occurs mostly by diffusion, rather than advection ("pseudo-immobile" pore-space). A critical challenge is the development of advanced experimental protocols that probe the pore scale structure in carbonates in a continuous range across over seven decades of length scales (from 10 nm to 10 cm) and to integrate information at these different scales. The objective of this PhD project is to develop and test an experimental workflow to quantify microporosity in carbonate rock cores, spatially.

There have been numerous laboratory methods to calculate porosity and pores connectivity in rock cores; examples of commonly used techniques are employing gas expansion (helium pycnometry), mercury immersion porosimetry and fluids aturation (API, 1998). Although these methods provide an accurate estimation of the porous volume, they all fail to deliver a spatial representation of the pore structure inside the rock, especially in rocks with very complex and varied porosity such as carbonates, plus some of these methods are destructive avoiding further analysis of the same sample. Application of imaging technologies such as Xray computed tomography are non-invasive techniques that are being implemented in recent times to look into the rock samples, allowing to visualize directly the pore-grains configuration, determine petrophysical properties and evaluate fluid transport phenomena. However, the main drawback of these techniques is the image resolution limit as is set at approximately 1 micron, this is particularly problematic when the objective is to study microporosity because at sub-micron resolution is not possible to accurately distinguish if the region scanned contain connected or isolated pores. Different methods have been proposed to overcome this issue, (lin et al, 2016) suggested saturating the core samples with highly concentrated brines to enhance the contrast in the images between the filled pores and the grains and then apply differential imaging between saturated and dry scans. This method helps to identify regions potentially containing microporosity but uncertainty remains if the brine can saturate the entire microporous regions and the possibility of the method to be destructive altering the internal structure of the sample.

The solutions proposed in this project are to systematically examine new methods in digital imaging as well as optimize methods currently used to resolve sub-resolution porosity. It is suggested to explore the use Synchrotron-based omography, this technology delivers higher spatial resolution compared to micro CT. Similarly, opaque gases (Xenon, Krypton) will be taken into consideration for saturation of the rock, this is beneficial as the gases could reach all the microporous space as compared to liquids, having the advantage of generating better contrast. Emerging methods including K-edge substraction (Mayo et al, 2015) and grating interferometry (Blykers et al,2021) will be tested and compared against traditional experimental methods. If the application of these methods prove to be successful to resolve sub-resolution microporosity, A multi-scale imaging workflow can be proposed, where lower resolution imaging techniques are used to resolve macroporosity and to decide regions of interest containing unresolved porosity to apply high resolution methods. This will allow a more comprehensive characterization of the rock avoiding underestimation of porosity calculation due to unresolved microporosity.