2.1 CO2 plume migration in the storage reservoir

Subsurface monitoring techniques play a vital role in identifying CO2 plume location, pressure propagation, and reservoir and seal integrity. These techniques can detect CO2 and compare observations with the predicted fate and transport results from modelling efforts. Many techniques can be imported from oil and gas exploration and reservoir management disciplines. A variety of techniques is also available to assess the condition of the well and ensure that the well itself does not provide a leakage pathway for CO2 migration.

However, no techniques are available to measure the CO2 in situ with precision. Therefore, it is not possible to directly quantify CO2 in the injection zone. Hence, it is necessary to use indirect or inferential methods to document that the storage site is performing as expected and that CO2 and brine are not escaping the storage reservoir in unacceptable directions and at unacceptable rates.

For geological storage, CO2 is injected at depths of ³ 800 m so that it will be present as a supercritical fluid under typical temperature and pressure conditions prevailing at these depths. Since compressibility and density of supercritical CO2 are smaller in comparison to those of saline formation water, the pore space in a saline aquifer will be filled with a less compressible and less dense fluid after substituting formation water by injected CO2. This contrast in properties is useful for different geophysical monitoring techniques. The situation is more complicated in depleted hydrocarbon reservoirs due to the large variations in the physical properties of oil, and since CO2 will modify the physicochemical properties of the oil in short time scales.

A recent overview of the different geophysical monitoring techniques can be found in Sayers and Wilson (2010). Estimates of CO2 detection limits for some of the most commonly used geophysical methods are given by JafarGandomi and Curtis (2011). Tab. 2-1 gives a summary of the most common monitoring techniques to monitor CO2 injection and follow the migration of the CO2 plume.

Tab. 2-1. Geophysical methods commonly used for monitoring CO2 injection and tracking CO2 plume migration.

Measurement method

Physical parameter(s)/

General characteristics in terms of tracking CO2 plume

Seismic

Seismic velocities, density

High spatial resolution

Geoelectrical

Electrical resistivity

Intermediate spatial resolution

Electromagnetic

Electrical resistivity

Intermediate spatial resolution

Gravity

Density

Low spatial resolution, although an advantage is that the response is linear

 

in depth

2.1.1 Seismic reflection

In seismic measurements surface sources (e.g. dynamite, vibrating machines or air gun arrays for onshore and offshore us...

2.1.2 Gravity

Gravity measurements are most useful for monitoring CO2 injection in saline aquifers. The CO2 will push away the brine a...

2.1.3 Geoelectrics and electromagnetics

One method for investigating the resistivity of the subsurface is to use geoelectrical measurements, which are based on ...

2.1.4 Well Logging / Wireline Logging

One of the most common methods for evaluating geological formations is the use of well logs. Logs are conducted by lower...

2.1.5 Satellite interferometry and other techniques for surface movement detection

Monitoring of surface uplift can be used as an indirect method for mapping of the area affected by CO2 storage, which ca...