The stored carbon dioxide can be monitored using an array of direct and remote-sensing technologies on or above the surface of the Earth and in the borehole. The technologies record properties such as pressure, temperature, electrical resistivity and sound responses in injection and observation wells. Other monitoring involves seismic, microseismic and petrophysical well logs and geophysical sampling to track movement of carbon dioxide under the ground before, during and after injection.
Baseline surveys of the distribution, type and origin of any existing carbon dioxide in a potential storage site is carried out through soil-gas sampling and other analyses. Geochemical sampling at the surface allows rapid detection of any seepage or leakage, in the unlikely event that this occurs.
Seismic is a method of exploring the underlying strata of the earth, based on how sound waves are reflected by fluid in the subsurface. It uses variation in structure, density and velocity to build a picture of the subsurface.
Seismic techniques can be used to characterise the storage site and to monitor the CO2 during and after injection. The technique uses generated long-wavelength sound waves which travel through the earth. These waves, called seismic waves, are reflected by changes in the rock and then recorded as they return to the detectors.
The seismic waves are generated by a variety of means. In off-shore surveys, they are often generated by airguns. On land they can be generated by small dynamite explosions just under the surface, or, more commonly in modern surveys, by vibration. The vibrations are generated by a special plate that is mounted under a large truck. The plate is pressed against the ground at regular intervals and vibrated, generating sound waves that travel through the subsurface and are reflected back by the underground formations, such as a boundary between layers in the earth’s subsurface. Smaller units called mini-vibroseis may be used where the area to be imaged is at shallower depths. Less commonly, the seismic waves may be generated by dropping a heavy weight onto the ground, but this method tends to be restricted to research applications.
For surface data collection, the sound waves are detected by sensors on the surface. In off-shore seismic surveys they are known as hydrophones and on land they are called geophones. This data is analysed using complex computer software to form a picture of the subsurface. In addition to detecting the major boundaries between different types of rocks and outlining the subsurface structures, features that may be detected include faults and fractures, gas-water contact, and indications of possible CO2 migration paths.
Surface seismic data can produce 2D, 3D and 4D images of the subsurface depending on the method of collecting the seismic images. If the source and receiver are moved along in a straight line, a 2D image is obtained. If a two dimensional arrangement on the surface of source and receivers is used, a 3D image is produced. Creating 3D images at different times gives a 4D image, with time as the fourth dimension. This is also called time-lapse 3D seismic imaging.
Vertical seismic profiling
Vertical seismic profiling collects data from waves generated at the surface and collected via geophone located in a well. The geophone records the energy of the downward travelling wave and the energy of the reflected wave on its way back to the surface. By varying the relative positions of the source and the receiver, different geological images can be produced, all of which are high resolution pictures of the subsurface close to the borehole. Again, the array of the sources determines if the image is 2D or 3D.
In addition to using seismic sources, measurements are taken of the natural, low level seismic activity of the earth to detect any subsurface stress. These are called microseismic surveys and can indicate small changes in the subsurface stress field brought about by man-made activity such as the extraction of oil and gas or the injection of carbon dioxide.