The Seismoelectric Method (eBook)

Theory and Application

Andr& amp; eacute; Revil, Abderrahim Jardani, Paul Sava, Allan Haas (Autoren)

eBook Download: EPUB

2015 | 1. Auflage
264 Seiten
Wiley-Blackwell (Verlag)
9781118660300 (ISBN)

Infos

The seismoelectric method consists of measuring electromagnetic signals associated with the propagation of seismic waves or seismic sources in porous media. This method is useful in an increasing number of applications, for example to characterize aquifers, contaminant plumes or the vadose zone. This book provides the first full overview of the fundamental concepts of this method. It begins with a historical perspective, provides a full explanation of the fundamental mechanisms, laboratory investigations, and the formulation of the forward and inverse problems. It provides a recent extension of the theory to two-phase flow conditions, and a new approach called seismoelectric beamforming. It concludes with a chapter presenting a perspective on the method.
This book is a key reference for academic researchers in geophysics, environmental geosciences, geohydrology, environmental engineering and geotechnical engineering. It will also be valuable reading for graduate courses dealing with seismic wave propagation and related electromagnetic effects.

André Revil is Associate Professor at the Colorado School of Mines and Directeur de Recherche at the National Centre for Scientific Research (CNRS) in France. His research focuses on the development of new methods in petrophysics, and the development of electrical and electromagnetic geophysical methods applied to geothermal systems, water resources, and oil and gas reservoirs.
Abderrahim Jardani is Associate Professor at the University of Rouen, where he also obtained his PhD in Geophysics 2007. His research interests centre on environmental geophysics, mathematical modeling of hydrologic systems and inverse problems.
Paul Sava is an Associate Professor of Geophysics at Colorado School of Mines. He specializes in imaging and tomography using seismic and electromagnetic wavefields, stochastic imaging and inversion, computational methods for wave propagation, numeric optimization and high performance computing.
Allan Haas is currently working at hydroGEOPHYSICS, Inc. as a Senior Engineering Geophysicist.  He graduated with a PhD in Geophysics at the Colorado School of Mines, on December 13, 2013.  During his PhD research, Allan investigated the measurable electrical signals associated with leakages in wells, hydraulic fracturing, and subsurface fracture flow. 

Textauszug

CHAPTER 1
Introduction to the basic concepts

The goal of the first chapter is to introduce some of the key concepts required to understand the seismoelectric theory that will be developed for the saturated case in Chapter 2 and for the partially saturated and two-phase flow cases in Chapter 3. These key concepts include the electrical double layer theory and the reasons why an electrical (streaming) current density is produced when the pore water flows relative to the skeleton formed by the solid grains. In the context of the seismoelectric theory, the propagation of seismic waves will be responsible for the relative flow of pore water, and the resulting source current density will be responsible for electromagnetic (EM) disturbances. We will provide a short history of the seismoelectric method as well as its basic concepts. We will also give an introduction to wave propagation theory. At the end of this chapter, we will also provide some simulations using a simplified version of the seismoelectric theory that is based on the acoustic approximation. These models will illustrate, in a simple way, the key concepts behind the seismoelectric method, especially the difference between coseismic signals and seismoelectric conversions. Finally, we will present a preliminary model of seismoelectric phenomena pertaining to the Biot–Frenkel theory of linear poroelasticity.

1.1 The electrical double layer

As discussed later in Section 1.4, the existence of seismoelectric effects is closely related to the existence of the electrical double layer at the interface between the pore water and the skeleton (made of the elastic minerals). In the presence of several immiscible fluids in the pore space, seismoelectric effects can be also associated with the existence of an electrical double layer at the interface between the pore water and these other fluids such as air or oil. Therefore, we believe that it is important to start this book with an extensive description of what the electrical double layer is for silica and clay minerals that are in contact with an electrolyte composed of water molecules and ions. We will focus on silica and clays but the electrical double layer theory has been also developed for carbonates (Cicerone et al., 1992; Strand et al., 2006; Hiorth et al., 2010) and other types of aluminosilicates such as zeolites (van Bekkum et al., 2001).

The electrical double layer is a generic name given to electrochemical disturbances existing at the surface of minerals in contact with water containing dissolved ions. The electrical double layer comprises (1) the Stern layer of sorbed ions on the mineral surface (Stern, 1924) and (2) the diffuse layer of ions bound to the surface through the coulombic force associated with the deficiency or excess of electrical charges on the mineral surface and the Stern layer (Gouy, 1910; Chapman, 1913). The sorbed ions of the Stern layer possess a specific affinity for the mineral surface in addition to the coulombic interaction (specific is usually used to include all types of interactions that are not purely coulombic). In the case of the diffuse layer, the ions are interacting with the mineral surface only through the coulomb interaction.

The readers that are interested to understand the seismoelectric effect but that are not interested by the interfacial electrochemistry can skip Sections 1.1.1 and 1.1.2 and can go directly to Section 1.1.3 of this chapter.

1.1.1 The case of silica

1.1.1.1 A simplified approach

Figure 1.1 sketches the surface of a silica grain coated by an electrical double layer. When a mineral like silica is in contact with water, its surface becomes charged due to chemical reactions between the available surface bonding and the pore water as shown in Figure 1.2. For instance, the silanol groups, shown by the symbol >SiOH, of the surface of silica (where > refers to the mineral crystalline framework), behave as weak acid–base (amphoteric sites). This means that they can lose a proton when in contact with water to generate negative surface sites (>SiO−). They can also gain protons to become positive sites (>SiOH2+). Putting water in contact with a fresh silica surface leads to a slight acidification of the pore water, as shown in Figure 1.2, which explains why silica is considered to be an acidic rock. At the opposite end, a mineral like carbonate will generate a basic pH (>7.0) in the pore water.

Figure 1.1 Sketch of the electrical double layer at the pore water–mineral interface coating a spherical grain (modified from Revil & Florsch, 2010). The local conductivity σ(χ) depends on the local distance χ from the charged surface of the mineral. The pore water is characterized by a volumetric charge density corresponding to the (total) charge of the diffuse layer per unit pore volume (in coulombs (C) m−3). The Stern layer is responsible for the excess surface conductivity ∑S (in siemens, S) with respect to the conductivity of the pore water σf, while the diffuse layer is responsible for the excess surface conductivity ∑d. These surface conductivities are sometimes called specific surface conductance because of their dimension, but they are true surface conductivities. The Stern layer is comprised between the o-plane (mineral surface) and the d-plane, which is the inner plane of the electrical diffuse layer (OHP stands for outer Helmholtz plane). The diffuse layer extends from the d-plane into the pores. The element M+ stands for the metal cations (e.g., sodium, Na+), while A− stands for the anions (e.g., chloride, Cl−). In the present case (negatively charged mineral surface), M+ denotes the counterions, while A− denotes the coions. The fraction of charge contained in the Stern layer with respect to the total charge of the double layer is called the partition coefficient f.

Figure 1.2 Formation of the electrical double layer in the case of silica. In the present case, a neutral silica surface is brought in contact with a neutral pore water solution composed of cations M+ and anions A−. The silanol surface groups at the surface of silica release a certain number of protons in the pore water, making the solution slightly acidic. Some of the cations from the pore water are adsorbed in the Stern layer. The surface charge density and the Stern layer charge density are compensated in the diffuse layer. In a sandstone, the bulk pore water is neutral (no net charge density), and only the diffuse layer is not neutral and more precisely characterized usually by an excess of (positive) charges.

It follows that the mineral surface charge of silica appears to be pH dependent. It is typically negative at near-neutral pH values (pH 5–8) and possibly positive or neutral for very acidic conditions (pH <3). The simplest complexation reactions at the surface of silica can be summarized as (e.g., Wang & Revil, 2010, and references therein)

(1.1)

(1.2)

where are the two equilibrium constants associated with the surface sorption and desorption of protons. This 2-pK model considers that two charged surface species, namely, >SiO− and >SiOH2+, are responsible for the surface charge density of silica. That said, the reaction in Equation (1.1) is often neglected in a number of studies because the occurrence of the positive sites, >SiOH2+, can only happen at low pH values (typically below pH <3 as mentioned briefly previously).

We also assume that the pore water contains a completely dissociated monovalent salt (e.g., NaCl providing the same amount of cations Na+ and anions Cl−). In the following, a “counterion” is an ion that is characterized by a charge opposite to the charge of the mineral surface, while a “coion” has a charge of the same sign as the mineral surface. The typical case for silica is to have a negative surface charge, and therefore, the counterions are the Na+ cations and the coions are the Cl− anions. Note however that the sorption of cations is characterized by a high valence and a strong affinity for the silica surface (for instance, Al3+) and can reverse the charge of the mineral surface (surface and Stern later together) and therefore can reverse the sign of the charge of the diffuse layer. The sorption is described by the following reaction:

(1.3)

where KM corresponds to the equilibrium constant for this reaction. Sorption is distinct from precipitation, which involves the formation of covalent bonds with the mineral surface. This sorption can be strong (formation of an inner-sphere complexes with no mobility along the mineral surface) or weak. In the “weak case,” the formation of the Stern layer is a kind of condensation effect demonstrated by molecular dynamics. A weak sorption example is the case of a hydrated sodium. In this example, the sorbed counterion Na+ keeps its hydration sphere, and it forms a so-called outer-sphere complex with the mineral surface (e.g., Tadros & Lyklema, 1969). Such counterions are expected to keep some mobility along the mineral surface, responsible (as briefly explained in Section 1.3) for a low-frequency polarization of the mineral grains in an alternating electrical field. The layer of ions formed by the sorption of these...

Produkt-Details

• Erscheint lt. Verlag: 13.4.2015
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Geowissenschaften ► Geologie
• Themenwelt: Naturwissenschaften ► Geowissenschaften ► Geophysik
• Themenwelt: Naturwissenschaften ► Physik / Astronomie
• Themenwelt: Technik ► Umwelttechnik / Biotechnologie
• Schlagworte: Angewandte u. ökonomische Geologie • Angewandte u. ökonomische Geologie • Applications • Aquifers • Book • Concepts • consists • Contaminant • earth sciences • Economic & Applied Geology • Electromagnetic • Example • First • full • fundamental • Geophysics • Geophysik • Geowissenschaften • increasing • Method • Number • Overview • Porous • propagation • Seismic • seismoelectric • Signals • Sources • useful • Waves
• ISBN-13: 9781118660300 / 9781118660300