A B S T R A C TThe determination of clay content in near-surface formations is crucial for geotechnical, hydrogeological and oil-contamination studies. We have developed a technique for estimating clay content that consists of the minimization of the difference between the theoretically calculated and measured soil resistivities as a function of water salinity. To calculate the resistivity, we used a model that takes into account the electrochemical processes in the clay micropores. The experimental measurements of soil resistivity were performed on soil samples, completely saturated by brines at different concentrations of NaCl salt in the range 0.6-100 g/l, to obtain the resistivity versus salinity curve. The parameters obtained with this curve inversion are the clay content, the total porosity and the cation exchange capacity. To verify the new technique, we determined clay concentrations of artificial mixtures of calibrated sand and clay. The relative mean error in the clay content does not exceed 20% for a 5% fitting error of the resistivity versus salinity curves. Such evaluations allow the correct separation of the main lithological groups (sand, sandy loam, loam, and light, medium and heavy clay).We applied this technique to estimate the petrophysical parameters of soils (clay content, porosity and cation exchange capacity) at various sites in Mexico. The results improved the interpretation of the vertical electrical soundings, the lithological soil characterization and the delineation of oil-contaminated areas.

[1] A technique for the joint modeling of the electrical conductivity and acoustic velocities in porous rocks is proposed. The technique is based on the model of twocomponent media, composed by grains, which constitute a solid frame and pores saturated by a fluid. For this model we used symmetrical effective medium approximation method that provides the simulation of the acoustic and electrical parameters for multicomponent systems with equally treated constituents. The individual element of each component as a pore or grain was approximated by an ellipsoid. The aspect ratios for grain and pore ellipsoids are introduced as a function of porosity. By applying this technique we performed the calculation of acoustic velocities and electrical conductivity for carbonate formations with a primary pore system. For such formations the experimentally determined acoustic and electrical parameters are described by the empirical regression equations for the P and S wave velocities versus porosity and Archie's law. The ellipsoid aspect ratios were obtained by minimizing the differences of both predicted P wave velocity and conductivity with experimental data. The results obtained demonstrate that electrical conductivity is more sensitive to the pore and grain geometry than acoustic velocities. To ensure the conductivity for the low porosities, the form of pores tends to a needle shape. The simulation technique developed is the base for the petrophysical inversion of well log data to reconstruct the microstructure of porous rocks.

A B S T R A C TAn approach to determining the effective elastic moduli of rocks with double porosity is presented. The double-porosity medium is considered to be a heterogeneous material composed of a homogeneous matrix with primary pores and inclusions that represent secondary pores. Fluid flows in the primary-pore system and between primary and secondary pores are neglected because of the low permeability of the primary porosity. The prediction of the effective elastic moduli consists of two steps. Firstly, we calculate the effective elastic properties of the matrix with the primary small-scale pores (matrix homogenization). The porous matrix is then treated as a homogeneous isotropic host in which the large-scale secondary pores are embedded. To calculate the effective elastic moduli at each step, we use the differential effective medium (DEM) approach. The constituents of this composite medium -primary pores and secondary poresare approximated by ellipsoidal or spheroidal inclusions with corresponding aspect ratios.We have applied this technique in order to compute the effective elastic properties for a model with randomly orientated inclusions (an isotropic medium) and aligned inclusions (a transversely isotropic medium). Using the special tensor basis, the solution of the one-particle problem with transversely isotropic host was obtained in explicit form.The direct application of the DEM method for fluid-saturated pores does not account for fluid displacement in pore systems, and corresponds to a model with isolated pores or the high-frequency range of acoustic waves. For the interconnected secondary pores, we have calculated the elastic moduli for the dry inclusions and then applied Gassmann's tensor relationships. The simulation of the effective elastic characteristic demonstrated that the fluid flow between the connected secondary pores has a significant influence only in porous rocks containing cracks (flattened ellipsoids). For pore shapes that are close to spherical, the relative difference between the elastic velocities determined by the DEM method and by the DEM method with Gassmann's corrections does not exceed 2%. Examples of the calculation of elastic moduli for water-saturated dolomite with both isolated and interconnected secondary pores are presented. The simulations were verified by comparison with published experimental

We considered clay content in loose soil as the factor mostly influencing on hydraulic conductivity (filtration coefficient). We collected and analyzed some published experimental data about hydraulic conductivity relation with soil lithology and clay content in the form of grain size. Also we performed some theoretical modeling modifying well-known formulas to include clay content in them. Experimental and calculated data showed quite good coincidence. Correlation between hydraulic conductivity and clay content seemed better, than correlation between hydraulic conductivity and resistivity. We created some approximation formulas relating filtration coefficient with clay content. Clay content in soil can be estimated on soil resistivity obtained from VES data interpretation and from groundwater salinity found from its resistivity. Then filtration coefficient is determined on clay content. Some examples of this method practical application at clean and oil contaminated areas are presented. We considered anomalies of decreasing filtration coefficient in contaminated zones not as a real effect, but as a good indicator of contamination, though in several publications there were some indications of hydro geological changes in soil properties due to oil contamination.

A technique for the primary and secondary porosity estimation is proposed. This technique is based on a resistivity model of carbonate formations with double porosity and the analysis of numerous core data. The resistivity model consists of an isotropic conductive matrix with a primary porous system and inclusions, which represent secondary porosity (vugs and fractures). Fracture systems are described by oblate ellipsoids of different sizes (which have the same orientation and aspect ratio) or by thin plates. These rock structures are characterized by an anisotropy of electrical properties. The components of the resistivity tensor have a nonlinear relationship with the matrix porosity and parameters of fracture systems (orientation and porosity). The model with spherical inclusions (Maxwell-Garnett approach) is applied to obtain the resistivity of vugular formations. The fluid type, saturation of matrix and secondary pores influence significantly the formation resistivity and anisotropy coefficient. When both pore systems are completely saturated by the same conductive fluid (the case of core analysis), the effective formation factor depends only on the matrix formation factor and the secondary porosity value. The statistical analysis of core data has shown that Archie's equation with cementation exponent m=2, is the best approximation to describe the matrix formation factor for carbonate formations in Mexico. For verification of the porosity separation technique, the fracture and vuggy porosities have been estimated for core with detailed description of pore structure. Introduction The correct evaluation of carbonate formation with double porosity requires separately determining the primary and secondary porosity. Type, distribution and porosity value of secondary-pore systems (vugs or fractures) influence significantly the estimation of saturation, permeability and hydrocarbon reserves. For porosity separation in the heterogeneous carbonate formations, the resistivity data from log and core analysis are widely used. Two different approaches that are applied to describe the resistivity of multiporosity system can be distinguished. The first technique uses the traditional presentation of the formation factor in the form of modified Archie's equation as the function of the total porosity. The cementation exponent varies considerably in the carbonate formations with complex porous system. For its correct calibration the correlation analysis of core data for each lithological type is recommended.1,2 Frequently, to describe the experimental distribution of the formation factor obtained from core data, the variable value of cementation exponent as a function of the total porosity has to be assumed.3,4 The estimation of type and value of secondary porosity applying such methodology is qualitative and it is based on the analysis of the parameters of modified Archie equation.5,6 The influence of porosity variation on the cementation exponent using effective-medium theory was studied by Sen et al.7 In the alternative approach the resistivity model of doubleporosity formation is presented as the host medium with primary porosity in which inclusions of different forms (secondary pores) are placed. The formation factor is expressed as a function of two arguments: the primary and secondary porosities.