Site Selection for GCS: Insights That Analogues Can Offer
Geological Carbon Storage (GCS) is considered the most reliable method of carbon sequestration, though its feasibility is restricted to large-scale point sources where in-situ capture and storage can occur. Depleted hydrocarbon fields are attractive targets for carbon storage; the same geological formations that are effective at trapping oil and gas over million-year timescales are also capable of trapping CO2. Additionally, a significant proportion of infrastructure previously used for hydrocarbon extraction can be repurposed or reused for storage, particularly in fields where Enhanced Oil Recovery (EOR) is operant. While not specifically aimed at long-term CO2 storage, EOR, which involves pumping CO2 into the subsurface to recover residual oil, achieves a significant degree of permanent carbon storage via residual CO2 trapping and is presently the only method of industrial-scale carbon sequestration in operation.
This technical feasibility is promising for large-scale carbon storage ventures in depleted or decommissioned hydrocarbon fields, which have a global CO2 storage capacity of 900-1200Gt. However, not all fields are suited to carbon storage, with leakage being the foremost concern; a 0.5% per annum leakage leads to 63% degassing over 200 years, while 1% leakage leads to near total loss over the same timescale. This is mitigated by the natural changes to trapping mechanisms over time, which increase storage security: free CO2 is initially constrained by structural and stratigraphic trap geometries, but the dominant trapping mechanism shifts towards residual trapping over 100-year timescales and chemical trapping over 1000-year timescales, whereby it is immobilised and cannot escape without deliberate engineering. Subsequently, for storage to be successful, leakage rates of <0.1% must be maintained to achieve 99% retention over 100-year timescales – secure short-term storage is key to the long-term immobilisation of carbon.
Several factors affect the short-term integrity of CO2 reservoirs. CO2 must be stored as a supercritical fluid, which restricts targets to deep hydrocarbon fields >800m in depth, or into reservoirs where pressures are equivalently high. Trap geometries and seal properties also dictate the long-term migration of the CO2 plume; seal thickness, a proxy for capillary pressure, affects trap integrity, while caprock topology and fault association influence lateral migration and leakage via surrounding faults or wells. Gas fields are favoured over oil fields for GCS due to the pressure conditions under which they exist. Idealised reservoirs might include deep and steep four-way closures with minimal fault associations and thick seals, while fault blocks or horizontally unconstrained reservoirs may present increased leak hazards, as might reservoirs with thin caprocks, which are more prone to fracturing. Even so, higher risk sites might prove to be adequate reservoirs depending on fault behaviour (sealing vs leaking), the availability of secondary seals, and the increased proportion of residually trapped CO2 in horizontally unconstrained fields. Here, careful evaluation and post-injection monitoring is necessary, but risks can be mitigated against.
The prevailing method of site assessment is the bow-tie risk assessment, a non-specific statistical evaluation of failure events based on similar occurrences across the petroleum industry. Hazards typically include acid reaction, diffusion, injection stress, faults, lateral migration and well distributions, which together can be categorised into CO2 behaviour, subsurface geology and drilling history. Subsurface geology plays the most important role in site selection, and first-order site suitability can be distinguished using seal thickness, depth, and fault association alone. Simplistically, fields like Goldeneye, a domed structural gas field with a thick un-faulted seal, run lower risks than those like Sleipner, a laterally unconstrained stratigraphic saline aquifer which presented risks with CO2 migration. Given that structurally similar fields are expected to present similar risks, the division of existing GCS projects into high-to-low risk types will allow analogous fields to be easily found and risked for prospective GCS.
Analogues have an additional use in GCS site selection. Information on subsurface geology is widely available in the petroleum industry, though coverage varies regionally and on a case-by-case basis. However, due to the global breadth of data on petroleum systems, analogues can be used where information is sparse. Seal parametrics, which are less well constrained than those of the underlying reservoirs, can be estimated from depositional environments, a more regionally consistent parameter (see Gibson-Poole et al, 2009; Root, 2007). Lagoonal seals, for example, are thicker than deltaic seals by several orders, and so might be better suited for GCS projects. Similar assumptions can and have been linked to sedimentary basins worldwide to estimate CO2 storage capacities. In this way, analogues can allow parameters relevant to GCS to be backfilled for site selection, presenting a great economic advantage over systems where the same breadth of data is lacking.
As the number of GCS projects increases, these parameters will contribute to a framework for multiparameter benchmarking, allowing more GCS-specific site selection metrics to be formulated. Using depth constraints alone the bMark database contains 7815 candidate fields for deep carbon storage, with the large majority (72%) currently producing. Of these, 450 fields involve some form of gas injection, while 40 explicitly use CO2-EOR.
Carbon Capture and Storage (CCS) is a critical step in decarbonising global energy. It is a necessary component of 2-degree climate models, which require both a decrease in fossil fuel usage and large-scale CO2 removal from the atmosphere. Sustainable sources, while key to diversifying the global energy mix, are presently insufficient in bridging the growing energy demand gap, and hydrocarbon usage is still increasing by 2.5% each year11.
The petroleum industry contributes to 9% of global emissions from scope 1-2 alone, and a further 33% from scope 312, CCS is a necessary step in mitigating against climate change, and complying to the 2-degree climate models of the Paris 2015 agreement.
Carbon Capture and Storage (CCS) is a critical technology in decarbonising global energy.
In order to meet the 2-degree climate commitments of the Paris 2015 agreement, both a reduction in fossil fuel usage and large-scale carbon storage must occur. However, given a growing global energy demand, hydrocarbon usage is instead increasing by 2.5% each year, and sustainable alternatives have thus far been insufficient in bridging the energy gap. Given that the petroleum industry contributes to 9% of global emissions from scope 1-2 alone, and a further 33% from scope 3, it is a necessesity.
As earlier stated, Geological Carbon Sequestration (GCS) is a critical component in decarbonising global energy, though its feasibility is restricted to large-scale point sources where in-situ carbon capture and storage can occur. Depleted hydrocarbon fields are particularly attractive targets for this: the same geological formations that are effective at trapping oil and gas over million-year timescales are also capable of trapping CO2.