Seismo-electrics, electro-seismics, and seismo-magnetics for earth sciences

International audienceThe seismo-electromagnetic method (SEM) is used for non-invasive subsurface exploration. It shows interesting results for detecting fluids such as water, ice, oil, gas, CO_2 , and also to better characterise the subsurface in terms of porosity, permeability, and fractures. However, a limitation of this method is the low level of the induced 5 signals. We first describe SEM's theoretical background, and the role of some key parameters. We then detail recent studies on SEM, through theoretical and numerical developments, and through field and laboratory observations, to show that this method can bring advantages compared to classical geophysical methods

Elimination of LWD (Logging-While-Drilling) Tool Modes Using Seismoelectric Data

Borehole acoustic logging-while-drilling (LWD) for formation evaluation has become an indispensable part of hydrocarbon reservoir assessment (Tang et al., 2002; Cittá et al., 2004; Esmersoy et al., 2005). However, the detection of acoustic formation arrivals1over tool mode contamination has been a challenging problem in acoustic LWD technology. This is because the tool mode contamination in LWD is more severe than in wireline tools in most geological environments (Tang et al., 2002; Huang, 2003). In this paper we propose a new method for separating tool waves from formation acoustic waves in acoustic LWD. This method is to measure the seismoelectric 2signal excited by the LWD acoustic waves. The acoustic waves propagating along the borehole or in the formation can induce electric fields. The generated electric field is localized around the wave pulses and carried along the borehole at the formation acoustic wave velocity. The LWD tool waves which propagate along the rigid tool rim can not excite any electric signal. This is due to the effectively grounding of the drill string during the LWD process makes it impossible to accumulate any excess charge at the conductive tool – borehole fluid interface. Therefore, there should be no contribution by the tool modes to the recorded seismoelectric signals. In this study, we designed the laboratory experiments to collect simulated LWD monopole and dipole acoustic and seismoelectric signals in a borehole in sandstone. By analyzing the acoustic and electric signals, we can observe the difference between them, which are the mainly tool modes and noise. Then we calculate the similarity of the two signals to pick out the common components of the acoustic and seismoelectric signals, which are the pure formation modes. Using the seismoelectric signals as reference, we could filter out the tool modes. The method works well. To theoretically understand the seismoelectric conversion in the LWD geometry, we also calculate the synthetic waveforms for the multipole LWD seismoelectric signals based on Pride’s theory (Pride, 1994). The synthetic waveforms for the electric field induced by the LWD-acoustic-wave along the borehole wall demonstrate the absence of the tool mode, which is consistent with the conclusions we get in the experimental study

A review on electrokinetically induced seismo-electrics, electro-seismics, and seismo-magnetics for Earth sciences

The seismo-electromagnetic method (SEM) can be used for non-invasive subsurface exploration. It shows interesting results for detecting fluids such as water, oil, gas, CO2, or ice, and also help to better characterise the subsurface in terms of porosity, permeability, and fractures. However, the challenge of this method is the low level of the induced signals. We first describe SEM's theoretical background, and the role of some key parameters. We then detail recent studies on SEM, through theoretical and numerical developments, and through field and laboratory observations, to show that this method can bring advantages compared to classical geophysical methods.Facultad de Ciencias Astronómicas y Geofísica

Modeling Of Seismoelectric Effects In A Borehole

We present a method to simulate the propagation of seismic and converted electromagnetic waves generated by a mechanical borehole source embedded in a layered poroelastic medium. The electroseismic conversions occur at both the borehole wall and the layer boundaries. Most studies in electroseismic effects have been modelled and tested with seismic sources and detectors (geophones and antennas) at the surface. In this paper, we investigate the case of a seismic source in a borehole and receivers either at the surface or embedded in the medium. The method is formulated as a boundary element technique (where the poroelastic displacement and relative flow Green's functions are calculated by the discrete wavenumber method. The singular properties of the Green's functions are determined analytically using static Green's functions to regularize the integrals. This is necessary to calculate the element's self interaction. The borehole is cylindrical and its axis rulls normal to the interfaces. The coupled electroseismic effects in the layered media are included by using the global matrix technique. The developed method is an extension of the model of Biot-Rosenbaum, who applied the wavenumber integration technique to investigate the effect of formation permeability on Stoneley waves, using Biot's theory to model the wave propagation effects of a homogeneous permeable formation surrounding a borehole. We extend the Biot-Rosenbaum model by including the effect of a heterogeneous permeable formation surrounding the borehole. The effect of formation permeable zones (or fractured zones) on Stoneley waves can now be investigated. The other modification is the inclusion of conversions of mechanical into electromagnetic waves at mechanical and/or electrical contrasts in the poroelastic formation. The converted electromagnetic fields are sensitive to large permeability contrasts and fluid chemistry contrasts inside a reservoir. Using the electroseismic method downhole will provide more information about permeability/permeability contrasts in the formation, as well as additional lithological information (salinity of the fluids)

Using Borehole Electroseismic Measurements To Detect And Characterize Fractured (Permeable) Zones

We present a new type of field measurement capable of detecting and characterizing fractured (permeable) zones intersecting a borehole. The method is based on measuring electrical fields generated by a borehole Stoneley wave. In this paper, we describe the measurement technique, present field data, and propose a theoretical model, which correctly predicts amplitudes and phases of the electrical fields measured in the borehole experiment. The theoretical model and the field data demonstrate that the measurements of the Stoneley-wave-induced electrical fields can yield information about the interconnected porosity, and possibly about the permeability of the formation around the borehole. We derive an estimate of the interconnected porosity from the field data, and show that it correlates well with the density of fractures intersecting the borehole. Our results suggest that the borehole electroseismic method can be developed into a logging or a VSP tool, with possible applications in reservoir characterization.Massachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu

Seismo-electrics, electro-seismics, and seismo-magnetics for earth sciences

The seismo-electromagnetic method (SEM) is used for non-invasive subsurface exploration. It shows interesting results for detecting fluids such as water, ice, oil, gas, CO2, and also to better characterise the subsurface in terms of porosity, permeability, and fractures. However, a limitation of this method is the low level of the induced signals. We first describe SEM's theoretical background, and the role of some key parameters. We then detail recent studies on SEM, through theoretical and numerical developments, and through field and laboratory observations, to show that this method can bring advantages compared to classical geophysical methods.Este artículo de discusión ha sido evaluado y publicado en Solid Earth 7(1). Se puede acceder al mismo desde el enlace "Documento relacionado".Facultad de Ciencias Astronómicas y Geofísica

Frequency-dependent streaming potentials: a review

The interpretation of seismoelectric observations involves the dynamic electrokinetic coupling, which is related to the streaming potential coefficient. We describe the different models of the frequency-dependent streaming potential, mainly the Packard's and the Pride's model. We compare the transition frequency separating low-frequency viscous flow and high-frequency inertial flow, for dynamic permeability and dynamic streaming potential. We show that the transition frequency, on a various collection of samples for which both formation factor and permeability are measured, is predicted to depend on the permeability as inversely proportional to the permeability. We review the experimental setups built to be able to perform dynamic measurements. And we present some measurements and calculations of the dynamic streaming potential

Borehole seismoelectric logging using a shear-wave source: Possible application to CO2 disposal?

International audienceThe behaviour of CO2 deposition sites-and their surroundings-during and after carbon dioxide injection has been matter of study for several years, and several geophysical prospection techniques like surface and crosshole seismics, geoelectrics, controlled source electromagnetics among others, have been applied to characterize the behaviour of the gas in the reservoirs. Until now, Seismolectromagnetic wave conversions occuring in poroelastic media via electrokinetic coupling have not been tested for this purpose. In this work, by means of numerical experiments using Pride's equations-extended to deal with partial saturations-we show that the seismoelectric and seismomagnetic interface responses (IR) generated at boundaries of a layer containing carbon dioxide are sensitive to its CO2 content. Further, modeling shear wave sources in surface to borehole seismoelectric layouts and employing two different models for the saturation dependence of the electrokinetic coefficient, we observe that the IR are sensitive to CO2 saturations ranging between 10% and 90%, and that the CO2 saturation at which the IR maxima are reached depends on the aforementioned models. Moreover, the IR are still sensitive to different CO2 saturations for a sealed CO2 reservoir covered by a clay layer. These results, which should be complemented by the analysis of the IR absolute amplitude, could lead, once confirmed on the field, to a new monitoring tool complementing existing ones

Borehole Acoustics and Logging and Reservoir Delineation Consortia

Research activity in the Borehole Acoustics and Logging/Reservoir Delineation Consortia continue to focus on the development of geophysical methods to detect and characterize geological conditions which control fluid flow in a reservoir. This report presents a summary of our results from the past year. Three major areas of research are presented: subsurface fracture characterization, modeling and imaging of complex structures, and reservoir logging applications

Seismoelectric wave propagation numerical modelling in partially saturated materials

International audienceTo better understand and interpret seismoelectric measurements acquired over vadose environments, both the existing theory and the wave propagation modelling programmes, available for saturated materials, should be extended to partial saturation conditions. We propose here an extension of Pride's equations aiming to take into account partially saturated materials, in the case of a water-air mixture. This new set of equations was incorporated into an existing seismoelectric wave propagation modelling code, originally designed for stratified saturated media. This extension concerns both the mechanical part, using a generalization of the Biot-Gassmann theory, and the electromagnetic part, for which dielectric permittivity and electrical conductivity were expressed against water saturation. The dynamic seismoelectric coupling was written as a function of the streaming potential coefficient, which depends on saturation, using four different relations derived from recent laboratory or theoretical studies. In a second part, this extended programme was used to synthesize the seismoelectric response for a layered medium consisting of a partially saturated sand overburden on top of a saturated sandstone half-space. Subsequent analysis of the modelled amplitudes suggests that the typically very weak interface response (IR) may be best recovered when the shallow layer exhibits low saturation. We also use our programme to compute the seismoelectric response of a capillary fringe between a vadose sand overburden and a saturated sand half-space. Our first modelling results suggest that the study of the seismoelectric IR may help to detect a sharp saturation contrast better than a smooth saturation transition. In our example, a saturation contrast of 50 per cent between a fully saturated sand half-space and a partially saturated shallow sand layer yields a stronger IR than a stepwise decrease in saturation