Geologic CO₂ Storage
TTS v1.0 - Annex G
Geologic CO₂ Storage injects supercritical CO₂ into deep saline aquifers, depleted oil and gas reservoirs, or basaltic formations more than 800 metres below surface, where it is permanently trapped beneath an impermeable caprock through structural, residual, and dissolution trapping mechanisms. This annex serves a dual role in the Teravent system - it is both a standalone pathway for industrial point-source CO₂ and the underlying storage protocol referenced by Direct Air Capture, Bioenergy with CCS, Carbon Capture Utilisation & Storage, and Bio-oil Geological Storage.
Submit Storage Project View TTS v1.0 Annex G →How this pathway works
Geologic CO₂ Storage permanently disposes of supercritical CO₂ by injecting it into deep subsurface rock formations under an impermeable caprock seal. Unlike In-situ Mineralisation (Annex E), where CO₂ chemically converts to solid carbonate minerals, this pathway relies on physical trapping mechanisms: structural trapping beneath the caprock, residual trapping as isolated droplets within pore spaces, solubility trapping as CO₂ dissolves into formation brine, and - over multi-century timescales - a slower secondary mineral trapping process. Suitable formations include deep saline aquifers, depleted oil and gas reservoirs (repurposed after resource extraction ends), and basaltic rock bodies with sufficient porosity for supercritical-phase injection.
Under the Teravent Technology-Based Carbon Standard (TTS v1.0) Annex G, this pathway serves two distinct functions. As a standalone pathway, it credits the geologic storage of CO₂ captured from industrial point sources (issuing TTC-D Reduction credits, since the CO₂ source is typically fossil industrial flue gas). As a shared storage component, Annex G defines the common storage integrity, monitoring, and post-closure requirements referenced by Direct Air Capture (Annex A), Bioenergy with CCS (Annex B), Carbon Capture Utilisation & Storage (Annex C), and Bio-oil Geological Storage (Annex H) - each of which inherits its own credit type (TTC-R or TTC-D) from its host pathway rather than from Annex G itself.
Three methodology variants are approved for standalone registration under Annex G, differentiated by storage formation type. Each specifies its own site characterisation, injection monitoring, and reservoir-specific reversal risk profile.
TTS v1.0 - Annex G
This pathway is governed exclusively by the Teravent Technology-Based Carbon Standard (TTS v1.0). No external registry, standard, or methodology is referenced or incorporated. All requirements - additionality, quantification, durability, safeguards, and credit issuance - are defined within TTS v1.0 and Annex G specifically.
Annex G as the storage backbone
Rather than each capture pathway defining its own separate storage integrity rules, TTS v1.0 centralises geological storage requirements in Annex G. Four other annexes reference it directly for the storage stage of their own project boundary:
A project registering under any of these four host annexes must still meet all Annex G site characterisation, injection monitoring, storage confirmation, and post-closure requirements at its storage site - the difference is that credit issuance, credit type, and pathway coding follow the host annex, not Annex G.
Three approved methodology variants
TTS v1.0 Annex G approves three discrete methodology types for standalone Geologic CO₂ Storage registration, differentiated by the storage formation type. A project may combine multiple formation types only where each is independently metered under a separate methodology code.
Deep saline aquifers - porous sedimentary rock formations saturated with brine too saline for drinking water or agricultural use - represent the largest global geologic CO₂ storage capacity by volume. CO₂ is compressed to supercritical phase and injected through one or more wells, where it migrates into pore space beneath the caprock, displacing formation brine and gradually undergoing structural, residual, and solubility trapping. This is the most widely used standalone storage methodology and the default storage route for most DAC and BECCS facilities.
- Continuous CO₂ mass injection rate via calibrated wellhead metering at ±2% accuracy
- Formation pressure and temperature monitoring at injection and observation wells
- Periodic pressure falloff testing to confirm caprock integrity and plume containment
- Satellite InSAR ground-deformation monitoring for larger facilities
- Groundwater quality monitoring at shallow aquifers above the injection zone per DNSH provisions
Depleted oil and gas reservoirs offer a well-characterised alternative to greenfield saline aquifers - the caprock seal has already demonstrated multi-million-year hydrocarbon containment, and existing wells and seismic data can often be repurposed, lowering site characterisation costs and risk. Eligibility requires independent confirmation that the reservoir is fully depleted and that injected CO₂ will not be used for enhanced oil recovery.
- Continuous CO₂ mass injection rate via calibrated wellhead metering at ±2% accuracy
- Legacy wellbore integrity survey covering all penetrations of the storage complex, remediated prior to injection where needed
- Reservoir pressure monitoring relative to original virgin pressure and depletion history
- Chain-of-custody documentation confirming zero EOR use of injected CO₂
- Induced seismicity monitoring per Module 5 seismic risk protocol
This methodology covers CO₂ injection into basaltic formations where the project relies primarily on structural and residual trapping mechanisms - the same physical containment logic as saline aquifer or depleted reservoir storage - rather than pursuing dissolved-phase injection engineered specifically for rapid mineral conversion (which instead registers under In-situ Mineralisation, Annex E, INM-M01). Where a project can subsequently demonstrate confirmed mineralisation of a portion of the injected CO₂, that fraction may transition to Annex E crediting at a later verification.
- Continuous CO₂ mass injection rate via calibrated wellhead metering at ±2% accuracy
- Formation pressure, plume extent, and structural trap integrity monitoring
- Periodic geochemical sampling to assess mineralisation progress and potential transition to Annex E
- Induced seismicity monitoring per Module 5 seismic risk protocol
- Groundwater quality monitoring at any overlying potable aquifers per DNSH provisions
Which emission sources must be counted
TTS v1.0 Module 3 requires a full lifecycle GHG emissions inventory within the project boundary, deducted from gross injected CO₂ to arrive at the Net TTC figure. A source may be excluded only where its inclusion would reduce the net carbon benefit (conservative assumption) and this is documented in the PDD. For component-role registrations, upstream capture emissions are accounted under the host annex's own boundary, not duplicated here.
Measurement, reporting
& verification
Teravent's Science Advisory Board assesses each pathway against four MRV dimensions. Geologic CO₂ Storage benefits from decades of commercial CCS operating experience and a mature monitoring toolkit, giving it very high MRV confidence across all dimensions.
Quantification requires continuous CO₂ mass flow metering at the injection wellhead, accurate to ±2%, using instrumentation independently calibrated by an accredited VVB. Storage confirmation follows a mature, multi-decade CCS monitoring toolkit: pressure falloff testing to confirm caprock integrity, satellite InSAR ground-deformation monitoring for larger facilities, dedicated monitoring wells to track plume migration and pressure evolution, and periodic seismic surveys to confirm containment within the modelled storage complex. Site characterisation reports, including caprock integrity assessment and reservoir simulation, are required at registration and updated at each major verification milestone.
Demonstrating additionality
TTS v1.0 Module 2 requires all projects to pass a three-test additionality framework, with a Technology Readiness Level (TRL) screen applied ahead of the common practice test. Because geologic storage is a mature technology relative to other TTS pathways, standalone storage projects are more likely to require the full common practice survey rather than an automatic TRL-based pass.
Leakage types & deductions
TTS v1.0 Module 3 defines three leakage types applicable across all engineered pathways. Standalone Geologic CO₂ Storage projects are primarily subject to energy-source leakage and induced fossil extraction risk, the latter carrying particular weight given the pathway's overlap with depleted hydrocarbon reservoirs.
Buffer pool & reversal risk
All Annex G credits carry Class III Geological permanence (>1,000-year storage horizon). Buffer pool contributions protect credit buyers against reversal events - unintended CO₂ migration or seepage from the storage complex through the caprock, an unidentified fault, or a compromised wellbore. Buffer rates are set by the project's Storage Integrity Risk Rating (SIRR).
| Methodology | SIRR Rating | Buffer Pool Rate | Primary Reversal Risks |
|---|---|---|---|
| GCS-M01 Saline Aquifer | Low | 2–8% | Caprock breach; unidentified fault reactivation; wellbore integrity failure |
| GCS-M02 Depleted Reservoir | Very Low | 2–7% | Legacy wellbore integrity; proven caprock lowers structural risk relative to greenfield sites |
| GCS-M03 Basaltic Structural | Medium | 3–10% | Pre-mineralisation dissolved-phase migration; less-characterised trap geometry than sedimentary formations |
Key registration criteria
Projects must meet all of the following minimum requirements to qualify for registration under TTS Annex G - whether standalone or as the storage component of a host pathway. Additional methodology-specific requirements are detailed in the Annex G methodology tables for each GCS-M code.
Sustainable Development
Goal alignment
All Teravent registered Geologic CO₂ Storage projects must complete an SDG impact assessment at registration and at each verification period. Three SDGs are systematically tracked for this pathway. Projects may apply for co-benefit quality labels where independently verified indicators are met.
Deployment scope: Global - sedimentary basins with proven saline aquifers or depleted hydrocarbon reservoirs, and basaltic terrains, exist on every populated continent, making Annex G the most geographically flexible storage backbone in the Teravent system.
Ready to register your
storage project?
Submit a Project Concept Note under TTS v1.0 Annex G to begin your registration - whether standalone or as the storage component of a DAC, BECCS, CCUS, or Bio-oil Storage project. Select the GCS-M code matching your storage formation, install continuous wellhead metering to ±2% accuracy, and appoint an accredited VVB to validate your PDD and storage integrity plan.