CO2-brine-caprock interaction: Reactivity experiments on mudstone caprock of South-west Hub geo-sequestration project

Journal Pre-proof CO2-brine-caprock interaction: Reactivity experiments on mudstone caprock of South-west Hub geo-sequestration project D.W. Jayasekara, P.G. Ranjith, W.A.M. Wanniarachchi, T.D. Rathnaweera, D. Van Gent PII:

S0920-4105(20)30106-6

DOI:

https://doi.org/10.1016/j.petrol.2020.107011

Reference:

PETROL 107011

To appear in:

Journal of Petroleum Science and Engineering

Received Date: 20 October 2019 Revised Date:

27 January 2020

Accepted Date: 28 January 2020

Please cite this article as: Jayasekara, D.W., Ranjith, P.G., Wanniarachchi, W.A.M., Rathnaweera, T.D., Van Gent, D., CO2-brine-caprock interaction: Reactivity experiments on mudstone caprock of Southwest Hub geo-sequestration project, Journal of Petroleum Science and Engineering (2020), doi: https:// doi.org/10.1016/j.petrol.2020.107011. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

CRediT author statement D. W. Jayasekara:: Conceptualization, Methodology, Investigation, Visualization, Writing -

Original Draft P.G. Ranjith: Resources, Supervision, Project administration and Funding acquisition W. A. M. Wanniarachchi T. D. Rathnaweera:: Formal analysis and Writing - Review & Editing, D. Van Gent: Sample analysis, geological informations, sample sourced

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Cover Page

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Manuscript Title: CO2-brine-caprock interaction: Reactivity experiments on mudstone

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caprock of South-west Hub geo-sequestration project

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Authors’ names:

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D. W. Jayasekara1, P.G. Ranjith*1, W. A. M. Wanniarachchi1, T. D. Rathnaweera1, D. Van

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Gent2

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1

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3800, Australia.

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2

Department of Civil Engineering, Monash University, Building 60, Melbourne, Victoria,

Department of Mines and Petroleum, 100 Plain Street, East Perth, WA 6004, Australia.

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*Corresponding author:

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Prof. Ranjith PG

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Deep Earth Energy Laboratory, Monash University, Building 60,

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Melbourne, Victoria, 3800, Australia.

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Phone/Fax: 61-3-9905 4982

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E-mail: [email protected]

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Abstract

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This study aims at focusing on the geo-chemical reactions of caprock upon injection of

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supercritical CO2 (ScCO2) under deep saline aquifer’s conditions. The caprock samples were

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obtained at a depth of 979 m in the DMP Harvey-2 well, which is recognized as mudstone in

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the Lesueur Formation of the South-west Hub geo-sequestration project located at south of

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Perth in Australia. Geo-chemical reactions were conducted in a reaction chamber to

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determine the chemical reactivity of brine-saturated caprock under actual reservoir conditions

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(40 °C, 10 MPa and salinity 4.5%). The reaction was conducted at 10 MPa ScCO2 pressure

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and allowed for 37 weeks. The reacted fluid samples were subjected to several chemical

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analyses, including alkalinity tests, pH measurements and inductively-coupled plasma optical

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emission spectrometry (ICP-OES) tests. The solid minerals were tested using X-ray

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Diffraction (XRD), Scanning electron microscopy (SEM) and Energy-Dispersive X-ray

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(EDX) to obtain a better understanding related to of their chemical characterization of the

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caprock. Finally, a geochemical model (PHREEQC) was developed to forecast mineral

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dissolution/precipitation equilibrium and redox reactions.

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Fluid chemistry showed that the concentration of major elements such as Ca, Mg and K

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increased with time due to the dissolution of minerals such as K-feldspar, anorthite, and

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chlorite. On the other hand, the release concentration of Si, Fe and Al ions decreased with

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time due to the precipitation of secondary minerals such as kaolinite, gibbsite, amorphous

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silica and Fe(OH)3. Accordingly, the dissolution of minerals is very significant compared to

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precipitation of secondary minerals in the short term which can increase the pore volume of

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the caprock.

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Keywords: South-west Hub geo-sequestration project, caprock, mudstone, minerals,

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chemical reactivity

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1. Introduction

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The increase of anthropogenic CO2 in the atmosphere has caused significant climate changes

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in the world (Nguyen et al., 2017). In order to mitigate global climate change, it is crucial to

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implement substantial emission reduction strategies to reduce anthropogenic CO2 in the

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atmosphere. Storing CO2 in deep saline aquifers plays a significant role in CO2 sequestration

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because of their high storage capacity, easy accessibility and extensive availability (De Silva

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et al., 2015; Rathnaweera et al., 2014). Generally, deep saline aquifers lie at a depth range of

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800- 2000 m and such a depth allows to retain the injected CO2 in its supercritical state due to

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the high pressure and temperature prevailing at the depth, which is higher than the critical

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point of CO2 (temperature >31.1 °C and pressure > 7.39 MPa) (Kharaka et al., 2009). Since

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the density and viscosity of CO2 are lower than those of the reservoir pore fluid, the injected

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CO2 undergoes viscous fingering and gravitational segregation while moving upward to the

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top of the reservoir rock. During this process, the contamination of upwardly moving CO2

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with overlying groundwater bodies is prevented by the caprock, a less permeable layer made

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up of siliciclastic sedimentary rocks such as mudstone, claystone, shale or siltstone (Shukla et

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al., 2010).

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Injecting CO2 into deep saline aquifers can cause hydro-dynamic, thermal, chemical and

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mechanical changes in the caprock. The main chemical variation which appears in the

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caprock is the loss of geochemical equilibrium between the caprock minerals and the pore

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fluid due to massive CO2 injection into the reservoir rock (Shao et al., 2010). The dissolution

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of supercritical CO2 in brine, making an acidic medium capable of reacting with the caprock

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matrix, leads to the dissolution of primary minerals such as quartz, plagioclase and biotite and

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the precipitation of secondary minerals such as analcime and carbonate minerals (Kaszuba et

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al., 2003). Such changes can significantly alter the porosity, morphology and permeability

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jeopardising the entire CO2 storage process (Liu et al., 2012). It is therefore important to

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understand the mechanics of the interactions of various caprock minerals with CO2 in order to

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characterise the caprock behaviour in both short- and long-term injection scenarios.

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In most cases, the effective minerals in terms of caprock integrity are clay minerals such as

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kaolinite, montmorillonite, illite and chlorite (Shao et al., 2010). Due to the presence of

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abundant clay minerals in the caprock, some characteristics such as ultra-low permeability,

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low diffusion and high capillary entry pressure enhance the sealing capacity of the caprock

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which reduces the risk of back-migration of injected CO2 into the surface and freshwater 3

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aquifers (Bennion and Bachu, 2007). Generally, clay minerals in caprocks can be divided into

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different groups based on their characteristics, as in Table 1, where different clay minerals act

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differently according to the prevailed conditions in the aquifer.

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A deep knowledge of the geo-chemical reactions in the caprock upon injection of CO2 is

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critical for the site selection process. Several studies have been recently conducted to evaluate

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the reactivity of caprock under different temperatures (Alemu et al., 2011; Credoz et al., 2009;

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Kaszuba et al., 2005; Liu et al., 2012; Xiao et al., 2017). This is important to investigate as

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temperature varies with the depth of the aquifer and has a significant influence on the

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reactivity of caprock. A detailed summary and the related key findings of each study,

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including geo-chemical models, are shown in Table 2. According to the previous studies, it is

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clear that the laboratory hydro-thermal tests conducted so far are based on higher

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temperatures (80-250 ºC) than the reservoir temperatures (30-50 ºC), and such high

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temperatures accelerate the silicate mineral reaction rate in sedimentary rocks to predict long-

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term mineral reactivity. For instance, Liu et al., (2012) conducted a study under the

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temperature of 200 ºC and pressure of 30 MPa and Alemu et al., (2011) performed a test at a

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temperature of 250 ºC and pressure of 11 MPa to accelerate the mineral reaction rates.

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However, the results of hydro-thermal reactions are not very reliable because high

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temperatures can change the mineral reaction mechanism (Liu et al., 2012). For example,

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when the aquifer temperature is less than 60 ºC, the carbonate caprocks, which contain

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dolomite and calcite start to dissolve in the presence of CO2, while at high temperatures (> 60

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ºC) precipitation can be expected as carbonate minerals (Song and Zhang, 2013). Moreover,

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smectite becomes unstable at the temperature of 80-100 ºC and starts to dissolve, proving the

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temperature effect on the reaction mechanism (Stephansson et al., 2004). Although Kohler et

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al. (2009) state that a lot of mineralogical changes due to temperature shift are related to

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dehydration, there are a few reactions that change its mechanism due to temperature. For

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instance, Alemu et al., (2011) conducted three different batch reactions at 80, 150 and 200 ºC

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for clayey shale which consisted of quartz 26 %, albite 8 %, pyrite 1 %, siderite 5 %, illite

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22 %, and chlorite 38 % and found that different mineral dissolutions and precipitations took

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place according to the temperature. Results depicted that albite, K-feldspar and illite

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precipitated at 80 and 150 ºC while dissolved at 200 ºC even though quartz behaviour was

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opposite of it. Moreover, siderite mineral dissolved at 80 ºC whereas it precipitated at 150

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and 200 ºC. Furthermore, the SiO2 concentration identified using brine chemistry showed that

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its concentration reduced with the increase of temperature. In addition, K-feldspar- NaCl 4

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water mixture is transformed into gibbsite, kaolinite, kaolinite + quartz, aragonite + quartz,

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and finally into albite as a result of mineral-water interactions (Bjørlykke et al., 1992). But, if

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the temperature is greater than 200 ºC, boehmite is formed instead of gibbsite formation in

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this reaction series (Fu et al., 2009). Thus, it is clear that the high temperature has an ability

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to change the reaction mechanism, which can induce different primary mineral dissolutions

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and secondary mineral precipitations (Liu et al., 2012). As a result, if we provide high

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temperatures in batch reactions which are greater than the reservoir conditions, unrealistic

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chemical reactions can result, which are different from the real aquifer reactions. In addition,

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the temperature has a great impact on the CO2 dissolution rate, changing the pH value in the

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solution. According to Alemu et al., (2011), clayey shale saturated in brine showed a low pH

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value due to high dissolution of CO2 at lower temperatures. As a result, high silica and iron

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(Fe) concentrations were observed in the solution because low pH creates a highly acidic

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medium. Therefore, conducting experiments at simulated aquifer temperatures is crucial to

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identify the realistic chemico-mineralogical changes of caprock in deep saline aquifers (Liu et

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al., 2012).

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Although there are several studies done on caprock (hydro-thermal experiments) (Alemu et

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al., 2011; Anabaraonye et al., 2019; Credoz et al., 2009; Kaszuba et al., 2005; Kohler et al.,

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2009; Liu et al., 2012), very limited studies have focused on the reactivity of caprock under

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reservoir conditions (especially the temperature < 50 ºC) (Dávila et al., 2017; Ellis et al.,

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2011; Ilgen et al., 2018; Pearce et al., 2019; Tarkowski et al., 2015). Among them, the

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reactivity tests conducted for mudstone are very limited (Pearce et al., 2019; Tarkowski et al.,

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2015; Wdowin et al., 2014). Thus, the understanding regarding chemico-mineralogical

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behaviour of mudstone caprock is narrow, which needs to be investigated in detail.

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Tarkowski et al. (2015) conducted reactivity tests for 20 months at room temperature under

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CO2 pressure of 6 MPa for mudstone obtained from Zaosie Anticline which is considered as a

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potential CO2 storage site. Results depicted that the surface area and porosity of mudstone

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were increased while bulk density was reduced after the batch reaction. Similarly, Wdowin et

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al. (2014) also conducted reactivity tests for mudstone obtained from Chabowo Anticline at a

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temperature of 25 ºC and pressure of 6 MPa for 18 months and found that there were

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dissolution of feldspar and precipitation of kaolinite. However, the use of powdered sample

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to accelerate the chemical reactions, identification of possible chemical reactions and

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pressurizing the chamber with supercritical CO2 could improve the quality of these studies. In

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addition, Pearce et al. (2019) conducted a combined reactivity and µCT study on mudstone 5

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obtained from Surat Basin, Queensland, Australia to identify the relationship between the

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permeability changes and reactivity of caprocks after CO2 injection. The pressure and

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temperature in lined reactors were 12 MPa and 60 ºC respectively and found that there was a

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slight dissolution of chlorite, feldspar and calcite, which could change the permeability. But

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still, the findings would be more meaningful if authors could conduct geo-chemical

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modelling to verify the possible reactions. According to Pearce et al. (2019), the reactivity

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studies conducted on caprocks are very rare, especially under low salinity conditions. Thus,

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reactivity tests were conducted in the current study including a geo-chemical modelling on

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crushed mudstone obtained from South-west Hub geo-sequestration project, Australia to

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increase the reaction rate providing reservoir temperature and pressure minimizing the

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drawbacks in previous studies (Kaszuba et al., 2005; Tarkowski et al., 2015; Wdowin et al.,

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2014). In addition, since the current study is based on low salinity formation fluid (4.5 %),

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the results are very important as limited studies have been done on low salinity conditions

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(Pearce et al., 2019). As a summary, this paper focuses on the evaluation of geo-chemical

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reactions in mudstone caprock upon CO2 injection to better understand the caprock behaviour

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under CO2 sequestration environment.

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Table 1: Characteristics of clay minerals present in the caprocks (Kloprogge, 2017; Shainberg

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and Levy, 2005) Clay group

Clay minerals

Cation exchange

Remarks

capacity (cmolckg-1) Kaolin

Kaolinite, dickite, halloysite,

about 1-10

and nacrite Smectite

Montmorillonite, nontronite

capacity 80-120

and beidellite Illite

Clay micas

low shrinkage

High swelling capacity

20-40

Non-expanding clay minerals

Chlorite

Clinochlore, pennantite, nimite and chamosite

10-40

Non-swelling minerals

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2. Experimental approach 2.1 Sample selection and description

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The caprock samples for the current study was obtained from Harvey-2 well of South-west

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Hub geo-sequestration project. The main idea behind the South-west Hub geo-sequestration

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project establishment was to mitigate the CO2 emissions in the area using CCS techniques. It

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is located in the Perth Basin, which extends over 1350 km along the southwestern margin of

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Australia (Olierook et al., 2014). The stratigraphy of the Perth Basin is shown in Figure 1.

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The storage site consists of two formations known as Upper Lesueur (Yalgorup or Myalup

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member) and Lower Lesueur (Wonnerup member) which were developed during the late

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Triassic period. The Wonnerup member, which consists of coarse sands can be used as a

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good reservoir to store CO2, and the Yalgorup member can act as a sealing layer due to the

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high presence of floodplains and paleosols (shaley and clayey sediments which can provide

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better resistance to CO2 leakage) (Stalker et al., 2013). In order to conduct a detailed analysis

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of the storage site, four wells known as DMP Harvey 1, 2, 3 and 4 were drilled (Figure 2) to

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obtain core samples and log data. The temperature measurements of different DMP Harvey

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wells are displayed in Figure 3.

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The caprock sample for reactivity tests was obtained from a stratum between 979.10 – 979.40

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m that belongs to Myalup member in the DMP Harvey-2 well, which was identified as

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mudstone. The physical and mineralogical properties of the selected mudstone are shown in

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Table 3. For the reactivity test, the mudstone sample was crushed in order to enhance the

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reaction rate by increasing the surface area (Kweon and Deo, 2017). The crushed caprock

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sample was dry-sieved to retain the fraction between 425 and 75 µm.

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Table 2: Recent studies of the chemico-mineralogical behaviour of caprock due to CO2 injection

Purpose To identify the limestone caprock reactivity after interaction with mixed CO2-brine fluids under different physicochemical conditions (Credoz et al., 2009).

To

evaluate

Testing methods Key findings Reacted brine- Inductively coupled Intact caprock mineralogy plasma atomic emission • 45% calcite, 15% mixed-layer illite/smectite (I/S), 10% kaolinite, 10% spectroscopy (ICP –AES). quartz, 5% gypsum, 5% pyrite and 5% other components. Reacted solid fractions- Caprock-CO2 (aq)-brine tests Simultaneous thermal analysis • Fe-dolomite totally dissolved while calcite destabilized. There, carbonate (STA), X-ray powder diffraction minerals are the most reactive minerals. (XRD), scanning electron • New form of mixed carbonated mineral precipitated (composition was not microscopy (SEM), energy determined). dispersive spectrometry (EDS). • New smectite precipitated due to I/S destabilization and kaolinite dissolution. I/S + Kaolinite → Illitic I/S + Fe-Mg-Smectite • Pyrite was significantly dissolved and a new carbonated mineral precipitated. Here HCO3- ions in the solution substituted for S2- ions (All reactions happened in 365 days at a temperature and pressure of 150 °C and 0.1 MPa respectively). Caprock-CO2(Sc)-brine tests • Massive dissolution of Fe-dolomite (after 30 days at a temperature and pressure of 80 °C and 15 MPa respectively). • Destabilization of pyrite (after 90 days at a temperature and pressure of 80 °C and 15 MPa respectively). • Kaolinite dissolved, but I/S was stable (after 90 days at a temperature and pressure of 80 °C and 15 MPa respectively). • Smectite in the mixed-layer decreased so that the relative illite fraction increased (after 45 days at a temperature and pressure of 80 °C and 15 MPa respectively).

CO2–brine–shale Reacted solid fractions- SEM and Intact caprock mineralogy 8

caprock interactions in Eau Claire XRD tests Formation using hydrothermal experiments (Liu et al., 2012).

To evaluate long-term geochemical Chemical modelling reactions of caprock in the presence PHREEQC software of CO2 (Gaus et al., 2005)

• Major minerals are quartz, orthoclase, illite and minor mineral is chlorite. Caprock-CO2 (aq)-brine tests • Minor dissolution of K-feldspar and anhydrite. • Precipitation of illite and/or smectite, and siderite in the vicinity of pyrite. (All reactions happened after 60 days at a temperature and pressure of 200 °C and 30 MPa respectively). using Intact caprock mineralogy • 24.7% mica/illite, 21.5% quartz, 18% kaolinite, 12.3% plagioclase, 8.8% smectite, 4.1% chlorite, 2.8% pyrite, 2.1% K-feldspar, 1.6% siderite, 1% calcite and 3.1% other components. Short term (over 9 years at a temperature and pressure of 37 °C and 10.13 MPa respectively) • Calcite dissolution. Long term (over 15,000 years at a temperature and pressure of 37 °C and 10.13 MPa respectively) • Albite dissolution forming kaolinite and dawsonite. • Anorthite dissolution forming kaolinite. • Dissolution of illite and smectites, and precipitation of kaolinite, chalcedony, K-feldspar and large amounts of carbonate.

To investigate the effect of Reacted brine - Inductively coupled Intact caprock mineralogy mineralogical compositions on the plasma mass spectrometry (ICP – • 13% quartz, 6% plagioclase, 19% chlorite, 26% illite, 7% ankerite and 29% reactivity of shale (Alemu et al., MS) calcite. 2011) Reacted solid fractions Caprock-CO2(Sc)-brine tests X-ray fluorescence (XRF), XRD • Ankerite dissolution. • Precipitation of calcite. • Chlorite dissolution. • Illite dissolution to form smectite (All reactions happened after 5 weeks at a temperature and pressure of 250 °C and 11 MPa).

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To evaluate the reactive behaviour of CO2(Sc) –brine-caprock under physical-chemical conditions (Kaszuba et al., 2005)

Reacted brine- Inductively coupled Intact caprock mineralogy plasma atomic emission • 65% illite, 27% quartz, 5% feldspar and 3% kaolinite. spectroscopy (ICP –OES) and ICP- Caprock-CO2(Sc)-brine tests MS • Precipitation of magnesite and siderite (after 45 days at a temperature and Reacted solid fractions- SEM, EDS pressure of 200 ºC and 20 MPa).

To identify the dissolution and Reacted brine- ICP-MS, pH precipitation characteristics of measurements phlogopite, a clay mineral in Reacted solid fractions- atomic caprocks (Shao et al., 2010) force microscopy (AFM) analysis, SEM-EDX, XRD and X-ray photoelectron spectroscopy (XPS)

Mineralogy • Phlogopite sample Phlogopite -CO2(Sc)-brine tests • Phlogopite dissolution and precipitation of amorphous silica and kaolinite as secondary minerals (after 6 days reaction at a pressure of 10.3 MPa and temperature of 95 ºC).

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MIDDLE EARLY

180

JURASSIC

160

200

ALBIAN

Coolyena Group

Osbourne Formation

Warnbro Group

Leederville Formation South Perth Shale

Seal

Reservoir

Shore

Source

APTIAN BARREMIAN HAUTERIVIAN VALANGINIAN BERRIASIAN

TITHONIAN KIMMERIDGIAN OXFORDIAN CALLOVIAN BATHONIAN BAJOCIAN AALENIAN TOARCIAN PLEINSBACHIAN SINEMURIAN

Gage sandstone

Parmelia Group

Yarragadee Formation

Cadda Formation Cockleshell

Cattamarra Coal Measures

Gully fromation

Eneabba formation

HETTANGIAN

240

LATE EARLYMIDDLE

220

TRIASSIC

RHAET IAN

NORIAN CARNIAN

Myalup Member Lesueur Sandstone

LADINIAN ANISIAN

Wonnerup Member

SCYTHIAN

Sabina Sandstone

DZHULFIAN

UFIMIAN

ARTINSKIAN SAKMARIAN ASSELIAN

PRECAMBRIAN

192

Willespie Formation

MIDIAN

KAZANIAN KUNGURIAN

EARLY

280

PERMIAN

260

LATE

CHANGHSINGIAN

CONTINENTAL DEPOSITS

LATE LATE

140

STRATIGRAPHIC UNITS

CENOMANIAN

NEOCOMIAN

120

EARLY

100

AGE

CRETACEOUS

Ma

Sue Group

Redgate Coal Measures Ashbrook Sandstone Rosabrook Coal Measures Woodynook Sandstone Mosswood Formation Basement

Figure 1: Stratigraphy of the Perth basin (Backhouse and Crostella, 2000)

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193

Figure 2: The location of four Harvey wells in South-west Hub geo-sequestration project

194 Temperature (ºC) 0

20

40

60

80

Depth (m)

0 500

Harvey-1

1000

Harvey-2

1500

Harvey-3

2000

Harvey-4

2500 3000

195 196

3500

Figure 3: Temperature of different Harvey wells (ODIN_Reservoir_Consultants, 2015)

197 198 199 200

12

201

Table 3: Physical and mineralogical properties of mudstone obtained from Harvey-2

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well Rock Type

Mudstone

Colour

Dark grey

Mineralogical composition

Quartz – 32%, Goethite- 6%, Orthoclase -9%, Illite -20%, Kaolinite -6%, Chlorite -13%, Plagioclase -12% and other minerals in minor percentages (pyrite and montmorillonite)

Mean porosity

9.5 (+/- 4.8) %

Mean permeability

0.7 

Grain density

2.64 g/cm3

203 204

2.2 Experimental set-up and procedures

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A reaction chamber was utilized to achieve the steady-state reaction of the brine-caprock-CO2

206

system, as shown in Figure 4, which consists of reactor cell, thermal unit, gas inlet and outlet.

207

The maximum temperature and pressure that can be applied in the chamber is 100 ºC and 12

208

MPa, respectively. The reactor volume capacity is 1.25 L, and its inner diameter is 100 mm.

209

Gas flowing from the cylinder is pressurized up to desired pressure using a Teledyne ISCO

210

500D syringe pump and injected to the reactor cell. The interior pressure of the chamber is

211

measured using a GS4200 pressure transducer. In addition, the whole apparatus including the

212

chamber, tubing (316 stainless Steel), valves and o-rings (perfluoroelastomer polymer) is

213

made up of non-corrosive materials which do not react with CO2 and acidic medium to avoid

214

gas leaking and corrosion of the set-up. First, the crushed mudstone sample (75-425 µm) was

215

placed in the chamber and brine was poured into it. As identified using a formation tester in

216

the field, 4.5% (NaCl) salinity brine was used for this test series. The crushed rock sample

217

was stirred well in the synthetic brine to enhance dissolution. The reaction chamber was then

218

heated to 40 ºC, which is considered to be the temperature in the field using a heat band and

219

CO2 was injected into it under 10 MPa pressure. The temperature of 40 ºC and pressure of 10

220

MPa were maintained throughout the test series, and it was confirmed by monitoring the

221

temperature and pressure time to time using an infrared thermometer and pressure gauge

222

respectively. Thus, the crushed mudstone sample reacted under supercritical CO2, because the 13

223

used temperature (40 ºC) and pressures (10 MPa) are higher than the critical temperature

224

(31.1 ºC) and pressure (7.39 MPa) of CO2. Moreover, the purity level of injected CO2 is

225

99.9 %, and its moisture is less than 100 ppm. In different time intervals (2, 5, 9, 18 and 37

226

weeks), the solid and liquid samples were collected after cooling and releasing the pressure in

227

the reaction cell since a drain valve for sampling is absent in the current reaction chamber

228

(it’s an experimental limitation here). The brine samples collected after each reaction period

229

were filtered using 0.45 µm filter paper to separate the solution from the crushed rock sample.

230

The collected solid samples were analysed using XRD, SEM and EDX tests. The pH and

231

alkalinity measurements were taken for each collected solution. In addition, ICP-OES tests

232

were conducted to find the elemental concentration.

233

Figure 4: Schematic diagram of the reaction chamber used for caprock-brine-CO2

234

interaction

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2.2.1

X-ray diffraction (XRD)

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Powder XRD testing was conducted using the Bruker D8 cobalt XRD instrument at Monash

237

University, which consists of an X-ray source, an X-ray detector and the sample holder

238

(Figure 5a). For the current test, the X-ray diffraction patterns were recorded in the range of

239

5°≤ 2θ ≤70° and the minerals were quantified using the crystallography open database in the 14

240

DIFFRAC EVA 4.3 software using a semi-quantitative method. Accordingly, the intact

241

sample consisted of a high clay content with illite, chlorite and kaolin at around 42%.

242

Plagioclase is a mixture of anorthite and albite. The halite deposition on the CO2-reacted

243

caprock samples was considered to be an experimental artefact because the sample was not

244

washed with deionized water before the experiment.

245

2.2.2

Scanning Electron microscopy (SEM) analysis

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The intact and reacted mudstone (crushed) samples were scanned under a FEI Nova

247

NanoSEM 450 FEGSEM (Figure 5b) which is available at the Monash Centre for Electron

248

Microscopy (MCEM) to identify the mineral structure changes of rock samples which were

249

collected after 2, 5, 9, 18 and 37 weeks reaction. Since the intact and reacted powder samples

250

were non-conductive, they were coated with Iridium (Ir), a highly conductive metal, to avoid

251

the accumulation of static charges on the specimen surface or simply ‘surface charging’

252

which can cause problems such as reducing the landing energy of incident electrons (Kim et

253

al., 2010). In order to identify the elements in the caprock samples, EDX analysis also was

254

conducted. a).

b).

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Figure 5: a). Bruker D8 cobalt XRD instrument at Monash University, and b). FEI Nova

256

NanoSEM 450 FEGSEM

257

2.2.3

Finding alkalinity of reacted brine samples

258

The brine samples were titrated with H2SO4 to find the total alkalinity. For that purpose, the

259

alkalinity test kit (model AL-DT) was used, which consists of a digital titrator, standard

260

titration cartridges (H2SO4), a phenolphthalein indicator powder pillow and a Bromcresol

261

green-methyl red indicator powder pillow. The brine sample volume, titration cartridge type

262

according to the N factor and the digital multiplier were selected based on prior estimation of

263

the alkalinity range. First, the phenolphthalein indicator was added to the sample, but no

264

colour change could be observed. Second, the green-methyl red indicator was added to the 15

265

brine sample and titrated with H2SO4 acid (cartridge 1.6N) until the green turned to a light

266

pink end point. The number of digits recorded was used to calculate the alkalinity, as shown

267

in Eq.1. Figure 6 shows titration using the alkalinity test kit and pH measurement.

268

Digits required × digit multiplier = mg/l CaCO3 total alkalinity

269

2.2.1

(1)

Inductively coupled plasma optical emission spectrometry (ICP-OES)

270

The ICP-OES tests were conducted in the PerkinElmer Flagship facility at Monash

271

University, Melbourne (Figure 7) for each solution collected at different time intervals to

272

identify the chemical elements. The calibration standard curves were derived using internal

273

standards and the element concentrations of solutions were found. a).

274

b).

Figure 6: a) Titration of solution using alkalinity test kit, and b) pH measurement

16

275

Figure 7: ICP-OES set-up and the real-time colour viewing of ignited plasma using

276

plasma camera

277

2.2.2

278

2.2.5.1

Geo-chemical modelling Calculation of saturation indices

279

Geo-chemical modelling was built with PHREEQC software version 3.4.0 using the

280

LLNL.dat database. It is a computer program written in C programming language, which can

281

be used to perform different geochemical calculations, such as finding saturation indices (SIs),

282

advective-transport and inverse modelling (Parkhurst, 1995). SIs were calculated for each

283

reacted sample at different time intervals, depending upon the measured elemental

284

composition of the saturated solution. Hence, the ability of the solution to dissolve or

285

precipitate mineral types confirms the chemical reactivity of caprock minerals in the presence

286

of CO2 and brine. Generally, the SI is positive when the mineral tends to precipitate, negative

287

during mineral dissolution and zero when the mineral and solution are at chemical

288

equilibrium. SI can be calculated using the chemical activities of dissolved ions of minerals

289

such that Ion Activation Products (IAPs) and the solubility product (Ksp), as shown in Eq.2.

290

 = log ( )

291



(2)



2.2.5.2

Batch geo-chemical modelling

292

Batch geochemical modelling estimates the possible geochemical reactions due to caprock-

293

brine-CO2 interactions. Since there is no flow in batch calculations, the dimensions are zero.

294

Here, the dissolution and precipitation reactions were calculated based on mineralogy of

295

caprock, CO2 solubility, initial brine properties and kinetic rates. The kinetic calculation was

296

run for mudstone which was reacted with CO2 and brine for 37 weeks at 40 °C and 10 MPa.

297

The simulation consists of two steps as equilibrating brine with caprock minerals at 40 °C

298

and then, pressurizing the system with CO2 up to 10 MPa. The model was simulated for 37

299

weeks to find the chemico-mineralogical reactions of caprock with time. In order to calculate

300

the mineral dissolution and precipitation rate, a general rate equation (Eq 3) derived using

301

transition state theory was used (Alemu et al., 2011; Helgeson et al., 1984).

302

 =  () ( ) [1 − "$%]

#

$

(3)

17

303

Where ' represents the mineral index and  is dissolution (positive value)/precipitation

304

rate (negative value) of mineral which is measured in mol m-2 sec-1.  denotes the reactive

305

surface area, () is the rate constant which depends upon temperature,  refers to proton

306

activity, ( is the order of the reaction with respect to protons, ) represents equilibrium

307

constant for mineral water reaction and * depicts the corresponding ion activity product. The

308

rate constants, () at any temperature can be calculated using the rate constants at 25 ºC,

309

+, (mol m-2 sec-1) incorporating Arrhenius law as shown in Eq 4 (Lasaga, 1984).

310

() = +, exp [

311

Where  denotes the universal gas constant (8.314 J mol-1 K-1), 9: is the activation energy (J

312

mol-1) and  is the temperature in Kelvin. The kinetic data, +, obtained from different

313

literature was converted into 40 ºC and then the model was simulated for 37 weeks. Due to

314

non-reliability of kinetic rate constants, the model was run for several times incorporating

315

data published in different studies to find most accurate values (trial and error method). The

316

most suitable kinetic rate constants for each mineral were selected after comparing the

317

elemental concentration of reacted solution obtained from laboratory and model results. Table

318

4 shows the selected kinetic rate constants and other data used to calculate the

319

dissolution/precipitation rates of each caprock mineral under CO2 and brine saturation. Here,

320

the specific surface areas of minerals were calculated assuming the spherical shape with a

321

diameter of 2 µm for all clay minerals and 63 µm for the rest of the minerals.

012 3

4

4

](5 − +67.4,)

322

(4)

Table 4: Data used in batch modelling Minerals

Mass

Rate log25

Rate log40

Ea (kJ

(mol/kg

(mol m-2

(mol m-2

mol-1)

of

-1

n

-1

sec )

sec )

1×10-14

5.45×10-14

Surface

Reference

area

for kinetic

2

-1

(m g )

rates

0.036

(Gherardi

water) Quartz

4.26

87.70

0

et al., 2007; Palandri and Kharaka., 2004) Illite

0.47

3.98×10-13

9.68×10-13

46

0.1

0.461

(Köhler et

18

al., 2003) Chlorite

0.19

7.76×10-12

3.34×10-11

88

0.5

1.140

(Palandri and Kharaka., 2004)

Anorthite

0.17

1×10-8

1.38×10-8

16.6

1.41

0.035

(Palandri and Kharaka., 2004)

Albite

0.18

1×10-12

3.51×10-12

65

0.5

0.036

(Palandri and Kharaka., 2004)

K-feldspar

0.26

8.71×10-11

2.37×10-10

51.7

0.5

0.0373

(Palandri and Kharaka., 2004)

Kaolinite

0.19

4.90×10-12

1.75×10-11

65.9

0.2

1.16

(Palandri and Kharaka., 2004)

Goethite

0.54

1.15×10-13

3.42×10-8

86

0.5

0.022

(Palandri and Kharaka., 2004)

Pyrite

0.07

4×10-11

1.35×10-10

62.76

0.5

0.036

(Xu et al., 2005)

Montmorillo nite

0.02

1.7×10-13

3.34×10-13

35

0.5

0.045

(Gherardi et al., 2007; Palandri and Kharaka., 2004) 19

323 324 325

3. Results 3.1 Fluid chemistry

326

The ions (Si, Al, Fe, Ca, Mg and K) which were absent in the initial NaCl solution were

327

found at 2, 5, 9, 18 and 37 weeks due to the dissolution of reacting minerals, as shown in

328

Figure 8. According to the ICP-OES results, the concentration of Na+ ions in the solution did

329

not change significantly with time compared to the initial solution concentration. However,

330

Ca2+, Mg2+ and K+ ions were significantly released to the solution during chemical reactions.

331

The concentration of Si in the solution reached 10.67 ppm after a 2-week reaction, and it was

332

reduced by 46% when the reaction was continued for 5 weeks and doubled again after 9

333

weeks compared to 5th week. After that, the Si ion concentration gradually reduced up to 3.87

334

ppm after a 37-week reaction with CO2. In addition, the Al concentration which appeared at

335

the 2nd week as 0.78 ppm was approximately doubled after 9 weeks and then reduced by up

336

to 0.62 ppm in the solution after reacting for 37 weeks. The release of Fe ions was very

337

similar to the Al ion concentration in the solution, possibly because of secondary mineral

338

precipitation such as aluminosilicate minerals or transitional products with Fe ions. 100000

Concentration Log(ppm)

10000 Si

1000

Al Fe

100

Ca 10

Mg K

1

Na

0.1 0

10

20

30

40

Reaction time (weeks)

339 340

Figure 8: Changes in ion concentrations of the solution with reaction time

341

The predicted model and experimental results for pH and alkalinity values are depicted in Figure

342

9. In the presence of CO2, alkalinity continues to increase throughout the reaction time, such that

343

the alkalinity at the 37th week was around 5 times greater than that at the 2nd week. There is a 20

344

good relationship between the model and experimental alkalinity values (relative error 2%). On

345

the other hand, the pH value of the solution at the beginning was around 4.0 and it gradually

346

increased up to 5.0, which can be considered as a 25% increment. Although the model result

347

deviated from the experimental values (relative error 24 %), both show an increment with time

348

due to the consumption of H+ in other chemical reactions. 6

4

3 4

pH

2.5

3

pH using model

2

2

pH

1.5 1

Alkalinity (meq/l)

1

Alkalinity (meq/l)

3.5

5

0.5 Alkalinity (meq/l) using model 0

0 0

5

10

15

20

25

30

35

40

Weeks

349 350

Figure 9: Laboratory and model results for pH and alkalinity measurements as a

351

function of reaction time

352

3.2 Reaction of minerals in the mudstone sample

353

The mineralogical composition of intact mudstone obtained from XRD results shows that the

354

major minerals are quartz, illite, orthoclase, plagioclase and chlorite. In addition, goethite,

355

pyrite, montmorillonite and kaolinite are present in minor percentages (Vatankhah). Figure

356

10 provides the mineral composition variation of intact and reacted mudstone over 37 weeks.

357

Accordingly, the mineral composition of reacted mudstone changed, but no significant

358

mineral variation could be observed in quartz, illite, pyrite, albite and montmorillonite. All

359

the other minerals decreased with time, with the exception of kaolinite. Kaolinite dissolved in

360

the first 5 weeks and then its content almost doubled at the end of the 37th week compared to

361

the intact sample. However, the XRD results were obtained using semi-quantitative analysis

362

with a ±2% error. In addition, SEM and EDX results are shown in Figure 11. Using the EDX

363

results, the chemical characterization of the sample was identified, and the minerals present

364

were verified.

21

Mineral Composition (%)

35 30 25 20 15

decrease

Intact

increase

2 weeks

decrease

10

5 weeks

5

9 weeks

0

18 weeks 37 weeks

Minerals

365

Figure 10: XRD results of intact and reacted mudstone after reactivity tests a).

b).

22

366

Figure 11a) SEM results of Intact, and b) two-week reacted mudstone samples c).

d).

367

Figure 11c) SEM results of five-week, and d) nine-week reacted mudstone samples

23

e).

f).

368

Figure 11e) SEM results of 18-week, and f) 37-week reacted mudstone samples

369

3.3 Geochemical modelling

370

3.3.1

Calculation of saturation indices

371

In order to find the dissolution, precipitation and equilibrium state of minerals in the reacted

372

samples, saturation indices (SIs) were calculated using the brine compositions obtained from

373

the ICP-OES tests. The predicted SIs of the PHREEQC model results are shown in Table 5.

374

According to the results, the mudstone sample reacted with CO2 was under-saturated with

375

respect to K-feldspar, illite, chlorite, montmorillonite, anorthite and albite for 37 weeks while

24

376

the solution was saturated with respect to gibbsite and kaolinite after 9 weeks reaction with

377

CO2. However, chlorite, anorthite and illite have higher saturation indices compared to other

378

minerals, which suggests that their dissolution rates are higher. Since the SIs of goethite and

379

quartz are very close to zero, these minerals are stable (in equilibrium) in the presence of

380

weak carbonic acid at low temperatures. In addition, gibbsite mineral appeared in the latter

381

part of the test series.

382

Table 5: Saturation indices for major minerals involved in chemical reactions Mineral

383

2 weeks

5 weeks

9 weeks

18 weeks

37 weeks

Quartz

0.02

-0.25

-0.01

-0.39

-0.42

Goethite

2.24

0.83

0.27

-1.52

-0.9

K-Feldspar

-7.19

-7.3

-2.59

-1.57

-1.51

Illite

-12.21

-11.86

-2.95

-0.17

-0.17

Kaolinite

-6.39

-6.05

0.52

2.7

2.6

Chlorite

-28.33

-27.37

-20.91

-17.84

-11.97

Montmorillonite

-9.5

-9.61

-2.58

-1.02

-1.08

Anorthite

-21.37

-20.65

-12.1

-8.7

-8.57

Albite

-7.5

-7.73

-3.01

-2.05

-2.14

Gibbsite

-

-

-2.73

1.13

1.33

3.3.2

Batch geo-chemical modelling

384

The element composition of the solution with reaction time was estimated using batch geo-

385

chemical (kinetic reaction) model, as shown in Figure 12. There is a good relationship

386

between model and laboratory results except for Al and Fe ions which do not match with the

387

measured concentration patterns over reaction time. In the model results of the solution

388

concentration, the changes occur rapidly in the first few weeks and afterwards, the ion

389

releasing rate is very small.

25

Al modelled

100,000

Fe modelled Si modelled

Concentration Log (ppm)

10,000

Mg modelled K modelled

1,000

Ca modelled Na modelled

100

Al experimental Fe experimental

10

Si experimental Mg experimental

1 0 0

10

20

30

40

K experimental Ca experimental

Reaction time (weeks)

Na experimental

390 391

Figure 12: The comparison between experimental and modelled results of ion

392

concentrations in CO2-mudstone-brine reacted solution.

393 394

4. Discussion 4.1 Dissolution of primary minerals in caprock

395

The results of reactivity tests conducted on caprock samples obtained from the South-west

396

Hub geo-sequestration project provide a qualitative understanding of caprock-brine-CO2

397

interactions in deep saline aquifers. Since the reaction temperature is quite low (40 ºC), rapid

398

geo-chemical reactions were not observed in the test series. According to the results, almost

399

all the mineral content is reduced in the presence of carbonic acid due to dissolution, with the

400

exception of kaolinite. Kaolinite content was increased after 5-week reaction due to

401

kaolinization (Knut, 1984). The change in clay mineral content was less than that of chlorite,

402

k-feldspar and anorthite mineral dissolution. Quartz was stable and did not show any change

403

with ScCO2 at 40 °C. In addition, kaolinite content increased compared with the original

404

amount and new minerals precipitated with elements of Si, Al and Fe. Therefore, Fe, Al and

405

Si ion concentration in the solution reduced with reaction time, as shown by the ICP-OES

406

results (Figure 8). Thus, it is important to investigate the net pore volume change due to geo-

407

chemical reactions because high porosity can increase the back migration of injected CO2,

408

which is a negative point for CO2 sequestration.

26

409

CO2 tends to dissolve in brine forming carbonic acid, which is considered to be a weak acid

410

(Ni et al., 2014) and it easily decomposes into bicarbonate ions, as shown in Eq. 5 (Leonenko

411

and Keith, 2008). Due to the rapid release of H+ ions into the solution at the beginning, the

412

initial pH was around 4.0, and it increased with time due to the exchange of H+ ions with

413

caprock minerals such as K-feldspar, goethite, anorthite and chlorite. In addition, the

414

alkalinity which measures the acid-buffering capacity increased rapidly during the reaction

415

period due to the high HCO3- concentration in the solution. According to Reuss et al. (1987),

416

the alkalinity of the solution does not increase because of the HCO3- ions until an exchange

417

reaction or silicate weathering consumes the H+ ions. Thus, we can conclude that in these

418

reaction series, such kind of exchange reactions have taken place as alkalinity increased with

419

time.

420

 0 ;<+(:=) + ?+ <(@) ↔?+ ;
(5)

421

Diffusion and advection are the main mechanisms used by injected CO2 to saturate the

422

caprock in deep saline aquifers. Therefore, the caprock-CO2 reactions are considered as rock-

423

dominated reactions where the fluid flow is reduced (Kaszuba et al., 2013). Of the minerals

424

present in the selected caprock, the chlorite mineral is more reactive in the presence of

425

carbonic acid. The dissolution rate of chlorite at 40 °C and pH 4 is around 3.34 × 10-11

426

mol/m-2sec-1 (Black et al., 2015). Chlorite dissolution in the presence of CO2 causes

427

continuous release of Al, Mg and Fe, as shown in Eq 6, verifying the ICP-OES, XRD and

428

PHREEQC model results. Therefore, chloride is more reactive than clay minerals and has

429

mineral reactivity ranging from 10-14 to 10-10 mol/m-2sec-1 (Du et al., 2018). In addition, the

430

mineral reactivity of anorthite and albite at 40 °C and pH 4 are approximately 1.38×10-8 and

431

3.51×10-12 mol/m-2sec-1, respectively (Black et al., 2015; Oelkers and Schott, 1995; Palandri

432

and Kharaka., 2004). Therefore, anorthite dissolves (Eq. 7) faster than albite (Eq 8) releasing

433

Ca, Si and Al, but albite dissolves comparatively more rapidly than quartz (Welch and

434

Ullman, 1996), and the dissolution rate is approximately 5.45 × 10-14 mol/m-2sec-1 in the

435

solid quartz phase (Black et al., 2015; Gherardi et al., 2007). According to the ICP-OES

436

results, the Ca2+ ion concentration increased with time, implying continuous anorthite

437

dissolution throughout the reaction time. The dissolution of anorthite is also consistent with

438

the XRD and PHREEQC model results. Moreover, Amrhein and Suarez (1992) conducted

439

experiments to investigate the long-term (4.5 years) reactivity of anorthite under controlled

440

CO2 conditions and found that the final reaction rates are 200 times slower than the short-

27

441

term dissolution. Therefore, it is crucial to identify the long-term reaction rates in order to

442

predict caprock reactivity over thousands of years.

443

Moreover, according to the PHREEQC model results, illite dissolves slowly, and the mineral

444

reactivity is around 9.68 × 10-13 mol/m-2sec-1 at 40 °C and pH 4 (Black et al., 2015; Köhler et

445

al., 2003) as shown in Eq. 9, releasing K+ and Al3+ ions. Although such a mineral content

446

change was not observed in the XRD results, the percentage variation was so low that it could

447

be considered as a quantification error. Similar patterns were observed for montmorillonite

448

and goethite minerals. According to the XRD results, goethite dissolves with the presence of

449

H+ ions in the solution, which can be described as shown in Eq. 10 (Cornell et al., 1976).

450

Therefore, it can be considered as one of the sources of Fe3+ ions in the solution. However,

451

there is no compatibility between the model results and the XRD results in a few cases. The

452

geo-chemical model results can vary from the actual results due to limitations in the

453

PHREEQC model, such as treating CO2 as an ideal gas and non-reliability of kinetic rate

454

constants. Hence, it is essential to implement the model with new modifications such as

455

introducing a CO2 solubility model and building the model with the most accurate kinetic rate

456

constants.

457



[I  A BCD.6 EF.G HF.4 HF.4 + 0.1H + + A., EF., ]<4F (
458

0.1H A + 3.5?D I
(6)

459

;E+ I+ <7 + 8?  → ;+ + 2E A + 2I<+(:=) + 4?+ <

(7)

460

QEIA <7 + ?  + + ?+ < → Q + 2?D [I
(8)

461

)F.G, E4.G, [IA., EF., <4F ](
(9)

462

H<
(10)

463

The brine solutions obtained at different time intervals were under-saturated with respect to

464

K-feldspar (from the 2nd week to the 37th week), or in other words, k-feldspar tended to

465

dissolve in the presence of carbonic acid at 40 °C. Moreover, according to the XRD test

466

results, the reacted solid minerals had a much lower percentage of K-feldspar at the end of

467

the reaction period. According to Du et al. (2018), such reactions can make micro-fractures

468

and round, oval and square dissolution pores in the mineral surfaces due to the pressure and

469

weak carbonic acid. As a result, the porosity and permeability reduce with time increasing

470

CO2 leakage risk which is a disadvantage for CO2 sequestration.

6

28

471

4.2 Precipitation of secondary minerals

472

Generally, the dissolution/precipitation of kaolinite can be studied by observing Si and Al ion

473

concentrations in the solution and usually the release of Si ions is greater than Al ions, as

474

shown in Figure 8 (Polzer and Hem, 1965; Wieland and Stumm, 1992; Xiong et al., 2009).

475

According to the ICP-OES results, the Al and Si ion concentrations increased up to the 9th

476

week due to the dissolution of kaolinite, as given in Eq. 11 (Polzer and Hem, 1965; Wieland

477

and Stumm, 1992) and later, the solution became supersaturated with respect to kaolinite.

478

This is supported by the results of the geo-chemical model. According to Nagy et al. (1991),

479

the kinetic rates of kaolinite during dissolution and precipitation at pH 3 and 40 °C are -2.02

480

× 10-11 and 2.29× 10-12 mol/m-2sec-1. Since kaolinite already existed in the caprock sample,

481

new formation of kaolinite could not be observed very clearly in the SEM images. However,

482

the kaolinite percentage almost doubled compared with the intact sample in the XRD test

483

results, possibly because of the dissolution of K-feldspar and the precipitation of kaolinite on

484

the caprock sample, as shown in Eq. 12. Therefore, at low temperatures in deep saline

485

aquifers, K-feldspar acts as one of the reactive minerals among silicate minerals which can be

486

subjected to kaolinization. In addition, according to the ICP-OES test results, K+ ion

487

concentration increases during the reaction time, because K-feldspar releases K+ ions and

488

forms kaolinite. Therefore, the saturation indices calculated by PHREEQC are compatible

489

with the mineral changes found in the reacted solid minerals. The same phenomenon was

490

observed in several other studies too (Wollast, 1967; Yuan et al., 2019). A number of studies

491

have been conducted to investigate the alteration of K-feldspar at elevated temperatures and

492

pressures (hydro-thermal reactions), but the behaviour of K-feldspar at low temperatures and

493

pressures is different from that at high temperatures and pressures because the rates and the

494

products are totally different in the reactions. According to Simpson et al. (1979) and Taylor

495

H. (1979), kaolinization, chloritization and hematization occur at low temperatures (<200 °C);

496

thus these kind of reactions cannot be observed at very high temperatures (>200 °C).

497

According to Yuan et al. (2019), when temperature and pressure are greater than 100 °C and

498

300 bars respectively, k-feldspar and kaolinite react and form illite and quartz instead of

499

kaolinization. Therefore, Wollast (1967) carried out experiments at room temperature and

500

pressure to identify K-feldspar behaviour and found that the release of Si and Al to the

501

solution and the exchange of H+ for K+ are the dominant observations for K-feldspar in the

502

presence of carbonic acid. As a result, a high K+ ion release was observed in the ICP-OES

503

results as shown in Figure 8. 29

504

The dissolution of kaolinite in the first stages of the reaction increases the porosity of the

505

caprock, which would enhance the permeability and back-migration of injected CO2 (She et

506

al., 2016). This is a disadvantage for caprock integrity. However, the porosity and

507

permeability decrease due to the re-precipitation of kaolinite as a secondary mineral after 9th

508

week. These phenomena have been observed in different studies (Nagy et al., 1991; Nagy and

509

Lasaga, 1993; She et al., 2016), and it is crucial to recognize the net gain in porosity due to

510

dissolution and precipitation to identify the permeability of caprock. During kaolinization,

511

one volume of K-feldspar forms approximately 60% kaolinite and 40% silicic acid (Knut,

512

1984). However, significant kaolinite precipitation cannot be seen under these conditions

513

because kaolinite formation in solutions with high K+ concentration and lower H+

514

concentration at lower temperatures is very slow. Once the solution becomes supersaturated

515

with amorphous silica, H4SiO4 precipitates on the caprock, reducing its porosity (Wollast,

516

1967; Yoshiro et al., 1974) which is an advantage for CO2 sequestration.

517

E+ I+ <, (
518

2)EIA <7(UVWXUY@:YZ) + 2?  + 9?+ < ↔ E+ I+ <, (
519

At the beginning, the pH value was low (around 4.0) due to ?+ ;
520

rapid release of Al, Si and Fe ions could be observed as a result of mineral dissolutions such

521

as kaolinite, k-feldspar, goethite and chlorite. However, Si, Fe and Al ion concentrations were

522

reduced (due to kaolinization and precipitation of H4SiO4) after 9th week. Another reason

523

may be the formation of Si(OH)4, gibbsite and Fe(OH)3 colloidal precipitations in the solution

524

(Wei and Taian, 1998). According to Ni et al. (2014), the dissolution rate of Si was maximum

525

at 45 ºC and Si(OH)4 was formed, reducing the Si concentration. In addition, the presence of

526

Al ions in the solution led to the precipitation of gibbsite, resulting in a decrease of Al ion

527

concentration in the solution. This result complies with the geo-chemical model. Therefore,

528

secondary precipitations of minerals in the clay matrix leads to porosity reduction, decreasing

529

the diffusion and reaction rate, which is a positive characteristic for caprock integrity in long-

530

term scenarios.

531

5.

532

The results of reactivity tests conducted on caprock samples obtained from the South-west

533

Hub geo-sequestration project provide a qualitative and quantitative understanding of

534

caprock-brine-CO2 interactions in deep saline aquifers. In this reactivity test series, in situ

(11)

Conclusion

30

535

conditions at a depth of 979 m (40 °C and 10 MPa) were provided for 37 weeks to recognize

536

possible field geo-chemical reactions in the caprock, rather than conducting hydro-thermal

537

experiments which give unreliable results due to the absence of such high pressures and

538

temperatures in the field. The following key findings were made:

539•

Injection of CO2 can cause a reduction of pH in brine up to 4.0. In the beginning, the ion

540

release rate is accelerated due to high H+ activity. However, with time, the H+ concentration

541

reduces because different chemical reactions consume it. As a result, the ion release rate

542

gradually decreases. Thus, the mineral changes in caprock become significant in geological

543

time scale so that, the long term caprock integrity should be evaluated.

544•

The caprock minerals react with injected CO2 causing precipitations and dissolutions

545

depending upon aquifer temperature and pressure. In the current study, considerable

546

dissolution was observed in chlorite, k-feldspar and anorthite minerals, in contrast minerals

547

such as quartz are not reactive in the presence of carbonic acid at low temperatures (40 °C).

548

In addition, the kaolinite content increases compared to the original amount and new minerals

549

precipitate with elements of Si, Al and Fe at aquifer temperature and pressure decreasing pore

550

volume of caprock. Thus, net pore volume change should be evaluated after the reaction time

551

period to identify whether the caprock is at risk or not.

552•

In the beginning, the dissolution of minerals is very significant, and this enhances the pore

553

volume of the caprock, and eventually increases the porosity, diffusion and permeability.

554

This is a negative point for caprock integrity as it enhances the back- migration of injected

555

CO2. However, due to the precipitation of secondary minerals in the rock pore structure such

556

as kaolinite, the pore volume can be reduced to some extent, which can be considered as a

557

benefit for caprock in long-term scenarios. Thus, the net change of porosity due to dissolution

558

and precipitation of caprock is crucial to predict the ability of CO2 to flow through caprock.

559•

Due to high heterogeneity nature of caprock mineralogy, the geo-chemical reactions differ

560

from site to site, and it is a challenge to predict the chemical reactivity of caprock in different

561

aquifers. The geo-chemical reactions occurring in the caprock with the presence of CO2 is

562

site-specific and it is essential to investigate for the long-term integrity of CO2 sequestration

563

process. Therefore, the prediction of reactivity incorporating heterogeneity of caprock using a

564

conceptual geological model is highly recommended as future work.

31

565

6. Author contribution

566

Jayasekara: Investigation, Writing - Original draft, Software. Ranjith: Conceptualization.

567

Wanniarachchi: Methodology. Rathnaweera: Writing - Review and Editing. Van Gent:

568

Resources.

569

7. Declarations of interest

570

The authors declare no conflicts of interest.

571

8. Acknowledgement

572

Authors would like to acknowledge the West Australian Department of Mines, Industry

573

Regulation and Safety for the provision of the sample.

574

9. References

575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603

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747

35

Highlights •

Chemical reactivity tests conducted for mudstone providing reservoir conditions

•

Conducted mineral-brine analysis for 37 weeks including geo-chemical modelling

•

Dissolution of minerals such as K-feldspar, anorthite, and chlorite

•

Precipitation of minerals such as kaolinite, gibbsite, amorphous silica and Fe(OH)3

•

Dissolution of minerals is significant which can increase pore volume of caprock

Conflict of Interests We declare that there is no conflict of interests.