Quantitative Characterisation of Microporosity in Carbonate Rocks

数字技术在碳酸盐岩研究中的应用还很不成熟。扩大其适用范围的机会很大，特别是在对“微孔隙”空间分布的更好理解与对输运和多相位移过程的观察相结合的情况下。在这种情况下，微孔隙被认为是质量交换主要通过扩散而不是平流（“伪不动”孔隙空间）发生的分数孔隙空间。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）的连续范围内探测碳酸盐的孔隙尺度结构，并在这些不同的尺度上整合信息。该博士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的微孔隙度。有许多实验室方法可以计算岩心的孔隙度和孔隙连通性；常用技术的例子是采用气体膨胀法（氦气比重法）、汞浸孔隙法和流体饱和度法（API， 1998年）。虽然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，特别是在孔隙度非常复杂和变化的岩石中，如碳酸盐岩，而且其中一些方法是破坏性的，避免了对同一样品的进一步分析。成像技术的应用，如x射线计算机断层扫描，是一种非侵入性技术，最近被应用于观察岩石样品，可以直接看到孔隙颗粒结构，确定岩石物理性质并评估流体输送现象。然而，这些技术的主要缺点是图像分辨率限制为大约1微米，当目标是研究微孔隙时，这尤其成问题，因为在亚微米分辨率下，不可能准确区分扫描区域是否包含连通或孤立的孔隙。已经提出了不同的方法来克服这个问题，（lin等人，2016）建议用高浓度盐水使岩心样品饱和，以增强填充孔隙和颗粒之间的图像对比度，然后在饱和扫描和干扫描之间应用差分成像。该方法有助于识别可能含有微孔的区域，但如果盐水能够使整个微孔区域饱和，以及该方法可能破坏样品的内部结构，则仍然存在不确定性。本项目提出的解决方案是系统地研究数字成像的新方法，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成像技术，该技术与微型CT相比具有更高的空间分辨率。同样，不透明气体（氙、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边减法（Mayo等人，2015）和光栅干涉法（Blykers等人，2021）在内的新兴方法，并与传统实验方法进行比较。如果这些方法的应用被证明是成功的，那么可以提出一个多尺度成像工作流程，其中使用低分辨率成像技术来解决大孔隙度，并确定包含未解决孔隙度的感兴趣区域，以应用高分辨率方法。这将允许更全面的岩石特征，避免由于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.