Borehole seismoelectric logging using a shear-wave source: Possible application to CO2 disposal?
Introduction
Injection of large amounts of man-produced CO2 in depleted oil wells below the sea floor and in other apropriate geological formations has been used, for several years, as a means of reducing the carbon dioxide emissisons into the atmosphere. For example, CO2 is being injected in the Sleipner field in the North Sea since 1996 at a rate of 0.85 Mt per year (Ellis, 2010), and also beneath the Sahara desert, at In Salah in Algeria (Ringrose et al., 2009). The former has been a subject of extensive theoretical and experimental studies, including laboratory rock sample analysis, seismic monitoring, etc. We mention, from the large literature concerning this deposition site, the studies of Chadwick et al., 2009, Chadwick et al., 2010 where time-lapse seismic is employed to characterize CO2 plume development, and the studies of Gomez and Ravazzoli (2011), where CO2 content related to seismic attributes were investigated. Moreover, a test site in Ketzin, Germany, is being run and extensively studied in order to monitor the CO2 behaviour during and after injection, see Martens et al., 2012, Martens et al., 2013 and references therein. Scientists from different areas have been studying this topic, and a still open problem is to predict the behaviour of the gas once set into the reservoir. Will it remain stable? Will it migrate, and make its way back to the surface? How the stored CO2 can be efficiently monitored in order to avoid pollution of overlying aquifers by leaked gas, among other issues (Thibeau and Mucha, 2011) is still a topic of intense research.
Among other works implemented at Ketzin, Wiese et al. (2010) studied the hydraulic properties of the storage reservoir, Kazemeini et al. (2010) carried out some rock physics and seismic modeling studies of surface seismic CO2 monitoring, and cross-well seismic tomography has been also performed (Zhang et al., 2012); more recently Fischer et al. (2013) made laboratory studies of geochemichal changes induced in Ketzin rock matrix samples by the presence of the stored carbon dioxide, and Wiese et al. (2013) studied – at the same site – not only the geochemical but also the hydraulic changes induced in the overburden by deposited CO2. We can also mention that both seismic and electric methods are potentially appropriate to study the CO2 reservoir (Fabriol et al., 2011, Girard et al., 2011, Carcione et al., 2012). Martens et al. (2012) describe not only the results of different campaigns including seismic, surface and borehole monitoring, but also some seismic simulation runs in order to check previous models; on the other hand synthetic and field geoelectrical methods were applied to study possible gas migration (Kiessling et al., 2010). Moreover Ishido et al. (2013) have numerically investigated the application of self potential methods to monitor the migration of CO2 sequestrated into saline aquifers, concluding that the used methods are effective for sensing the approach of CO2 to the well casings deep within the subsurface. We finally point out that in recent studies it was shown that seismics was useful to detect CO2 saturation below 15% and that electrical resistivity was useful to detect CO2 saturation above 15% (Kim et al., 2013).
Seismoelectric signals are electrokinetically generated by the propagation of seismic waves within a porous material. They can be recorded using a seismic source and electric receivers. The seismoelectric strategy aims to combine the resolution of the seismics to the sensitivity of the electric methods to fluid content. A specific seismoelectric signal, denoted the interfacial response, is expected to be induced at contrasts between rock properties (Garambois and Dietrich, 2002), including different fluids and different fluid-contents. This signal is usually weak compared to the so-called coseismic signal, which is the seismo-electric signal travelling within the seismic wave directly induced by the source. Several authors have investigated the benefits of surface-to-borehole seismoelectric layouts to accomplish efficient measurements of the interfacial response, as opposed to layouts for which both the seismic source and the receiving electrodes are laid at the surface.
The aim of this work is to provide numerical evidence that borehole seismoelectrics can discern carbon dioxide concentrations in a broader range than seismics allow, detecting at the same time salinity contrasts, task up to now fulfilled by geolectrics. The pure SH seismic source considered in the present study could achieve a better resolution than the one obtained through the usual P-driven experiments because of shorter wavelengths.
We start our work by reviewing the most important theoretical concepts of seismoelectrics, and by proposing a possible appropriate field experimental setup. We follow by analyzing shear-wave driven interface responses generated between to two consecutive units saturated with water, using a one dimensional finite element method to approximate the solution to Pride's equations. We study the sensitivity of these responses to contrasts in relevant parameters, such as porosity, salinity and viscosity; and continue by investigating the coseismic waves and interface response amplitudes of tabular media when one layer is partially saturated with carbon dioxide, employing in this analysis different models to take into account this situation in the electrokinetic coupling. Finally, we consider a layered model including a seal layer, in order to simulate a realistic CO2 deposition site.
Section snippets
Theoretical background
The seismoelectric method relies on electrokinetically induced seismic-to-electric energy conversions occurring in fluid-containing porous media. The reader can find a tutorial on electrokinetics in Jouniaux and Ishido (2012).
Appropriate field experimental setup
Although performing a field experiment is beyond the scope of this paper, we would like to emphasize what would be the most appropriate geometry to be developed to detect seismo-electromagnetic conversions for CO2 disposal monitoring. The interfacial response can provide information about the formations at depth while the co-seismic signal provides only information of the soil in the vicinity of the electrodes. The challenge is therefore to isolate the interfacial response, which is often of
Modeling seismoelectric and seismomagnetic signals measured at depth using a shear-wave source
In this section we use a numerical simulator, which features infinite shear sources generating 1D wave fields in likewise layered media for the modeling of the seismoelectric conversions; see the appendix for details in the 1D SHTE formulation. We model the seismoelectric and seismomagnetic conversions induced by a shear-wave source within a tabular model consisting of a sand layer over a sandstone layer. We then describe the results of the horizontal displacement, the horizontal electric
Sensitivity of the interface response to contrasts in fluid and rock properties
In this section we describe the amplitude of the interfacial reponse induced by a S-wave source when some physical properties of the sandstone half-space are changed whereas an upper sandstone layer is kept with constant parameters.The properties of the upper sandstone layer (Sandstone II) are given in the third column of Table 1.
Effect of a contrast between water-saturated sand and sandstone with various CO2 saturations
In this section we model a contrast between an upper water-saturated layer and a lower semispace with various concentrations of CO2 at supercritical conditions. We describe the results of the modelling coseismic magnetic field and the electric and magnetic interfacial responses induced by a shear-wave source.
Seismo-electromagnetic conversions induced in a CO2 reservoir with a seal layer
Let us consider now a new model, shown in Fig. 10, in which we intersperse a 10 m deep seal layer of very low permeability among a 100 m deep layer whose top boundary is the air–soil interface, and a semispace in which CO2 saturation can be changed. Indeed clay layers can be present as thin intra-reservoir shales. They act as main barriers to the upward migration of CO2 beneath which the the CO2 accumulates at high saturations (Arts et al., 2004).
The three layers parameters are displayed in
Conclusions
In this paper we numerically analyzed shear wave driven seismoelectromagnetic conversions in a surface-to-borehole layout, using a one dimensional finite elements code. Sensitivity analysis of the S-EM IR for porosity, permeability, zeta potential and viscosity were performed for a simple tabular medium, and normalized responses were used in these analysis, in order to make them independent of the physical source used by the employed method.
It was observed that -as expected- no contrast in the
Acknowledgements
This work was partially supported by a CNRS (INSU) – CONICET International Collaboration Grant, and by Université de Strasbourg.
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