Abstract
Abstract. Hontomín (N of Spain) hosts the first Spanish CO2 storage pilot plant. The subsurface characterization of the site included the acquisition of a 3-D seismic reflection and a circumscribed 3-D magnetotelluric (MT) survey. This paper addresses the combination of the seismic and MT results, together with the available well-log data, in order to achieve a better characterization of the Hontomín subsurface. We compare the structural model obtained from the interpretation of the seismic data with the geoelectrical model resulting from the MT data. The models correlate well in the surroundings of the CO2 injection area with the major structural differences observed related to the presence of faults. The combination of the two methods allowed a more detailed characterization of the faults, defining their geometry, and fluid flow characteristics, which are key for the risk assessment of the storage site. Moreover, we use the well-log data of the existing wells to derive resistivity–velocity relationships for the subsurface and compute a 3-D velocity model of the site using the 3-D resistivity model as a reference. The derived velocity model is compared to both the predicted and logged velocity in the injection and monitoring wells, for an overall assessment of the computed resistivity–velocity relationships. The major differences observed are explained by the different resolution of the compared geophysical methods. Finally, the derived velocity model for the near surface is compared with the velocity model used for the static corrections in the seismic data. The results allowed extracting information about the characteristics of the shallow unconsolidated sediments, suggesting possible clay and water content variations. The good correlation of the velocity models derived from the resistivity–velocity relationships and the well-log data demonstrate the potential of the combination of the two methods for characterizing the subsurface, in terms of its physical properties (velocity, resistivity) and structural/reservoir characteristics. This work explores the compatibility of the seismic and magnetotelluric methods across scales highlighting the importance of joint interpretation in near surface and reservoir characterization.
Highlights
Geophysical characterization is the main tool to unravel the complex physical properties of the subsurface and is the central part of the characterization of geological reservoirs
Reflection seismics is a powerful geophysical tool for subsurface imaging and it is largely used to characterize the subsurface for resource exploration (e.g. Hart, 1999; Hesthammer et al, 2001)
The presence of the South and the East faults alter the geoelectrical behaviour of the different layers and produces the major differences observed between the models
Summary
Geophysical characterization is the main tool to unravel the complex physical properties of the subsurface and is the central part of the characterization of geological reservoirs. The selection of Hontomín as a pilot CO2 storage site motivated a multidisciplinary characterization of the area, aimed to unravel the main characteristics of the subsurface structures This includes geochemical (Elío et al, 2013; Nisi et al, 2013; Permanyer et al, 2013), geophysical (Rubio et al, 2011; Alcalde et al, 2013a, b, 2014; Ogaya et al, 2013, 2014; Ugalde et al, 2013; Vilamajó et al, 2013, 2015; Benjumea et al, 2016) and hydro/geomechanical (Canal et al, 2013; Martínez-Landa et al, 2013) studies. This work aims to integrate the seismic (Alcalde et al, 2013a, b; 2014) and MT (Ogaya et al, 2013, 2014) characterization results, together with the available log data, to expand our knowledge of the subsurface of Hontomín
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