The UK is in a strong position with respect to the potential for permanent sequestration of CO2 offshore in rock formations deep below the North Sea. This is because of two factors. Firstly the correct type of rock formations exist in places below the North Sea with the porosity to provide capacity for the CO2, permeability to allow its injection, and sealing formations to contain the CO2.
Secondly the UK has had an active oil and gas industry since the 1960s which now provides a significant knowledge base to enable an understanding of subsurface rock formations and fluid flow. Without this existing understanding, exploration for CO2 storage offshore UK would be of unjustifiably high expense. We are in the fortunate position that CO2 storage offshore UK is able to benefit by Standing on the Shoulders of Giants*.
Many parts of the UK North Sea have been subject to oil and gas exploration and production. The technologies required to understand the subsurface for oil and gas are essentially the same as those required for CO2 storage. Seismic imaging identifies subsurface horizons; drilling wells provides specific formation and fluid data. Production and injection during hydrocarbon field developments provides an understanding of dynamic reservoir behaviour; that is connectivity and fluid flows within the formation. Developing a field on the basis of seismic data and several exploration and appraisal wells is always a risky business; knowing what lies between the wells and how well connected the formations are is the critical question. It has been said that the only time it is possible to know a field’s oil and gas recoverable reserves with any certainty is at the end of field life. Up to that point the heterogeneities and other reservoir complexities make it difficult to predict dynamic reservoir behaviour, even if it is possible to calculate the hydrocarbons initially in place.
CO2 storage differs from oil and gas. For CO2 storage, fluid is injected, but no fluid is extracted. This increase in subsurface fluid volume must be accommodated, and requires significant regional connectivity through a saline aquifer. By contrast, in hydrocarbon developments, fluids are extracted and connectivity is important only in the immediate vicinity of the field containing the oil or gas, which is a relatively small area/volume in a regional context. Even so, it is common for reservoir faulting and heterogeneity to inhibit or prevent flow within a hydrocarbon field, requiring separate wells within different fault blocks or reservoir sectors. The regional connectivity required to accommodate CO2 storage is 100 to 1000 times greater in volume than that for a hydrocarbon development.
CO2 storage is an infant discipline. However, luckily for CCS, much of the expertise required to make CO2 storage work already exists in the oil & gas sector. Several factors influence the potential storage location, including;
- Proximity to CO2 emissions source
- Suitability of storage location
- Transport infrastructure between the locations
For early CCS projects it seems sensible that we should build upon existing oil and gas subsurface knowledge and make use of existing facilities, where doing so can help constrain the cost of CCS. Not all areas of the North Sea have been subject to the same level of oil and gas activity. Areas of low hydrocarbon prospectivity may have no wells and no field developments. This significantly reduces the available data for that area and is likely to result in unjustifiable expense (or unjustifiable project risk), for development as a CO2 store. It is important that a good understanding exists of regional connectivity (as this determines the storage capacity), formation injectivity and seal all exist before a CO2 store is developed. Regional connectivity is the most difficult (ie costly & time consuming) to appraise in the absence of existing regional data, as shown in the CO2 storage Site appraisal and development flow chart below;
CO2 Storage: Site appraisal and development flow chart
Of course it would be possible to inject CO2 and worry about connectivity (i.e. capacity) later (*take a Risk in the chart). The obvious drawback of this is that when problems arise, CO2injection would have to stop and this is likely to shut down the power plant or emissions source, curtailing revenue, breaching consents, commercial and funding agreements. Indications are that some early CO2 injection projects have already experienced this outcome, which potentially compromises future CO2 storage and CCS projects, through a loss of public confidence.
The debate continues in the UK about how to stimulate CCS and how to better define and develop UK storage potential. Obligations for CCS on new projects or for CCS readiness assessments to be completed, require a deep understanding of CO2 storage. Ensuring cost effective CO2 storage is critical to delivery of CCS at a price competitive with other forms of low carbon power. CO2 can be stored safely in deep subsurface horizons under the N Sea. Some formations will be better suited than others. Making use of existing knowledge and specifically areas where oil and gas developments provide regional understanding is critical to cost and risk effective CO2 storage. It is only by Standing on the Shoulders of Giants that CO2 storage offshore UK will be successful.
*This quotation is attributed to Bernard of Chartres in the twelfth century, although more usually recognised as a quotation by Isaac Newton in a letter to Robert Hooke in 1676 when he said “If I have seen further it is by standing on the shoulders of giants”. It is now more usually considered a metaphor with a contemporary interpretation meaning “one who discovers by building on previous discoveries”.)
This article was written by Sam Gomersall.