Cross-well seismic and electromagnetic tomography for CO 2 detection and monitoring in a saline aquifer Gualtiero Böhm a , José M. Carcione a,n , Davide Gei a , Stefano Picotti a , Alberto Michelini b a Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42c, 34010 Sgonico, Trieste, Italy b Istituto Nazionale di Geofisica e Vulcanologia (INGV), Via di Vigna Murata, 605, 00143 Roma, Italy article info Article history: Received 23 December 2014 Received in revised form 6 May 2015 Accepted 11 June 2015 Available online 18 June 2015 Keywords: Seismic tomography Electromagnetic tomography CO 2 detection abstract Geological storage is one of the solutions to avoid the emission of carbon dioxide to the atmosphere. This process requires a careful monitoring of the CO 2 bubble, which can be performed by means of seismic and electromagnetic (EM) methods, on the basis of seismic velocity, attenuation and electrical con- ductivity contrasts before and after the injection. A successful monitoring depends on many factors, for instance the depth and properties of the reservoir. To test the feasibility of detecting the gas, we have performed cross-well seismic and EM tomographic inversions on a synthetic data set generated from a realistic aquifer partially saturated with CO 2 . We use two different algorithms based on traveltime picks. The method is novel regarding the EM inversion. Besides seismic velocity and conductivity, we have also obtained the seismic quality factor by performing attenuation tomography based on the frequency-shift approach. The RMS differences between the inverted and true initial models show that the methodology (and the adopted acquisition geometry) allows us to obtain reliable results which agree well with the true petrophysical model. Moreover, we have used a forward optimisation method to recover saturation, porosity and clay content from the tomographic seismic velocities, Q values and electric conductivity, with errors less than 15%. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Carbon dioxide is being injected worldwide in hydrocarbon reservoirs and saline aquifers as one of the solutions to the greenhouse effect (Arts et al., 2004). It is essential to monitor the diffusion and location of the CO 2 to predict and prevent any leakage to the atmosphere. To this purpose, the most used non- invasive techniques are seismic and electromagnetic surveys (e.g., Carcione et al., 2006; Norman et al., 2008; Bourgeois et al., 2009; Picotti et al., 2012). The process consists of periodically acquiring the data and performing inversions to obtain the seismic velocity and electrical conductivity and infer from these properties the CO 2 saturation by using appropriate rock-physics models. The com- bined use of the seismic and EM methods can give more reliable results if the interpretation is based on suitable cross-property relations between seismic velocity and conductivity (Carcione et al., 2007, 2012; Picotti et al., 2012). Existing wells may be used to perform cross-well repeated surveys and tomographic analysis of the recorded data, as successfully done by Saito et al. (2006) in the Nagaoka site and Carcione et al. (2012). Xue et al. (2009) used time-lapse well-logging data including gamma-ray log, neutron log, and induction log during CO 2 injection tests in the Nagaoka site. The EM method is a novel transient technique proposed in Carcione et al. (2012) and it is based on traveltime picks of the EM signal in the log(t) domain, where t is the time variable. In this domain, the pick of the maximum amplitude is possible since diffusion fields resemble waves. Equivalently, the pick (traveltime) can be obtained as the time that the first derivative of the field is zero (Yu and Edwards, 1997). An alternative picking method is given in Lee and Uchida (2005). To our knowledge, the only crosshole experiments somewhat related to this technique have been performed by Wilt et al. (1995). The method has not to be confused with electrical resistivity tomography (ERT) (e.g., Chris- tensen et al., 2006; Picotti et al., 2013). The integrated geological model constitutes a porous descrip- tion of the geological formation, where grain properties, fluid types, porosity, clay content and permeability are explicitly con- sidered, defining characteristic values of the electrical con- ductivity, seismic velocities and seismic quality factors, before and after the CO 2 injection. We then apply two different inversion al- gorithms to obtain the P-wave velocity and electrical conductivity (Michelini, 1995; Böhm et al., 2000) and seismic P-wave quality Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/petrol Journal of Petroleum Science and Engineering http://dx.doi.org/10.1016/j.petrol.2015.06.010 0920-4105/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. E-mail addresses: jcarcione@inogs.it (J.M. Carcione), alberto.michelini@ingv.it (A. Michelini). Journal of Petroleum Science and Engineering 133 (2015) 245–257