Active monitoring of the storage site in the short-term, using multidisciplinary skills is essential to ensuring secure CO₂ storage in the long-term.
A comprehensive range of monitoring techniques has been set up over the first five years to monitor CO₂ storage at In Salah, including geochemical, geophysical, 3D and 4D seismic and satellite technologies.
These monitoring techniques assess areas such as overall plume migration, well integrity, caprock integrity, surface movement and pressure development over time. The continuous monitoring programme provides information on how the CO₂ migrates in the formation.
Technology breakthroughs have been achieved using the seismic and satellite imagery so that the movement of CO₂ deep below the surface of the Sahara is now monitored continuously and non-invasively.
Satellite airborne radar interferometry (InSar) is used to detect and monitor subtle changes in surface deformation caused by CO₂ injection. The satellite technology used at In Salah is very new and the results have been some of the most valuable of the programme to date.
Seismic surveys acquire vast amounts of data that take many months of work to interpret. 3D-processing can be used to provide an image of the positions of the rock formations, while 4D-processing is used to detect differences in the signals caused by changes in fluids between one survey and the next. Initial seismic data and image logs confirmed minor faulting and fracturing in some of the strata and helped build understanding of the CO₂ plume behaviour. The latest seismic data was acquired for $7 million in May 2009 and processing of the data is under way.
See below for a description of the following monitoring technologies: Satellite imaging; Well monitoring; Microseismic monitoring; Seismic monitoring.
For a more detailed description of these technologies please click here to download a copy of The Leading Edge: CO₂ Sequestration Monitoring and Verification Technologies Applied at Krechba, Algeria
Understanding the ground from space
A successful and cost-effective CO₂ monitoring technique at In Salah is satellite imaging, specifically interferometric synthetic aperture radar (InSAR). This is a novel application of an established remote surveying method for identifying ground movement over days, months and years. The technology detects subtle ground-deformation changes by comparing phase differences from successive satellite passes. InSAR is used widely to monitor natural hazards such as volcanoes and tectonic plate movement, as well as in structural engineering, where it is used to monitor subsidence and stability of structures.
With satellite imaging, a satellite passes over a given position on the earth’s surface and emits a pulse of electromagnetic radiation. The strength and delay of the returning signal are recorded to produce ground images. Repeated passes over the same location enable any change in surface deformation or height change to be detected within two millimetres of accuracy – all achieved via a satellite orbiting many thousands of kilometres in space.
Other key monitoring techniques are implemented inside the CO₂ wells in the storage system to measure physically and directly the migration and movement of the CO₂ after it has been injected. These techniques are integrated into the well engineering and operational procedures.
One technique is the addition into the injection wells of tracers – stable, easily detected chemical substances added to the CO₂ stream to follow its movement in the formation. In the case of In Salah, perfluorocarbon tracers are used that can be detected at very low concentrations, with a different type of perfluorocarbon added to each well. This chemical differentiation allows CO₂ migration in the storage system to be tracked extremely precisely.
In addition, tests on the fluid at the top of each well, wellhead fluid sampling, and pressure and formation analysis, detect any rise in pressure indicating migration of the injected CO₂ from the reservoir back up into the wells. The seven active wells on site are monitored continuously in this way, while the seven discontinued wells – drilled by other operators before the current In Salah development and long since out of operation – are checked every few weeks and used for data collection.
Among the other monitoring methods being employed at Krechba is microseismic monitoring. Microseismic monitoring is a passive listening technique that entails drilling holes about 100m deep into the ground and then suspending geophones. These listening devices detect very small movements in rock structures as they ‘creak’ due to changes in temperature and pressure which may result from CO₂ movement deep down below.
The results of a recent soil gas monitoring survey are also currently being analysed. For the survey readings of CO₂ levels in the gases present in the soil about one metre below the surface were taken using manual probes at around 100 locations. The results will be compared with a baseline survey from 2004.
A 3D seismic survey of the northern part of the reservoir was carried out in 2009. The 3D seismic imaging process uses sound waves generated from near the surface of the earth and listens for the echoes reflected back from the layers of rock making up the geological formation of the reservoir. The reflected signals return to the surface where they are analysed and adjusted for distortions to create a detailed ‘picture’ of the subsurface. This is much how ultrasound is used to create pictures of unborn babies in their mother’s wombs or in MRIs, but on a far larger scale.
At In Salah, the 3D seismic process looks for density differences between CO₂ and water in the saline formation to detect the movement of CO₂ deep in the reservoir. It will also search for any possible impact on the overburden or subsurface layers over the reservoir. The data from this 3D seismic survey will be compared to a seismic baseline shot in 1997 so that a time-lapse comparison can be made between the two sets of data.
Several other monitoring techniques are also in progress or planned, including observation wells, pressure surveys, electric well-logs, and monitoring in the shallow aquifer.