Geoelectrics

Electrical Resistivity Tomography (ERT)

Source: William Daily et al.: Electrical resisivity tomography. The Leading Edge, p. 438, May 2004

4-electrode measurements:

•  current injected between 2 electrodes

•  voltage measured between 2 electrodes

real resistivity distribution through inversion

resistivity depends on fluid saturation

applicable to CO₂ storage monitoring due to high resistivity contrast between brine and CO₂

ERT complements seismic methods:

•  high temporal resolution

•  high sensitivity at high CO₂ saturation

Vertical Electrical Resistivity Array (VERA) at Ketzin CO₂ Storage Site

3 boreholes, equipped with 15 permanent electrodes each

borehole separation 50 m / 100 m / 112 m

electrode spacing d_electrode = 10 m

spatial resolution:

0.5 d_electrode = 5 m near borehole

1.0 d_electrode = 10 m away from boreholes

VERA: Synthetic Data Example

reservoir model: 1 Wm aquifer, 3 Wm caprock, 10 Wm CO₂ plume (70 % CO₂ saturation), 90 m x 140 m x 20 m, 1 Wm host medium
forward simulation: ABMN and AMBN electrode configurations, finite difference, 2 % Gaussian noise
inversion: robust inversion, homogeneous start model (i. e. no a-priori information)
results: good lateral and vertical definition of plume near boreholes, good reproduction of model resistivities, poor sensitivity in central part of the model
coming soon: use of a-priori information for inversion (resistivity logging, seismics)

Resistivity and CO₂ Saturation

Archie equation r_rock = A r_brinef^-ms_brine^-n
aquifer resistivity 0.5 Wm to 1.5 Wm (logging)
brine resistivity 0.03 Wm (fluid sampling)
typical sandstone m = 2.0, n = 2.0, f = 0.20
resistivity ratio r_rock / r_aquifer = s_brine^-n = (1-s_CO2)^-2

VERA: Tentative Schedule

- development of concept for ERT instrumentation - 03/2006
- order ERT equipment - 04/2006
- report on complete permanent downhole monitoring system - 09/2006
- all systems tested and ready for installation - 12/2006
- 1st well completed - 05/2007
- systems deployed in 1st well - 06/2007
- start of CO₂ downhole baseline measurements - 10/2007
- start of CO₂ injection - 04/2008
- 1st monitoring report (installation, test, baseline, first results) - 07/2008

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