CO2 wettability of seal and reservoir rocks and the implications for carbon geo-sequestration

We review the literature data published on the topic of CO2 wettability of storage and seal rocks. We first introduce the concept of wettability and explain why it is important in the context of carbon geo-sequestration (CGS) projects, and review how it is measured. This is done to raise awareness of this parameter in the CGS community, which, as we show later on in this text, may have a dramatic impact on structural and residual trapping of CO2. These two trapping mechanisms would be severely and negatively affected in case of CO2-wet storage and/or seal rock. Overall, at the current state of the art, a substantial amount of work has been completed, and we find that: 1. Sandstone and limestone, plus pure minerals such as quartz, calcite, feldspar, and mica are strongly water wet in a CO2-water system. 2. Oil-wet limestone, oil-wet quartz, or coal is intermediate wet or CO2 wet in a CO2-water system. 3. The contact angle alone is insufficient for predicting capillary pressures in reservoir or seal rocks. 4. The current contact angle data have a large uncertainty. 5. Solid theoretical understanding on a molecular level of rock-CO2-brine interactions is currently limited. 6. In an ideal scenario, all seal and storage rocks in CGS formations are tested for their CO2 wettability. 7. Achieving representative subsurface conditions (especially in terms of the rock surface) in the laboratory is of key importance but also very challenging

KG2B, a collaborative benchmarking exercise for estimating the permeability of the Grimsel granodiorite - Part 1: Measurements, pressure dependence and pore-fluid effects

Measuring the permeability of tight rocks remains a challenging task. In addition to the traditional sources of errors that affect more permeable formations (e.g. sample selection, non-representative specimens, disturbance introduced during sample acquisition and preparation), tight rocks can be particularly prone to solid–fluid interactions and thus more sensitive to the methods, procedures and techniques used to measure permeability. To address this problem, it is desirable to collect, for a single material, measurements obtained by different methods and pore-fluids. For that purpose a collaborative benchmarking exercise involving 24 laboratories was organized for measuring the permeability of a single low permeability material, the Grimsel granodiorite, at a common effective confining pressure (5 MPa). The objectives of the benchmark were: (i) to compare the results for a given method, (ii) to compare the results between different methods, (iii) to analyze the accuracy of each method, (iv) to study the influence of experimental conditions (especially the nature of pore fluid), (v) to discuss the relevance of indirect methods and models and finally (vi) to suggest good practice for low permeability measurements. In total 39 measurements were collected that allowed us to discuss the influence of (i) pore-fluid, (ii) measurement method, (iii) sample size and (iv) pressure sensitivity. Discarding some outliers from the bulk data set (4 out of 39) an average permeability of 1.11 × 10−18 m² with a standard deviation of 0.57 × 10−18 m² was obtained. The most striking result was the large difference in permeability for gas measurements compared to liquid measurements. Regardless of the method used, gas permeability was higher than liquid permeability by a factor approximately 2 (kgas = 1.28 × 10−18 m² compared to kliquid = 0.65 × 10−18 m²). Possible explanations are that (i) liquid permeability was underestimated due to fluid-rock interactions (ii) gas permeability was overestimated due to insufficient correction for gas slippage and/or (iii) gases and liquids do not probe exactly the same porous networks. The analysis of Knudsen numbers shows that the gas permeability measurements were performed in conditions for which the Klinkenberg correction is sufficient. Smaller samples had a larger scatter of permeability values, suggesting that their volume were below the Representative Elementary Volume. The pressure dependence of permeability was studied by some of the participating teams in the range 1–30 MPa and could be fitted to an exponential law k = ko.exp(–γPeff) with γ = 0.093 MPa−1. Good practice rules for measuring permeability in tight materials are also provided

Gas Transfer Through Clay Barriers

© 2015 Elsevier Ltd Quantification of gas migration through fully water-saturated geological clay barriers is of relevance in different research fields, like the safe and long-term storage of radioactive waste or carbon dioxide, or natural hydrocarbon exploitation. Reliable forecasts require robust and well-documented experimental data that can be integrated into different petrophysical transport models. In order to provide a broad theoretical background, this chapter first outlines the fundamentals of diffusive gas transport in water-saturated rocks and viscous gas flow controlled by capillary pressure and dilatancy. This is followed by a summary of relevant literature data, providing an overview of applications in different fields and the common ranges of values of individual parameters. Molecular diffusion, driven by chemical potential gradients, is characterized by a low transport efficiency. But it is a ubiquitous process that can act over long periods of time and may then be of relevance on length scales up to hundreds of meters. In comparison, pressure-driven viscous gas flow has a much higher transport capacity. But in originally water-saturated rocks it requires the formation of interconnected gas-filled fluid pathways, either by drainage or dilatation. Below this percolation threshold gas flow remains completely diffusion controlled. The experimental determination of gas transport-related parameters in low-permeable rocks is very challenging. Experiments may take several weeks to months and often a clear discrimination between different transport processes is not possible. Various experimental techniques have been developed and applied, and they usually require specific interpretation methods. In the last section of this chapter, examples are given of experiments performed on different clay-rocks considered as potential host rocks for radioactive waste storage (Boom Clay, Opalinus Clay, Callovian–Oxfordian Clay). Laboratories involved in these studies were those at the Belgian Nuclear Research Centre (SCK-CEN, Belgium), Ecole Centrale de Lille (France), British Geological Survey (UK), and at RWTH-Aachen University (Germany).status: publishe

The Dependency of Diffusion Coefficients and Geometric Factor on the Size of the Diffusing Molecule: Observations for Different Clay-Based Materials

Copyright © 2017 Elke Jacops et al. In order to investigate in more detail the relation between the size of diffusing molecules and their diffusion coefficients (and geometric factors), diffusion experiments with gases of different size and tritiated water (HTO) have been performed on different clayey samples (Boom Clay, Eigenbilzen Sands, Opalinus Clay, Callovo-Oxfordian Clay, and bentonite with different dry densities). We observed that, for unreactive gases in clayey materials, the effective diffusion coefficient varies with the size of the diffusing molecule and this variation can be described by an exponential or a power law function. The variation of the geometric factor can also be described by an exponential function. The observed experimental relations can be used to estimate diffusion coefficients; by measuring experimentally in clay the effective diffusion coefficient of two unreactive dissolved gases with a different size, the diffusion coefficients of other dissolved gases (with a size in between the two measured gases) can be estimated by using the fitted exponential relationship.status: publishe

Interplay of molecular size and pore network geometry on the diffusion of dissolved gases and HTO in Boom Clay

© 2016 Elsevier Ltd Through-diffusion experiments in Boom Clay have been performed with uncharged molecules: tritiated water (HTO) and dissolved gases of different size (He, Ne, H2, Ar, CH4, Xe and C2H6), allowing information to be obtained on the relationship between the diffusion coefficient and the molecular size (characterized by a ‘kinetic diameter’ of the molecules). Experiments have been performed on both clayey and silty Boom Clay samples, to scope for the changes induced by grain size variations on the diffusion process. Experiments on clay cores taken perpendicular as well as parallel to the bedding plane have also been executed, providing additional information on the anistropy of the diffusion process. Empirical relations are proposed to capture the observed decrease of both the diffusion coefficient in water and the effective diffusion coefficient in the Boom Clay porous medium as a function of molecular size. In the same way, the behaviour of the geometric factor G as a function of size is estimated. Although silty samples have a noticeably higher hydraulic conductivity than clayey samples, the difference in diffusion coefficient is less obvious. The anisotropy factor is roughly the same for all investigated components, with an average value of 1.5.publisher: Elsevier articletitle: Interplay of molecular size and pore network geometry on the diffusion of dissolved gases and HTO in Boom Clay journaltitle: Applied Geochemistry articlelink: http://dx.doi.org/10.1016/j.apgeochem.2016.11.022 content_type: article copyright: © 2016 Elsevier Ltd. All rights reserved.status: publishe