Key Reports:

State-of-the-Art of Monitoring Methods to evaluate Storage Site Performance

2.1.3 Geoelectrics and electromagnetics

One method for investigating the resistivity of the subsurface is to use geoelectrical measurements, which are based on the injection of electrical currents into the ground. Geoelectrical measurements can detect changes in fluids (i.e. fluid substitution). The depth of penetration of a surface array depends on the rock and fluid properties. In addition, data quality drops as electrodes are placed further apart.

Saline water or brine can have a very low electrical resistivity. For example, at the Ketzin injection site the brine resistivity is approximately 0.05 Ωm (Kiessling et al., 2010). The injection of CO2Carbon dioxide into a saline aquifer will increase the bulk resistivity of the reservoir. However, dissolved CO2Carbon dioxide has very little effect on brine resistivity. Hence, this method is mainly suitable for detecting free CO2Carbon dioxide.

Fig. 2-3: Layout of the combined surface and downhole geoelectrical measurements at Ketzin, Germany (<span data-ps-type= Kiessling et al., 2010). Cross-hole measurements provide high-resolution images in the area between the boreholes.">

Fig. 2-3: Layout of the coCOmbined surface and downhole geoelectrical measurements at Ketzin, Germany (Kiessling et al., 2010). Cross-hole measurements provide high-resolution images in the area between the boreholes. The combination of downhole sensors and 16 surface dipole sensors (dipole length of 150m) is used to extend the observation area. The surface dipoles are arranged on two concentric circles with radii of 800 and 1500m. The injection is carried out in borehole Ktzi 201.

In Electrical Resistivity Tomography (ERT) electrical measurements are made at the surface or by electrodes in one or more boreholes. This method can be used to obtain "snapshot" images of relatively static subsurface conditions for site screening or characterisation. It can also be used to obtain a series of images showing relatively rapid changes. ERT works well in both the vadose (unsaturated) and saturated subsurface zones. The extensive data resulting from measurements taken between the electrode arrays are processed to produce electrical resistivity tomographs using state-of-the-art inversion algorithms. These calculated tomographs show spatial variations in electrical resistivity and these images can be used as a guide for focusing more detailed characterisation and monitoring evaluations (Newmark et al., 2001). In connection with the CO2Carbon dioxide injection and storage experiment at Ketzin geoelectrical measurements have been carried out using a combination of permanently installed surface and borehole sensors (Fig. 2-3), also including sensors in the injection borehole (Kiessling et al., 2010).

At Ketzin the cross-hole measurements provide the highest resolution data and give a smoothed image of the plume (Fig. 2-4). A combination of surface and downhole sensors was also used at Ketzin to extend the investigated subsurface volume. The geoelectrical measurements at Ketzin indicate a maximum increase in the reservoir resistivity of around 200 % after injection.

The resistivity can also be investigated using electromagnetic (EM) methods. The magnetotelluric method images subsurface structures by mapping spatial resistivity variations using electrical currents (or telluric currents) created by natural variations in the Earth's magnetic field, e.g. by long-period electromagnetic waves from distant thunderstorms. The Earth's naturally varying electric and magnetic fields are measured over a wide range of frequencies (0.0001 to 10,000 Hz). Concurrent measurements of orthogonal components of the electric and magnetic fields permit the calculation of the impedance tensor, which is complex and frequency-dependent. Using this tensor, it is possible to gain insight into the resistivity structure of the surrounding material (Cantwell, 1960). The magnetotelluric sounding method was used successfully for the mapping of geothermal reservoirs starting in the early 1980s and became a standard application. In recent years, magnetotellurics has also become increasingly popular in oil and mineral exploration. Since it can probe the earth to depths of several tens of kilometres it may also be applied for monitoring CO2Carbon dioxide injection. For this, further developments are required. The resolution of magnetotelluric surveys are, however, limited by the diffusive nature of electromagnetic propagation in the earth.

Fig 2_4 Fig. 2-4: Resistivity difference images for the portion of the reservoir between the three boreholes at Ketzin (Kiessling et al., 2010). The CO2Carbon dioxide plume spreading away from the injection borehole (Ktzi 201) is clearly visible as a high-resistivity zone

There are also possibilities to use a modified version of magnetotellurics in which man-made signals are used, so-called controlled source electromagnetics or CSEM (Bourgeois and Girard, 2010). This method is an established tool in marine work but has so far not been used much on land. An experiment at the CO2Carbon dioxide injection site in Ketzin has showed promising results, although there are still issues with noise (e.g. from power lines) disturbing the measurements and with the practical fieldwork procedure (Streich et al., 2011). The transient electromagnetic method (TEM) is another potentially useful monitoring tool, provided that the electrical current loop used to generate the signal is large enough (e.g. Carcione et al., 2012; Dodds, 2005). Further tests are needed to fully establish the electromagnetic methods as monitoring tools for CO2Carbon dioxide injection.