9 - Monitoring

Samleatlas-CO2
13.11.2014

Monitoring of injected CO2 in a storage site is important for two main reasons: Firstly, to see that the CO2 is contained in the reservoir according to plans and predictions, and secondly, that if there are deviations, to provide data which can be used to update the reserservoir models and support eventual mitigation measures.

A wide range of monitoring technologies have been used by oil and gas industry to track fluid movement in the subsurface. These techniques can easily be adapted to CO2 storage and monitor the behavior of CO2 subsurface. For example, repeated seismic surveying provides images of the subsurface, allowing the behavior of the stored CO2 to be mapped and predicted. Other techniques include pressure and temperature monitoring, down-hole and surface CO2 sensors and satellite imaging, as well as seabed monitoring. In this chapter we present some of the challenges related to CO2 storage and some of the available monitoring techniques.

 

Fig-9-1 - Download pdf

Fig-9-1

 

 


 

Seal considerations for CO2 storage
by prof. Per Aagaard, UiO

 

UiOThe main criteria for selecting a site for geological CO2 storage (IPCC report on Geological CO2) are adequate CO2 storage capacity and injectivity, safety and security of storage (i.e., minimization of leakage), and minimal environmental impact. A potential reservoir thus needs a seal or caprock above the reservoir, i.e. physical and/or hydrodynamic barriers that will confine the CO2 to the reservoir.

Typical rocks forming seals or caprocks offshore in Norway, are sediments like mudstones, shales or fine-grained chalks. The pores are water-filled, while the reservoir beneath may have oil, gas or supercritical CO2. The seal should prevent the migration of these fluids into the fine-grained caprock. To form an efficient seal, the rock has to have a small pore throat radius, giving them a high capillary pressure. This prevents the migration of fluids like oil and gas or supercritical CO2 into the caprock, because the capillary pressure is greater than the buoyancy effect.

The capillary sealing is normally sufficient to prevent migration of fluid CO2 into caprock, and a diffusion of CO2 dissolved in the pore water of the caprock will also have very limited penetration in time scales of less than thousands of years. But we know from oil and gas reservoirs that caprocks may leak, and seepage of small gas volumes is commonly observed above the big oil and gas fields on the Norwegian shelf. This occurs either through small fractures or faults, which may open up under certain conditions. The seepage process is slow due to a combination of capillary pressures and low permeability in the caprock and the fracture systems. During injection, the caprocks can in particular be affected by: 1) the pressure rise in the storage formation induced by the injection process, and 2) geomechanical and geochemical processes that may affect the integrity and safety of the storage formation. In tectonically active areas, leakage can be induced by earthquakes. This is not an important risk in the North Sea, as recorded earthquake foci are deep-seated.

Fine-grained sediments undergo major changes after their initial deposition as mud. First they are compacted due to the weight of overlying sediments, and later, as the temperature increases with burial depth, chemical reactions also create cement between the sediment grains. Thus there is a transformation from ductile mudstones to more brittle shale or chalk, which mechanically is stronger, but more likely to fracture. Generally, thicker mudstone/shale formations will make better seals, but even rather thin, young sediments have been shown to be effective caprocks. The shallow Peon gas field has a less than 200m thick seal of Pleistocene mud. Several groups are active in research on geomechanics and rock physics of caprock research in Norway under petroleum research programs.

The CO2 will react with the caprock, and there is considerable concern as to how these processes may affect the seal integrity. In addition, well cement may also deteriorate under reaction with CO2. There is quite some dedicated research on CO2 - caprock interaction, both internationally and nationally. In Norway, several research projects are run both under the CLIMIT program (SSC-Ramore) and within the SUCCESS and BIGCCS Centres for Environment-friendly Energy Research (FME).

 

 


Monitoring of CO2 injection and the storage reservoir — by Ola Eiken, Statoil

 

Monitoring of CO2 injection as well as acquisition and interpretation of various kinds of well and reservoir data are important for control during the injection period and afterwards. Firstly, monitoring gives feedback to the injection process; it can lead to adjustment of rates, guide well intervention or decisions on new injection wells. In case of unwanted reservoir behaviour, monitoring data can lead to a number of mitigation measures. Furthermore, monitor data are needed to confirm storage reservoir behaviour and are crucial for operating CO2 quota systems. To obtain public acceptance of a storage site and wide recognition of CCS as a measure to prevent climate change, monitoring will play an important role. Also, predictions of a storage site’s long-term behaviour (over hundreds or thousands of years) should be calibrated against monitor data. Finally, public regulations, such as the EU directive 2009/31/EC, Article 13, on the geological storage of carbon dioxide, require monitoring of the storage reservoir.

Monitoring data can be acquired in the injection well(s), in observation wells and by surface measurements. Crucial measurements at the well head are rate, composition and pressure/temperature. Downhole pressure/temperature measurements are of further value, because sensors closer to the reservoir give more accurate responses of pressure build-up during injection and of fall-offs during shut-ins. These can be used to constrain reservoir models and to predict maximum injection rates and storage capacity. Observation wells can, if they penetrate the storage reservoir, give data on pressure build-up and CO2 breakthrough. This is done by installing various sensors, by logging the reservoir interval regularly and by fluid sampling. Regional pressure development within a basin is of particular importance in large-scale storage. A number of surface measurement techniques can be applied. 4-D seismic has proven most successful on the industry-scale offshore projects of Sleipner and Snøhvit, yielding the geometry of the CO2 plume with high resolution, while gravimetry has given complementary information on CO2 in-situ density and dissolution rates in the formation water. Onshore, surface elevation and microseismic data have given valuable information on injection and storage, and these techniques can be extended to offshore applications. Cost is an important aspect of a monitoring program, and subsurface and surface conditions that vary from site to site make a tailor-made plan necessary for each site. Equipment reliability and a system of documentation which works over a time-span of generations are also important for a monitoring program. With a proper monitoring program, a leakage out of the storage complex should be detected long before CO2 reaches the sea floor or the surface, so that mitigating measures can be implemented.

 

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Fig-9-2
Figure of the Sleipner CO2 injection 4-D seismic monitoring. Upper left: sketch of the injection well and storage reservoir. To the right is a seismic section along the long axis of the plume (south-west to north-east) for different vintages and for a time-lapse difference. Note the lack of reflectivity on the seismic difference above the storage formation, showing no signs of leakage. Lower left: Maps of the development through time of cumulative amplitudes for all layers. By 2008 the area of the CO2 plume was about 3 km2, and it was steadily growing.

 

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Fig-9-3
Figure from the Snøhvit CO2 injection. Left: Cumulative injection (black line) and estimated bottom-hole pressure (blue line) spanning year 2009, showing pressure increase during periods of injection and pressure fall-off during stops. The timing of a 4-D seismic survey is shown in the figure. Right: A 4D seismic difference amplitude map of the lowest Tubåen Fm. level, showing highest amplitudes close to the injection point, and with decaying amplitudes outwards from the well – falling below the noise level about 1 km away.

 


Seafloor monitoring of sub-seafloor CO2-storage sites — by prof. Rolf Birger Pedersen, UiB

 

A leakage of CO2 from a storage reservoir can result from a failure during injection or due to a migration of CO2 from the reservoir to the seafloor along unforeseen pathways for fluid flow. Whereas the first would be detected by instrumentation at the injection sites, monitoring of the seabed may reveal the latter.

The flow of fluids from the subsurface, across the seabed and into the water column has been studied extensively since the late nineteen seventies - when deep-sea hydrothermal venting was first discovered. Since then, the instrumentation and procedures to locate and monitor the flow of fluids (i.e. gases and liquids) from the seafloor has been developed during research investigations both at hot vents and cold seeps. Therefore, when strategies and procedures for monitoring sub-seafloor CO2 storage sites are being developed today, they are based on over four decades of basic research of natural seafloor fluid-flow systems.

Within the sediments below the seabed, chemical compounds like CO2 and CH4 form naturally through microbial activity and sediment diagenesis. There is a natural flux of these and other fluids across the seabed. These fluxes range from widespread and slow diffusion processes, to focused fluid flow at discrete seepage sites. Fluid flow at seepage sites results in distinct topographic, geochemical and biological signatures on the seafloor, as well as chemical and physical imprints in the water column above. Any change in these natural fluid-flow-patterns may indicate the first warning of leakage. Thus the flow of natural, reduced pore water at existing or new seepage sites is expected to be a distinct, initial sign of CO2 seepage from a subsurface reservoir.

Seafloor monitoring programs are now being designed to detect CO2 leakages and such early warnings. These schemes include: 1) scanning of the water column with acoustic systems to reveal any changes in the release of gas bubbles from the seafloor; 2) acoustic imaging of the seafloor at ultrahigh resolution to detect topographic changes that might reveal the formation of new fluid escape pathways; 3) imaging of bacterial mats and fauna at seepage sites to document environmental changes related to fluid-flow, and 4) chemical analyses of sea- and pore-water at natural seepage sites to monitor changes in the composition of the fluids emanating from the seafloor.

This monitoring requires advanced instrumentation that is either already available or currently under development. Hull-mounted multi-beam systems that scan the water column while simultaneously mapping the seafloor are now available. With a beam width of five times the water depth, these systems scan large areas in short time spans, detecting even small releases of gas bubbles from the seafloor. Autonomic underwater vehicles (AUV), which can dive for 24 hours and move at speeds of up to four knots at heights of just a few meters above the seafloor, can image the seafloor with side scan sonar systems at 10 cm scale resolution. At such resolutions, the appearance of new fluid flow pathways can be detected by small changes in the seafloor topography.

Where reduced subsurface fluids seep out, microorganisms will colonize the seafloor. They utilize the chemical energy in the fluids and form distinct, white bacterial mats that easily are detected by optical imaging of the seafloor using AUVs and ROVs as platforms for the camera. Today, thousands of images can be geo-referenced and assembled in large photo-mosaics. Repeated seafloor imaging of areas with evidence of fluid flow will be used to monitor the seabed fluid flow regime through the behaviour of microbial colonies and the seafloor biota.

AUVs and ROVs may also carry sensors that directly measure dissolved CO2 and CH4 in the water just above the seafloor. At present, these sensors lack the sensitivity as well as a rapid enough response time to be effective monitoring tools. Sensors with the needed capability are under development, and in a few years’ time they will be available for use in combination with acoustic and optical methods to monitor the state of the seabed fluid flow pattern.

Monitoring of the seafloor at regular intervals with these types of methods will not only be capable of detecting direct CO2 leakages, but also the subtle changes in the seabed fluid flow pattern that may represent early warnings. If the monitoring reveals anomalies relative to the baseline acquired before the CO2 injection starts, then special measures should be taken to investigate these areas in more detail. A range of geochemical, geophysical and biological methods is available to examine if the changes are related to leakage from the CO2-storage reservoir rather than natural variations.

 

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Fig-9-4
Detection of gas bubbles by echo sounder systems. The figure shows the acoustic signature generated by CO2 bubbles being naturally released from the Jan Mayen vent fields. The CO2 bubbles are here seen as a blue flare that rises around 500 metres from the seafloor through "clouds" of plankton in the water column.

 


Seafloor monitoring of sub-seafloor CO2-storage sites

 

At such anomalies, a necessary next step may be to place instrumentation on the seabed to obtain time series data. Called seafloor observatories, these instruments are capable of relaying sensor data and images to onshore laboratories via satellite links or fibre optic cable-connections. Seafloor observatories are at the cutting edge of today’s marine sciences. Presently, cable based seafloor observatories for basic research are being deployed at natural seabed fluid flow sites in the Pacific. As part of these and other research programs, a range of specialised instrumentation has been developed to monitor natural seabed fluid flow systems. These include: 1) acoustic systems to monitor the flux of gases into the water column; 2) mass spectrometers and chemical sensors to measure fluid components; 3) high-definition camera systems to monitor seafloor biota responses; and 4) broad-band seismometers for detecting cracking events related to subsurface fluid flow. Whereas most of these technologies may be directly transferable to the monitoring of CO2 storage sites, some may need further development and adaptation.

In conclusion, the know-how and technology developed partly by research on natural seabed fluid flow systems is currently available and can be transferred to the monitoring of CO2-storage sites. Monitoring schemes can therefore be designed and implemented to document the integrity of these sites, as well as providing early warnings of developing leakage situations from sub-seafloor storage sites.

 

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Fig-9-5
Detection of seafloor fluid flow structures using side-scan sonar imaging. The image shows a fracture system in the seabed where fluids are slowly seeping out from the subsurface. (Scale: 50 metres between red lines)

 

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Fig-9-6
Detection of seafloor fluid flow using biologic signatures. The photo mosaic shows white bacterial mats that form a distinct biologic signature of fluid flow across the seabed. (sea star for scale)



Centre gor Geobioligy

 

 




Wells

By: The Petroleum Safety Authority Norway

 

  • A potential CO2 storage location can be penetrated by a number of adjacent wells that represent potential leakage sources.
  • Adjacent wells are defined as wells that might be exposed to the injected CO2. These wells can be abandoned wells as well as production, injection and disposal wells.
  • Adjacent wells can have well integrity issues that might allow CO2 to leak into the surroundings.

 

There are challenges concerning the design of these adjacent wells, since they were not planned to withstand CO2. The carbon dioxide in water is called carbonic acid and it is very corrosive to materials such as cement and steel. This situation can over time cause damage to downhole tubulars and mechanical barrier elements and lead to degradation of well integrity.

The general concern regarding CO2 injection wells is the need of a common recognized industry practice related to design of CO2 injection wells. This includes qualification of well barrier elements and testing related to CO2 for medium to long term integrity and low temperatures. A CO2 resistant design includes considerations related to CO2 resistant cement, casing, tubing, packers and other exposed downhole and surface equipment.

A common industry practice is also needed concerning plug and abandonment of CO2 injection wells and adjacent wells.

 

  • Proposed ISO standard related to CO2 injection well design and operation.
  • DNV – ”Guideline for risk managment of existing wells at CO2 geological storage sites” (CO2WELLS)

 

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Fig-9-7

 

 

Contents - CO2 storage atlas


Main page >


Preface >


1. Introduction >


2. Petroleum activity on the NCS >


3. Methodology >


4. The Norwegian North Sea >


5. The Norwegian Sea >


6. The Barents Sea >


7. Summary. Storage capacities of the NCS >


8. Storage options with EOR >


9. Monitoring


References >


The figures in the CO2 Storage Atlas can be downloaded in pdf-format by clicking on the figure in the text. You will also find all the figures in pdf- and png-format here: Figures >>

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