An analytical study of seismoelectric signals produced by 1-D mesoscopic heterogeneities

The presence of mesoscopic heterogeneities in fluid-saturated porous rocks can produce measurable seismoelectric signals due to wave-induced fluid flow between regions of differing compressibility. The dependence of these signals on the petrophysical and structural characteristics of the probed rock mass remains largely unexplored. In this work, we derive an analytical solution to describe the seismoelectric response of a rock sample, containing a horizontal layer at its centre, that is subjected to an oscillatory compressibility test. We then adapt this general solution to compute the seismoelectric signature of a particular case related to a sample that is permeated by a horizontal fracture located at its centre. Analyses of the general and particular solutions are performed to study the impact of different petrophysical and structural parameters on the seismoelectric response. We find that the amplitude of the seismoelectric signal is directly proportional to the applied stress, to the Skempton coefficient contrast between the host rock and the layer, and to a weighted average of the effective excess charge of the two materials. Our results also demonstrate that the frequency at which the maximum electrical potential amplitude prevails does not depend on the applied stress or the Skempton coefficient contrast. In presence of strong permeability variations, this frequency is rather controlled by the permeability and thickness of the less permeable material. The results of this study thus indicate that seismoelectric measurements can potentially be used to estimate key mechanical and hydraulic rock properties of mesoscopic heterogeneities, such as compressibility, permeability and fracture complianc

Efectos mecánicos inducidos por marea en el acuífero Puelche

Fil: Cuello, Julián E.. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Guarracino, Luis. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Monachesi, Leonardo B.. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Kruse, Eduardo Emilio. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo; Argentin

Un método numérico para la obtención de relaciones constitutivas en rocas fracturadas

En este estudio se determinan las relaciones constitutivas para rocas fracturadas no saturadas mediante un procedimiento numérico que imita las técnicas de medición utilizadas en laboratorio. En forma computacional se generan muestras tridimensionales de roca con fracturas verticales y horizontales aleatorias. Cada fractura es conceptualizada como una placa tridimensional cuyas propiedades hidráulicas son descritas por el modelo de van Genuchten. El procedimiento se basa en la solución numérica de la ecuación de flujo no saturado estacionario en una malla regular de un millón de elementos utilizando un método híbrido mixto de elementos finitos. Los valores de saturación y conductividad hidráulica relativa se calculan numéricamente a partir de los resultados de la simulación para distintos valores de altura de presión. Las relaciones simuladas son ajustadas utilizando dos expresiones analíticas recientemente propuestas para rocas fracturadas no saturadas. Ejemplos numéricos muestran que ambos modelos pueden ajustar razonablemente las relaciones de saturación simuladas.In this study constitutive relations for unsaturated fractured rocks using a numerical procedure that mimics the laboratory technique of measurement are determined. Three-dimensional rock samples with random vertical and horizontal fractures are computationally constructed. Each fracture is conceptualized as a three-dimensional plate whose hydraulic properties are described by the van Genuchten model. The procedure is based on the numerical solution of the steady-state unsaturated flow equation in a three-dimensional cubic domain, on a regular mesh of one million elements, using a hybridized mixed finite element method. The saturation and hydraulic conductivity values are numerically computed from the simulation results, for seve-ral values of pressure head. The simulated relations are fitted using two recently proposed analytical expre-ssions for unsaturated fractured rocks. Numerical examples show that both models can match reasonably the simulated saturation relations.Fil: Monachesi, Leonardo B. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata (UNLP), La Plata; ArgentinaFil: Guarracino, Luis. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata (UNLP), La Plata; Argentin

Electromagnetic/Acoustic Coupling in Partially Saturated Porous Rocks: An Extension of Pride’s Theory

International audienceIn this paper a set of equations governing the electromagnetic/acoustic coupling in partially-saturated porous rocks in the low-frequency regime is derived. The equations are obtained by volume averaging of fundamental electromagnetic and mechanical equations valid at the porescale, following the same procedure as the one developed in the seminal paper of S. Pride for porous media where the fluid electrolyte fully saturates the pore space. In the present approach it is assumed that the porous rock is partially saturated with a wetting-fluid electrolyte (water) Electromagnetic/acoustic coupling in partially-saturated porous rocks and a non-wetting fluid (air). We also assume that an electromagnetic/mechanical coupling exists at the water-solid and water-air contact surfaces through adsorbed excess charges balanced by mobile ions in the water. The proposed approach is valid at the low-frequency regime, where capillary pressure perturbations can be safely neglected. The governing equations thus derived are similar to the ones obtained by Pride with the main difference that the various coefficients, including the electrokinetic coupling coefficient and electric conductivity appearing in the transport equations are functions of the water saturation and depend on electrical and topological properties of both electric double layers