Geoelectrical methods for monitoring geological CO2 storage: First results from cross-hole and surface–downhole measurements from the CO2SINK test site at Ketzin (Germany)
Introduction
The research project CO2SINK – CO2 Storage by Injection into a Natural Saline Aquifer at Ketzin – has been established as pilot project of CO2 storage close to the town of Ketzin (about 40 km West of Berlin), Germany. One of the main objectives is to test and to evaluate different monitoring methods for assessing CO2 migration within the reservoir during and after injection. Three boreholes, one injection well and two observation wells, have been drilled about 50 and 100 m apart, in perpendicular triangle geometry (Fig. 1) into an anticlinal structure to depths of about 800 m (Prevedel et al., 2008, Prevedel et al., 2009). The site is located in the Northeast German Basin, which contains a sedimentary succession of Permian to Quaternary sediments (Hoth et al., 1993). Halokinesis of Upper Permian salt and the development of salt pillows, walls and diapirs caused a deformation of the Mesozoic sequence (Trusheim, 1957). The Ketzin test site is situated in the eastern part of the Roskow–Ketzin double anticline, which was formed above an elongated salt pillow situated at a depth of 1500–2000 m. The overburden of the salt is comprised of geologic formations of the Germanic Triassic (Buntsandstein, Muschelkalk, and Keuper) and of the Lower Jurassic, Tertiary, and Quaternary (Förster et al., 2006). The CO2 is injected into the Triassic (Middle Keuper) Stuttgart Formation, which is situated at about 630–710 m depth.
A total of up to 75 tons of CO2 per day in supercritical state are injected at about 640 m depth into a saline aquifer within the Stuttgart Formation. The storage site and its complex geology offer a challenging task for the various monitoring methods deployed in the temporally and spatially resolved detection of CO2 in the reservoir (Giese et al., 2009).
In particular, continuously applied geophysical methods can help to ensure that impacts of CO2 storage on natural resources, such as groundwater and ecosystems, can be mitigated with shortest possible response time such that the local population will stay unaffected by the sub-surface CO2 injection (Benson and Cook, 2005). Established methods for sub-surface CO2 monitoring are time-lapse 3D seismic imaging (Arts et al., 2004, Chadwick et al., 2006), high resolution cross-well seismic imaging (Spetzler et al., 2008, Saito et al., 2006), time-lapse gravity (Nooner et al., 2007, Alnes et al., 2008), time-lapse well logging (Xue et al., 2006, Xue et al., 2009), passive seismic monitoring (White, 2009), and satellite imaging techniques (Vasco et al., 2008, Onuma and Ohkawa, 2009). The methods have been deployed in form of case studies with synthetic modelling as well as with real field data evaluation for the CO2 injection sites Sleipner, Weyburn, In Salah and Nagaoka.
Electrical resistivity tomography (ERT) has been evaluated for monitoring the resistivity changes caused by CO2 injection and migration in geological reservoirs (Christensen et al., 2006, Ramirez et al., 2003). The electrical resistivity of the rock–brine–CO2 system depends in good approximation mainly on CO2 saturation, permeability, and brine resistivity. The distribution of the resistivity in the reservoir is thus a potential CO2 indicator, independent of other geophysical parameters.
For the Ketzin test site, a vertical electrical resistivity array (VERA), as shown in Fig. 1, was developed and permanently deployed together with a distributed temperature sensing (DTS) cable as “smart-casing” technology for the CO2SINK wells (Prevedel et al., 2009). The well Ktzi201 was in addition equipped with a heating cable for active heating experiments. The VERA consists of 45 permanent ring-shaped steel electrodes (15 in each well), placed on electrically insulated steel casings in the depth range of 590–740 m with a spacing of about 10 m. In this section, the 5.5 in. steel casings are externally coated with a two-component material (Ryt-Wrap™) consisting of an epoxy matrix and a polyphenylene sulfide (PPS) membrane for electrical insulation in the open hole section (patent pending).
There have been logging campaigns in both the uncased and cased wells before the start of the CO2 injection. These data give insight into the general reservoir architecture and provide – together with core data – a detailed lithological and petrophysical characterization of the target zone (Norden et al., 2008). These data were used to define realistic and yet sufficiently simple boundary conditions for the modelling and for testing the reliability of the inversion results.
Several studies have demonstrated the successful use of surface DC geoelectric methods for structural imaging and for monitoring of sub-surface processes. In the study by Storz et al. (2000), DC surface measurements were deployed for large-scale structural investigations. Examples for the detection of 3D transport processes or monitoring magmatic caused CO2 uplift zones are the studies of Schütze and Flechsig (2002), Schütze et al. (2002) and Flechsig et al. (2008).
Therefore, 16 electrical surface dipoles were deployed additionally around the Ketzin wells to facilitate surface–surface, and surface–downhole measurements. They enlarge the lateral extension of the observed area which is assessed in comparison to pure cross-hole measurements (Fig. 2). This complex geoelectrical monitoring concept allows for interrogation of the target zone at different scales, from cm (logging) to km (surface–downhole ERT). This paper describes how the combined measurements can help to constrain the CO2 migration through the reservoir with increased confidence and discusses how anisotropy effects in the CO2 migration can potentially be detected in the enlarged hemispherical area, helping in deciphering directional inequalities in migration.
Section snippets
CO2 saturation estimates
In order to perform a feasibility study and to interpret the inverted resistivity distribution in the reservoir in terms of the local CO2 saturation we need to establish a general and yet sufficiently simple resistivity–saturation relation. An empirical equation was developed by Archie (1942) and the applicability was successfully tested in numerous field and laboratory studies. It describes the resistivity of a porous rock that is partly saturated with a conductive fluid by the porosity, the
Concept of resistivity field data acquisition
Our analysis scheme of the 3D cross-hole time-lapse data delivers tomograms of the resistivity distribution between the wells. The resistivity data provide information about the dimension of the CO2 plume and about the CO2 saturation independently of seismic methods. Base data sets have been measured prior to CO2 injection and monitoring data sets are registered while CO2 is being injected. 3D surface–downhole measurements are realized to detect effects of anisotropy in CO2 migration. The
Inversion results of ERT cross-hole data
To establish the baseline of the VERA experiment, we checked thoroughly the effects of the various setting parameter of the 3D-inversion algorithm (EarthImager 3D Manual, 2008). Our inversion strategy was developed to match the available information of the laboratory and logging results that both set limits for acceptable ERT data ranges and model boundary conditions. This approach largely constrains acceptable inversion results, even though we are comparing data with very different
Summary and outlook of the geoelectrical monitoring in Ketzin
Geoelectrical methods are applied for monitoring the geological CO2 storage into the natural saline aquifer at Ketzin. The feasibility of geoelectric cross-hole and surface–downhole measurements was investigated. ERT is not capable of imaging details of the CO2 saturation on length scales well below the electrode separation (10 m in Ketzin). It will rather give a smoothed image of the bulk CO2 distribution. Sensitivity and resolution of an array depend critically on several factors, such as
Acknowledgments
This ERT research work was EU-funded in part by CO2SINK project, and was also supported in part by COSMOS (CO2 Storage, Monitoring and Safety Technology) financed by the German Federal Ministry of Education and Research, and its R&D program “GEOTECHNOLOGIEN”. Special thanks are extended to Dr. William D. Daily from the Lawrence Livermore National Laboratory for his helpful comments and fruitful discussions in the phase of planning the VERA system.
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