CO2 underground storage

Why store CO2 ?

Carbon dioxide (CO2) produced by the combustion of fossil fuels (oil, gas, coal) is one of the most significant contributors to global warming.


One way to minimise the impact of CO2 on the climate is to isolate it from the atmosphere by reinjecting it into deep geological formations. The international community refers to this chain Carbon Capture and Storage (CCS). The Intergovernmental Panel on Climate Change (IPCC) presents CO2 capture, use and storage techniques as a necessity to keep the temperature rise below 1.5°C to 2°C.

CCS is carried out in three stages: the capture of CO2 at its point of emission (mainly from industrial source), its transport to its storage location, and its geological storage in suitable formations.

CO capture

Capture technologies aim at CO2 separation from the plan exhaust gas instead of releasing it to the atmosphere. They can be classified into three categories, according to the type of changes they induce in the emitting processes:

Without process modification

In these emitting processes, production is associated with  CO2 release (e.g. natural gas processing, hydrogen production, bioethanol production). Recovering CO2 for geological storage or any other industrial use thus requires no changes to the emitting process, an example being methane reforming plants in refineries.

Downstream changes to the CO2 emitting process

This technology is referred to as post-combustion capture. The CO2 is separated from the exhaust gas stream produced either by the combustion of fuels in air or by the process itself (for example, decarbonising in cement production or the reduction of iron oxides in the steel industry). The separation process downstream of the emitting process limits changes to the emitting process.

Upstream changes to the CO2 emitting process through the modification of fuels or oxidants before combustion

In pre-combustion capture, the initial carbon-rich fuel is transformed into a carbon-free synthesis gas. Decarbonising the inlet gas generates CO2, which is captured before its used, hence the term “pre-combustion”. The synthesis gas is then used in the industrial process that is being decarbonised. Hydrogen is the gas most often envisaged to play the role of decarbonised fuel.

In oxy-combustion capture, atmospheric air is replaced by pure oxygen, which avoids dilution of the CO2 formed during combustion by the nitrogen in the atmospheric air and facilitates its separation.

CO2 transport

Transport issues

Once separated and captured, the CO2 needs to be conditioned (purified, compressed) and transported to its geological storage location. Though it appears simple, transport can turn out to be complex as it depends on geographical and environmental, geopolitical and economic constraints that can vary substantially. Transport must be flexible to factor in variations in flow rates related to the CO2 source(s) and has to adapt to different geological storage options (deep saline aquifer, depleted hydrocarbon reservoirs). It also needs to account for industrial applications (including enhanced oil recovery) and all the combinations of types of storage or use.

Existing solutions

CO2 can be transported in a variety of ways – by pipeline, ship, truck or train – but for large volumes only the first two are suitable. Given the physical characteristics of the CO2 molecule, several solutions are implemented to increase the mass transported in a given volume, either by reducing temperature or increasing pressure to find conditions that increase the density of CO2.

Because of the large volumes generated by industrial process, CO2 is transported either compressed at room temperature or liquefied at low temperatures to increase its density. For pipeline transport, the CO2 is compressed to a dense form before being injected into the pipeline system.

CO2 in refrigerated liquid form is generally the preferred option for transport by ship. It can also be transported by pipeline, which pipelines need to be thermally insulated. But this generates a sizeable additional cost that is not necessarily offset by the savings made on compression.

For offshore transport, the CO2 can be liquid depending on the ambient sea temperature.

  • CO2 storage

CO2 storage

Geological context

The geological storage of CO2 is technically feasible in a wide variety of underground geological environments. These include depleted liquid or gaseous hydrocarbon reservoirs and deep saline aquifers, porous environments as found in sedimentary basins. Other environments may be considered, such as coal seams, either not used or unable to be used for coal extraction, and basaltic formations, research on which is being conducted to better understand their mechanisms and storage potential.

Geostock‘s expertise lies mainly in porous media such as hydrocarbon reservoirs and deep saline aquifers.

CO2 storage in porous media

The optimal injection strategy for a given CO2 storage project depends mainly on local geological considerations (the storage reservoir and its cap rock) and regional considerations (the storage complex as a whole).

The ability to achieve safe geological storage in a porous medium has been demonstrated by various projects in the USA, Norway, Algeria and Canada. These countries are implementing CCS projects that inject about one million tonnes of CO2 per year into different structures and at different depths.

In Europe, numerous projects are underway to store CO2 in geological formations under the North Sea in the Netherlands, Norway, the United Kingdom, Italy, Greece, and France.

An analogy can be drawn with underground natural gas storage, as both injection strategies are highly dependent on the case studied and require prior detailed studies to adapt to the specific geological conditions of the site.

Porous geological formations can provide a large storage capacity provided that:

  • the reservoir rock has been adequately characterised (thickness, porosity, permeability, geological nature);
  • the cap rock above the reservoir is continuous and reliable, ensuring the vertical containment of the stored CO2;
  • the storage reservoir has a minimum capacity and sufficient injectivity for the total volumes and flow rates of CO2 required;
  • site-specific objects or conditions such as faults, fractures, existing wells, mechanical stresses, vertical and horizontal stratigraphy and hydrodynamics do not have an adverse effect on CO2 containment.

For deep saline aquifers, the injection strategy needs to factor in the management of overpressure and displaced water volumes. This can be predicted and sized with the help of detailed site characterisation and large-scale 3D modelling, which are also necessary for storage safety management.

  • CO2 storage

For depleted hydrocarbon reservoir storage, the injection strategy can be based on practices established by the oil industry, which has used CO2 injection for many decades to improve reservoir productivity. Enhanced oil recovery mechanisms during CO2 storage may prove economically beneficial.

CO2 storage projects are large-scale ventures requiring investments in excess of €500M, and substantially more for offshore projects. These projects are established in phases:

  1. Studies and design to define the outline of the project and obtain the necessary authorisations for its implementation;
  2. Construction and development of the project;
  3. Commissioning, operation and closure of the storage facility.

Services offered by Geostock


Design and conceptual sizing

Geostock offers to its clients recommendations for CO2 capture and separation by studying the emitting process(es) and analysing the applicable technologies. The key sizing parameters to be considered concern the characteristics of the CO2 source and their variation over the lifetime of the facility, and, where applicable, the possibilities of clustering all or part of the capture between sources located on the same industrial site.


Design, development and operations

  • For onshore storage, the most economical and efficient solution is to transport the CO2 in dense phase, i.e. above critical pressure. For small volumes (for example, pilot sites), other options can be considered, such as truck, train and gas phase pipeline transport. Conceptual studies provide the preliminary estimates, routing and key operating parameters of a pipeline or pipeline system for transporting CO2 from the capture site to the geological storage site, taking into account the source-to-storage distance, CO2 flow rates, storage pressure and injection rate, the possibility of reusing existing pipelines (for example, used for natural gas), and the environmental constraints along the pipeline route. Geostock can provide the basic design of the equipment required for transport, other than pipelines, namely compressors, pumps and instrumentation. Energy needs are assessed.
  • For offshore storage (i.e. in geological layers under the seabed), CO2 can be transported by pipeline or ship. For sea transport, Geostock can recommend an optimal implementation scenario based on numerous factors such as captured CO2 flows, conditions for injection into geological storage and ship tonnage. The analysis can also be supplemented by the need for intermediate storage along the entire transport chain, often necessary due to the discontinuous nature of sea transport.
  • Geostock can also make recommendations on pipeline maintenance and operation (inspections through various techniques, maintenance or replacement work, installation and management of corrosion protection systems, management of pumping stations, etc.).

Design phase

  • Analysis of options and pre-selection of favourable sites
    In the case of deep saline aquifer storage, Geostock is developing a regional analysis of geological reservoir formations and their overlying formations, based on public data and Geostock‘s own. This geological summary provides an assessment of the theoretical CO2 storage capacity and is supplemented by a precise identification of the environmental and operational constraints likely to limit the storage capacity in practice.
    For storage in depleted hydrocarbon reservoirs (onshore or offshore), potential reservoirs are also assessed to estimate the theoretical storage capacity, once again based on published data or Geostock’s own data. This is done by factoring in the expected end-of-production date, the temperature and pressure of the reservoir at the time of CO2 injection, and any data on the reservoir geology (shape, porosity, permeability, fluids, salinity, etc., and structural features such as faults and fractures) and its production history.
  • Analysis and optimisation of source-storage coupling
    This type of study uses a geographic information system (GIS) to superimpose a large amount of geo-referenced information to match CO2 sources with the storage resources identified via an optimised transport network. The analysis can be developed based on the simplest systems (one source and one storage facility) to an integrated scheme on a regional or industrial basin scale in order to optimise resources and costs.
  • Preliminary risk analysis
    This type of service concerns the identification of key risk events (e.g. CO2 leakage from storage reservoir) and the definition of site-specific monitoring and inspection processes.
    For deep saline aquifer storage in which CO2 (in supercritical or dissolved brine) and the overpressure associated with injection diffuse in the reservoir, the corresponding events and risk factors are identified and studied using a geo-referenced GIS-type database. The creation of a project-specific risk register allows for a qualitative assessment of risks. For each risk identified, one or more control and prevention/correction action(s) are proposed (depending on the impact, probability of occurrence and consequences) for each phase of the project, injection, closure and post-closure.
    For depleted hydrocarbon reservoirs, in addition to the geological context, the analysis is particularly concerned with the pressure reached in the reservoir at the end of injection and the state of the existing wells (in operation or abandoned). Similarly to deep saline aquifer storage, a risk register is established, together with a monitoring and prevention/correction action plan.
  • Analysis of environmental factors
    The aim of CCS is to reduce greenhouse gas emissions to the atmosphere and improve environmental quality. However, the implementation of the chain requires additional energy and water, especially for CO2 capture. The net energy and environmental benefit is therefore reduced by quantities that need to be provided (energy balance, carbon balance are used to asses the benefits for the whole CCS chain) and which can vary greatly from one project to another depending on location and technologies used. Where possible, Geostock recommends clustering emissions, as it minimises the carbon and energy footprint of the chain, and the use of other resources, such as water. Projects combining CCS with enhanced hydrocarbon recovery call for a review of fossil energy (directly responsible for CO2 emissions).
  • Simulations
    Prediction of the long-term behaviour, containment of CO2 in the reservoir, based on reactive transport simulations.
  • Sizing of surface facilities
    Initial sizing and detailed engineering of injection and monitoring wells (geometry, architecture and completion) and surface equipments
  • Preliminary project development schedule
    Geostock offers the creation of an overall project development schedule with the aim of phasing of actions, notably ordering/purchasing materials, the critical path, and interactions between tasks and the overall cost allocation (a level 2 schedule is generally sufficient for an initial analysis).
  • Economic assessment (costs and finances)
  • Miscellaneous services
    As an expert in the development of geological storage projects, Geostock also offers services in drilling supervision, seismic acquisition, well log acquisition, and the construction, delivery and commissioning of injection facilities.
  • Administrative engineering
    Geostock provides administrative engineering throughout the project.

Construction phase and project development

  • Exploration, pre-injection characterisation programme, risk analysis
  • Monitoring and control programme (baseline and injection operations)
  • Safety management strategy (overall project environment), including the implementation of preventive and corrective actions
  • Definition of operating principles (CO2 injection and inspection)
  • Optimised development schedule

Commissioning phase, operation and closure

  • Operations schedule
  • Site monitoring (surface, soil, reservoir, well), well monitoring and maintenance
  • Injection monitoring
  • Design of closure and dismantling works
  • Design of long-term monitoring (post-closure)
  • Files for administrative authorisations
  • Performance analysis of closure works
  • Post-closure monitoring

Come tell us about your project

Geostock provides a complete range of services covering the entire life cycle of an underground storage project.

Contact us

Cavité minée – Mined Roeck Cavern
Control room of operated site
Shalapa site operated by Geostock