Introduction
Global warming due to anthropogenic greenhouse gas production is widely regarded as one of the major issues threatening the planet (Solomon et al.
2009). One method for reducing excessive atmospheric CO
2 emission is carbon capture and sequestration (CCS); a general term for the capture, transport and storage of carbon dioxide from the atmosphere or large emission sources. An effective form of sequestration is geological sequestration, whereby carbon dioxide in gaseous, liquid, supercritical or dissolved form is injected underground into porous rock formations typically at depths larger than 1 km for long-term storage (Ansolabehere et al.
2007). The supercritical conditions of CO
2 exist above temperatures of 31.1 °C and pressure of 73.9 bar. Under these conditions, CO
2 exhibits properties of both liquid phase and gaseous phase. For example, CO
2 will occupy a container like a gas but with liquid density (Petrik and Mabee
2011; Metz et al.
2005).
Notably, suitable formations for CO
2 storage include deep saline aquifers, basalts, hydrocarbon reservoirs and un-mineable coal seams located at a depth of 800 m or above, where pressures and temperatures of the reservoirs keep the CO
2 in liquid or supercritical condition (Franceschina et al.
2015; Metz et al.
2005). If all the sedimentary basins worldwide are considered for CCS, the storage capacity for CO
2 in geological formations would be enormous; however, the acceptability of any specific storage site depends on many features such as proximity to CO
2 sources, possibility for leakages and other specific factors such as permeability and porosity (Folger
2009).
It is important to know that for CCS to become a successful climate change mitigation option, it must be possible to securely store CO
2 underground for millennia without leakage into the atmosphere; and to ensure that the CO
2 is successfully trapped, the reservoir sites must be monitored (Khudaida and Das
2014; Benson and Cole
2008). Geological CO
2 storage can occur through four main trapping mechanisms: physical barriers, capillary forces, solubility trapping, and mineralisation (Ansolabehere et al.
2007; Abidoye et al.
2015; Khudaida and Das
2014). Although carbon sequestration is an economically and ecologically viable method, the carbon must be effectively trapped to avoid leakages back into the atmosphere or undesirable migration to shallow aquifers via fractures, permeable pathways and nearby penetrable wells, whereby potable water could become contaminated (Abidoye and Das
2015b).
Monitoring techniques can also minimize the risks of CO
2 leakage, because it gives early warning of CO
2 storage problem, in other words, it quantifies the amount of CO
2 storage (Hartai
2012). Many geophysical techniques exist to monitor carbon sequestration (Kiessling et al.
2010; Hovorka et al.
2011).
Geoelectrical characterization of the carbon sequestration presents a simple and non-invasive monitoring method as it can relate the electrical properties of the rock formation to its water saturation (White et al.
2003; Abidoye et al.
2015), which is directly related to the CO
2 content of the reservoir. Two main electrical properties exist: the electrical conductivity,
σb (a metric for the current induced upon application of an electric field or ability of an aqueous solution to carry electric current) and the dielectric constant,
εb (a metric for the electrical polarization induced upon application of an electric field) (Keller
1966; Han
2011). These are suitable parameters for monitoring because of their sensitivity to changes in saturation of the water-CO
2 phase (Abidoye and Das
2015a). A time domain reflectometry (TDR) sensor, which is inexpensive and presents a reliable and simple technique to measure both
σb and
εb, is a potential sensor for the measurement of these parameters as it can be incorporated underground around the area of storage.
It is observed in the literature that seismic method also provides an effective monitoring technique to assess CO
2 plume. A repeated seismic survey is important for ensuring both containment and conformance monitoring. Seismic method has been shown to be an excellent monitoring method to quantify small amount of CO
2 saturations in the reservoir, but it has been less successful in determining the increase in CO
2 saturation (Furrea et al.
2017; Alfia et al.
2019). Electromagnetic and electric methods are also important tools for monitoring CO
2. They utilise the electrical and electromagnetic responses from the subsurface to determine the changes in CO
2 or water saturations. The methods involve the quantification of electric parameters such as resistivity and conductivity and, determining the correlations such as the Archie expression to relate these parameters to saturations. Methods that use these concepts are the electrical resistivity tomography (ERT), electromagnetic resistivity (ER), electromagnetic induction tomography (EMIT) among others (Ajayi et al.
2019). Dafflon et al. (
2012) explained the importance of electrical resistance tomography (ERT) in monitoring the migration of groundwater with dissolved CO
2. As the electrical response of rocks is highly independent of the mechanical response, electrical and seismic quantifications provide complementary estimates of CO
2 saturation (Daley
2019).
To date, most CO
2 sequestration projects involve injecting CO
2 in supercritical phase (Metz et al.
2005; Hosa et al.
2010; Abidoye and Das
2015a). Although the storage of CO
2 in supercritical form can be safer and more effective, it is important to consider the high cost of compression from gaseous to supercritical phase (Petrik and Mabee,
2011). In addition, the implication of injecting ScCO
2 under high pressure during the injection process can cause natural disasters such as earthquakes (Bachu
2000; Metz et al.
2005). Therefore, this work focused on using both the gaseous and supercritical CO
2. Research has shown that gaseous CO
2 has been used as an alternative to supercritical CO
2 (see, e.g., U.S Department of Energy
2008). Nonetheless, commercial scale of CO
2 storage in gaseous form is very unlikely because of its unfavourable risk assessment. This study is also important, because CO
2 stored in supercritical condition may leak due to faulty caprock and form a gaseous phase. Therefore, if monitoring sensors are situated at lower depth of the reservoir (i.e., 200–400 m), the sensor can detect the gaseous CO
2 when there is leakage. A wide range of geoelectrical monitoring methods have been developed to monitor the movement and storage of injected supercritical CO
2 (Rabiu et al.
2017; Abidoye and Das
2015a), yet there is an outstanding need to find out whether this robust tool can track gaseous CO
2 efficiently and effectively.
Characteristics of some of the geological rock formations currently used for carbon sequestration have been summarised in Table
1. The rock types being investigated are silica, limestone and basalt; with silica and limestone being the most ubiquitous rock types in deep saline aquifers (Bentham and Kirby
2005; De Silva et al.
2015). Basalt is also investigated as it offers the possibility of improving carbon mineralisation and thus provides permanent CO
2 storage (Matter and Kelemen
2009; Adam et al.
2011; Rabiu et al.
2017). Therefore, to effectively relate the geoelectrical characteristics to the water saturation, the factors influencing these electrical properties must be properly understood. Previous studies have shown that
σb and
εb are influenced by temperature, pressure, rock type, salt concentration, surfactants, permeability, mineralogy and clay content (Rabiu et al.
2017; Abidoye and Das
2015a,
b; Han
2011; Magill
2009; Carcione et al.
2012) when injecting liquid or supercritical CO
2 into a bed of sand particles. The purpose of this study is to understand how these factors affect the measurements, for which there is currently little data and significant uncertainty as to the extent to which these factors affect these parameters.
Table 1
Typical characteristics of deep saline aquifers used for carbon sequestration (Ranganathan et al.
2011; Rempel et al.
2011; De Silva et al.
2015; Metz et al.
2005)
Porosity | 0.18–0.30 |
Depth (km) | 0.6–2.7 |
T (°C) | 35–98 |
P (bar) | 70–285 |
Salt conc. w/w % | 5–20 |
pH | 5.4–8.1 |
The work of Rabiu et al. (
2017) characterised CO
2 sequestration in silicate, limestone and basalt using geoelectrical properties. But their investigation was limited on liquid and ScCO
2 phases. Earlier, the works of Kaszuba et al. (
2003) as well as Abidoye and Das (
2015a) investigated supercritical CO
2 trapping in porous materials. Therefore, it can be concluded that most of the existing publications focus on injecting liquid and supercritical CO
2 in porous media. However, the important question concerning whether gaseous CO
2 and scCO
2 have the same effect on geoelectrical properties remains unanswered. This work explores this gap in knowledge by investigating geoelectrical characterisation of CO
2 sequestration during the injection of gaseous CO
2.
Conclusions
The effect of different factors on σb–Sw and εb–Sw relationships in porous rock media were investigated using a TDR probe. Salt concentration, rock type and presence of surfactant had an observable effect on the relationships; however, different initial values of pH (with silica sand) produced no significant change. Higher salt concentrations were shown to result in higher σb and εb values for a given Sw, which was attributed to the greater number of ions present. For any given Sw, limestone was found to give higher values of both σb and εb, followed by basalt and silica, respectively. For σb, this can be explained by a greater level of dissociation of the rock, resulting in a greater number of ions in solution.
In the case of εb, the difference can be attributed to their respective chemical compositions. Presence of surfactant resulted in lower σb values at lower Sw values and higher σb values at higher Sw values compared with solution absent of surfactant. εb values were found to be lower in the presence of surfactant at any given Sw. These differences can be attributed to the increase in viscosity due to the surfactant. Although change in initial pH was found to produce no discernible change in the relationships with silica sand, it was hypothesized that an effect could be observed with a different rock type.
This work contributes to the understanding of the effect of salt concentration, pH, rock type and surfactant presence on σb–Sw and εb–Sw relationships. Measuring σb and εb is a viable option for monitoring CO2 storage sites, and consequently understanding how these geoeletrical parameters relate to Sw is essential. This enables an estimate to be made from the measured geoelectrical parameters, which in turn relates to the CO2 content of the storage site. Therefore, by monitoring the geoelectrical characteristics, changes in CO2 content indicative of leakage can be detected. The Archie equation was used to predict the experimental results and the outputs were corroborated with previous studies. Finally, a fit regression analysis was carried out on the experimental results and the model reveals a good reliability in the prediction of monitoring process in geological carbon sequestration.
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