Electrokinetic conversion

In their search for improved and new exploration tools, geophysicists have improved seismic and electromagnetic techniques. Since the 1930s it is known that there is a coupling between seismic and electromagnetic waves in the shallow subsurface of the earth. Electroseismic surveying and its reciprocal process, seismic-to-electromagnetic conversion, are methods for remotely identifying the presence of hydrocarbons in the subsurface of the earth. In this study we investigate electrokinetic coupling theoretically and experimentally. The origin of this effect lies in a very thin nano-layer which is conventionally present at all solid-fluid interfaces where an excess charge density with respect to the bulk charge density in the pore fluid exists. Any hydraulic disturbances of this nano-layer cause electric currents that are opposed by ionic counterflows generating electric fields. These effects become manifest when acoustic waves impinge upon the interface between two adjacent porous layers having different electro-mechanical properties. The theoretical basis for the coupling phenomena under investigation is conceptually imbedded in the framework of the combined Biot-Maxwell equations. An important aspect of the theory is the so-called dynamic (i.e., frequency-dependent) coupling factor. This coupling factor is studied in a dedicated experimental set-up where an oscillating flow through a porous material generates electric fields. We address the frequency-dependency of the coupling coefficient, the mathematical description and experimental detection results. We validate that the governing model is capable of modeling the coupling effect.Geoscience & EngineeringCivil Engineering and Geoscience

Minimum-phase Property of Memory Functions in the Wave Equation

Memory functions occur in the wave equation as time-convolution operators and generally account for the instantaneous and non-instantaneous responses of a medium. The specific memory function that is causal and stable, and the inverse of which is also causal and stable, is conventionally referred to as minimum phase. In this paper we present "extended minimum-phase relations" between the amplitude and phase spectra of a memory function that has different properties. The considered memory function and its inverse are both causal, but they do not need to be stable. We still address the function as minimum phase because the phase spectrum exhibits minimum group delay, like a conventional minimum-phase function. We have successfully tested the derived relations for the well-known Maxwell and Kelvin-Voigt models. The relations have potential applications in acoustics, seismology, poroelasticity, electromagnetics, electrokinetics and any other effective-medium theory that employs memory functions.Geoscience & EngineeringCivil Engineering and Geoscience

Study of shale wettability for CO2 storage

For a water-saturated cap-rock, which consists of a low-permeability porous material, the wettability of the reservoir rock-connate water- CO2 system and the interfacial tension (IFT) between CO2 and connate water are the significant parameters for the evaluation of the capillary sealing. Also, the amount of capillary-trapped CO2 depends on the wettability of reservoir rocks. The wettability of the rock matrix has a strong effect on the distribution of phases within the pore space and thus on the entire displacement mechanism and storage capacity. In this work, the equilibrium contact angles of water/shale system were determined with CO2 for a wide range of pressures at a constant temperature of 318 K by using the dynamic captive bubble method. The results reveal that intermediate-wet conditions and hence possible leakage of CO2 must to be considered at relatively high pressures, however, the salt concentration of the water in the shales plays an important role too. The results show that this estimate is highly dependent on the pore structure, fluid composition and pressure/temperature conditions of the reservoirs. These properties need to be first evaluated before estimates for shale capillarity is used.Geoscience & EngineeringCivil Engineering and Geoscience

Electrokinetic fields and waves: Theory, experiments and numerical modeling

Geoscience & EngineeringCivil Engineering and Geoscience