Carbon Capture and Storage: Palaeogeographical and Facies Data to Understand Suitability for Storage
Carbon capture and storage (CCS) is an option for reducing emissions to the atmosphere. CO2 storage requires that supercritical CO2 be injected into deep subsurface reservoir rocks over a long enough time period to make a difference to atmospheric CO2 concentration. Understanding the long term behaviour of CO2 in advance of development of these reservoirs is therefore vital to provide confidence for investors, developers, regulators and the public (Stephenson 2013, Stephenson et al. 2019, 2022). This will mean understanding the character of CCS targets, whether they be depleted gas fields or saline aquifers. To make a difference, CCS will need to be a big industry with hundreds or even thousands of operating storage sites, many in geological basins that have not yet been surveyed and where knowledge particularly of saline aquifers is scant. Programs like the Deep-time Digital Earth program (DDE) of the International Union of Geological Sciences that seek to develop linked databases and platforms will help to model target basins to help make CCS a reality.
Carbon Capture and Storage (CCS) is an integrated process involving three stages: (1) capture of CO2 from power stations and other large industrial sources, (2) transporting CO2 (usually in pipelines) to a storage site, and (3) permanent storage of CO2 in deep geological structures. In storage, CO2 is converted into a high pressure, liquid-like form known as supercritical CO2. Supercritical CO2 behaves like a runny liquid, and can be injected directly into sedimentary rocks in depleted oil fields, gas fields, or saline aquifers. Unmineable coal seams and some volcanic rocks may also be storage sites. Various physical structures prevent CO2 from escaping to the surface. These include impermeable caprocks and geochemical trapping mechanisms.
The largest capacity storage is in the world’s saline aquifers and these will likely take the majority of emissions in the long term once available depleted fields are filled. However there are several challenges with saline aquifer suitability. An important issue relates to the risk that small scale heterogeneity poses to injectivity. In areas where some survey work has been done, for example in Europe and the US, capacity estimates for saline aquifers tend to contain limited geological detail at the sub-seismic (metres to tens of metres) scale and so sedimentary heterogeneity at that scale is not very well known. This introduces considerable uncertainty. The second relates to less well surveyed parts of the world, for example many of the emerging economies, where very few exploration scale studies of saline aquifers have been done at all. This provides another level of uncertainty and slows down investment and progress.
Small scale sub-seismic heterogeneity
One of the key uncertainties is heterogeneity along the migration path of an injected CO2 plume. This heterogeneity can be caused by primary sedimentological and paleoenvironmental factors, diagenesis and tectonic and structural change (Newell and Shariatipour 2016). Although the oil industry has been dealing with heterogeneity for decades in relation to oil and gas extraction there are some crucial differences between CO2 injection and oil and gas extraction.
The first is obviously the fact of injection rather than extraction. The second is that CO2 is a reactive gas injected in supercritical form, and that CO2 is being placed as far as possible in geological conditions that encourage slow migration, dissolution and precipitation, and/or stratigraphic trapping. There is a balance to be struck between injectivity and the presence of enough heterogeneity to facilitate solubility trapping. An example is the Sleipner CO2 storage facility in the North Sea Utsira Formation, the longest running facility for CO2 storage. Repeat seismic surveys alongside specially tuned seismic has detected layers of rock with high CO2 saturation – where the CO2 has accumulated below thin sub-seismic mudstone layers creating bright spots within the Utsira Formation reservoir (Chadwick et al. 2012). 4D seismic has also allowed tracking of the rate at which buoyant supercritical CO2 rises through the reservoir, how and when it reached the overlying seal, and how it moved under the seal as more CO2 arrived. In the Utsira Formation, mudstone layers could be been seen as problems for injection – perhaps making less of the reservoir available to injection - but also have proved useful in slowing down upward migration, allowing more time for reactions to take place dissolving CO2 and leading to more long term storage through carbonation. Thus the low permeability layers promote ‘solubility trapping’ leaving the overlying seal (the physical trap) to do less of the work of confining the CO2.
So certain types of heterogeneity could be useful in CO2 storage, and some could impair injectivity. This means we have to understand it better. Studies of the recognition and primary controls on heterogeneity have been carried out in the UK on rocks that are primary targets for offshore CCS, particularly the Triassic Sherwood Sandstone Group.
Studies by Newell and Shariatipour (2016) and Newell (2006) showed how outcrop study of part of the Sherwood Sandstone Group (locally the Otter Sandstone Formation) of the Wessex Basin could be used to improve understanding of how rock heterogeneity controls the migration and trapping of buoyant CO2 plumes injected into dipping saline aquifers. In particular the transition from the Sherwood Sandstone Group (fluvial sandstone reservoir) to the Mercia Mudstone Group (playa lacustrine mudstone seal) was shown to be sedimentologically complex. The arrangement of inclined point-bar sandstones, horizontal splay sandstones and mudstone baffles were key to heterogeneity as was the arrangement of early diagenetic calcrete in the form of rhizocretions, calcrete sheets and calcrete conglomerates.
Thompson et al. (2019) used a combination of outcrops including the excellent orthogonal faces of a quarry outcrop at Frogsmouth Quarry in Cheshire coupled with borehole study to consider heterogeneity in the Helsby Sandstone Formation and the Wilmslow Sandstone Formation, again in the Sherwood Sandstone Group. Four broad facies types were identified: aeolian dune, dry interdune, damp interdune and wet interdune. The chief baffles to fluid flow were the wet and damp interdune units, and their extents and form were considered to be a function of their position within the dune field depositional environment.
Fig. 1. Translating outcrop data on small scale sedimentary environment to permeability models
Many of these characters of heterogeneity - of permeability - are ultimately a function of paleogeography and paleoenvironments at various scales. The challenge is to translate small scale features of ancient floodplains and dunefields into 3D permeability models and so this is essentially a data problem (Fig. 1). DDE is currently developing methods to hold and display stratigraphic data of this type, as well as tools to apply machine learning to problems of this type.
Saline aquifers of the emerging economies
The saline aquifers of many of the emerging economies are much less well understood and may not even have been mapped, particularly in the offshore. This uncertainty again slows down vital investment in CCS. Saline aquifers, the vast majority of which have not been explored for oil and gas have very little geological data. Seismic and sparse boreholes may allow simple guesses to be made about the suitability for CCS. However again the characteristics of these sedimentary rocks and the basins in which they sit - at a gross scale - are to some extent governed by their tectonic position, paleoclimatic controls during their deposition, and palaeolatitude.
Fig. 2. The DDE deeptime org platform may provide a high level tool for assessing basins for saline aquifers for CO2 storage
Many of these variables can be modelled in the technique of forward stratigraphic modelling so that variables can be used with appropriate algorithms to calculate gross facies variations on a basin scale. These mapped facies variations may allow predictions of CO2 storage suitability. This and other techniques will be made simpler and more accessible to the scientific community by the development and release of the DDE deep time org platform. This platform enables complex and sophisticated analysis of the evolution of large scale basins through modelling tectonic, palaeoclimatic and other variables. The platform working with suitable data could help in the highest level estimates of CO2 storage.
Carbon capture and storage (CCS) is a vital tool in carbon abatement, however the most likely largescale storage solution – the globe’s saline aquifers – are known poorly because they have not been explored much until now. Data in many areas is sparse but some of the primary sedimentological and paleoenvironmental controls on suitability, for example permeability, can be modelled using data collected from outcrop (for the small scale) and from large palaeogeographic-palaeoclimatic-tectonic models. DDE, as the IUGS’ big data programme, is considering the use of data for the development of CCS and other applications where the properties of rocks are important in the energy transition.
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