Quantitative Characterisation of Microporosity in Carbonate Rocks

数字化技术在碳酸盐岩研究中的应用还远未成熟。扩大其适用范围的机会很大，特别是在更好地了解“微孔隙率”的空间分布与对运移和多相驱替过程的观察相结合的情况下。在这种情况下，微孔隙率被认为是质量交换主要通过扩散而不是平流发生的分数孔隙空间(“伪固定”孔隙空间)。一个关键的挑战是开发先进的实验方案，在超过70年的长度尺度(从10纳米到10厘米)内连续探测碳酸盐中的孔隙尺度结构，并整合这些不同尺度上的信息。这个PHD项目的目标是开发和测试一种实验工作流程，以在空间上量化碳酸盐岩岩心中的微孔隙率。已经有许多实验室方法来计算岩心中的孔隙度和孔隙连通性；常用技术的例子是使用气体膨胀(氦比浊法)、汞浸泡孔压法和流体饱和度(API，1998)。尽管这些方法提供了对孔隙体积的准确估计，但它们都不能提供岩石内部孔隙结构的空间表示，特别是在具有非常复杂和多变的孔隙度的岩石中，如碳酸盐，而且其中一些方法具有破坏性，可以避免对同一样品进行进一步分析。X射线计算机断层成像等成像技术的应用是近年来正在实施的非侵入性技术，用于观察岩石样品，使其能够直接可视化孔隙-颗粒结构，确定岩石物理性质，并评估流体传输现象。然而，这些技术的主要缺点是图像分辨率极限被设置为大约1微米，当目标是研究微孔率时，这是特别有问题的，因为在亚微米分辨率下不可能准确地区分扫描的区域是否包含连通或孤立的孔。已经提出了不同的方法来解决这个问题，(Lin等人，2016)建议用高浓度盐水浸泡岩心样本，以增强填充的毛孔和颗粒之间的图像对比度，然后在饱和和干燥扫描之间应用差异成像。这种方法有助于识别可能含有微孔率的区域，但仍然存在不确定性，即盐水是否能饱和整个微孔区，以及该方法具有破坏性地改变样品内部结构的可能性。本项目提出的解决方案是系统地检查数字成像中的新方法，以及优化目前用于解决亚分辨率孔隙度的方法。建议探索使用基于同步加速器的断层扫描技术，该技术与Micro CT相比具有更高的空间分辨率。同样，不透明气体(氙气、氪)将被考虑到岩石的饱和度，这是有益的，因为与液体相比，气体可以到达所有的微孔空间，具有产生更好对比度的优势。将测试包括K边缘减法(Mayo等人，2015)和光栅干涉法(Blykers等人，2021)在内的新兴方法，并将其与传统实验方法进行比较。如果这些方法的应用被证明成功地解决了次分辨率微孔隙度，则可以提出多尺度成像工作流程，其中使用较低分辨率成像技术来解决大孔隙度，并确定包含未解析孔隙度的感兴趣区域以应用高分辨率方法。这将允许对岩石进行更全面的表征，避免因未解决的微孔隙度而低估孔隙度计算。

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.