Carbon capture and storage (CCS) is broadly recognised as having the potential to play a key role in meeting climate change targets, delivering low carbon heat and power, decarbonising industry and, more recently, its ability to facilitate the net removal of CO2 from the atmosphere. However, despite this broad consensus and its technical maturity, CCS has not yet been deployed on a scale commensurate with the ambitions articulated a decade ago. Thus, in this paper we review the current state-of-the-art of CO2 capture, transport, utilisation and storage from a multi-scale perspective, moving from the global to molecular scales. In light of the COP21 commitments to limit warming to less than 2 °C, we extend the remit of this study to include the key negative emissions technologies (NETs) of bioenergy with CCS (BECCS), and direct air capture (DAC). Cognisant of the non-technical barriers to deploying CCS, we reflect on recent experience from the UK's CCS commercialisation programme and consider the commercial and political barriers to the large-scale deployment of CCS. In all areas, we focus on identifying and clearly articulating the key research challenges that could usefully be addressed in the coming decade.

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Carbon capture and storage (CCS): the way forward

One of the critical factors that control the efficiency of CO2 geological storage process in aquifers and hydrocarbon reservoirs is the capillary-sealing potential of the caprock. This potential can be expressed in terms of the maximum reservoir overpressure that the brine-saturated caprock can sustain, i.e. of the CO2 capillary entry pressure. It is controlled by the brine/CO2 interfacial tension, the water-wettability of caprock minerals, and the pore size distribution within the caprock.

By means of contact angle measurements, experimental evidence was obtained showing that the water-wettability of mica and quartz is altered in the presence of CO2 under pressures typical of geological storage conditions. The alteration is more pronounced in the case of mica. Both minerals are representative of shaly caprocks and are strongly water-wet in the presence of hydrocarbons.

A careful analysis of the available literature data on breakthrough pressure measurements in caprock samples confirms the existence of a wettability alteration by dense CO2, both in shaly and in evaporitic caprocks. The consequences of this effect on the maximum CO2 storage pressure and on CO2 storage capacity in the underground reservoir are discussed. For hydrocarbon reservoirs that were initially close to capillary leakage, the maximum allowable CO2 storage pressure is only a fraction of the initial reservoir pressure.

Wettability alteration of caprock minerals by carbon dioxide

Interfacial tension measurements and wettability evaluation for geological CO2 storage

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: 

Sandstone and limestone, plus pure minerals such as quartz, calcite, feldspar, and mica are strongly water wet in a CO2-water system.
Oil-wet limestone, oil-wet quartz, or coal is intermediate wet or CO2 wet in a CO2-water system.
The contact angle alone is insufficient for predicting capillary pressures in reservoir or seal rocks.
The current contact angle data have a large uncertainty.
Solid theoretical understanding on a molecular level of rock-CO2-brine interactions is currently limited.
In an ideal scenario, all seal and storage rocks in CGS formations are tested for their CO2 wettability.
Achieving representative subsurface conditions (especially in terms of the rock surface) in the laboratory is of key importance but also very challenging.

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

CO2/water interfacial tensions under pressure and temperature conditions of CO2 geological storage

A novel apparatus, PeDro, for high-pressure measurement of the interfacial tension using the pendant drop method is presented. This apparatus was constructed for accurate measurements in the two-component system of water and compressed carbon dioxide. The main influences on accurate and reproducible results have been investigated, and the results have led to an optimized experimental setup and a quasi-static measuring method. PeDro was calibrated against well-established data for water + air. The experimental error of measurement is smaller than 2%. In this two-phase system measurements were conducted in the ranges (278 to 335) K and (0.1 to 20) MPa. The interfacial tension showed a pronounced dependence on pressure and temperature. A regression function has been found which describes the experimental data in the range investigated with high precision.

Andrea Kögel

Papers

Interfacial Tension at Elevated PressuresMeasurements and Correlations in the Water + Carbon Dioxide System