Abdoul Nasser Yacouba will present his mid-term thesis work on the Characterization of heterogeneous limestone from the O-ZNS- A Multi Hydrogeophysical approach
June 9
10:00
E018
N/A
Abstract
The O-ZNS (Observatories de la Zone Non Saturée) site has been designed in the Centre-Val-de-Loire region (France) for monitoring and characterizing the dynamics of the vadose zone (VZ) of the Beauce aquifer. This VZ is made of mainly by lacustrine limestones is characterized by multiscale heterogeneities (presence of cracks, fractures, pores, cavities and karstification). In order to predict fluid flow, heat transfer, and aquifer recharge through this VZ, these heterogeneities have to be integrated into geological concepts and models. This PhD project aims at (i) understanding and classifying the microstructural and petrophysical properties at laboratory scale; (ii) predicting these properties through quantitative geophysical parameters and; (iii) developing new geophysical models through coupled approaches.
Based on well logs analysis from O-ZNS site, limestone samples (1.5, 2.5 and 4 cm diameter) from three defined facies have been cored. Porosity, density, pore structure and permeability measurements were carried out based on gravitational saturation method, mercury injection method and steady flow permeability method respectively. Geophysical measurement was then performed. First, acoustic measurements with P- and S-waves at 0.5 and 1 MHz in both wet and dry conditions and then, complex resistivity measurement at different water salinity (on going).
The results show a large dispersion of the petrophysical properties (porosity, density and permeability). This dispersion of petrophysical properties is interpreted in terms pore type heterogeneities and mineralogy variations. We show that it is possible to discriminate two facies as they highlight opposite petrophysical behavior. The results show also the difficulty to directly link the permeability to the porosity. However, by integrating the pore diameter as a second controlling factor, it is possible to better constraint the modelling.
Acoustic velocity results show coherent VP and VS values for limestones. We find also dispersed and heterogenous values in both wet and dry conditions especially for VS. However, there is a saturation effect which results in a decrease of the acoustic velocities when the samples are saturated. In overall, there is a consistency with the petrophysical behavior and one can expect good correlation between transports properties and acoustic velocities. The defined petroacoustic models can be linked to the type of heterogeneities characterizing each facies. Finally, using the appropriate petroacoustics, we are able to discriminate some facies and therefore predict their transport properties.
The next step of this work is to integrate complex resistivity data in order to see if they can be linked to the transport proprieties and the heterogeneities of our facies which may improve their discrimination. Based on the utility of the complex resistivity data, we can develop a more sophisticated models including transport, acoustic and electric properties.
It also planned to look deeper the frequency effect on the acoustic velocities by using at least three different frequencies going from ultrasound to sonic frequencies. This study will enhance our capacity of upscaling from laboratory to field scales.