Olivar A. L. de Lima

A generalized Maxwell-Wagner theory for membrane polarization in shaly sands

The effects of charged clay platelets on the frequency dependent electrical properties of shaly materials are analyzed using simplified models for the membrane polarization around charged spheres immersed in electrolytic solutions, under a thin double layer approximation. The polarization is defined through two possible mechanisms: (1) a surface conductivity related with a modified Stern double layer model (S‐model) according to Schurr‐Schwarz theory; (2) a coupled electro‐diffusional mechanism occurring in a Guoy‐Chapman double layer using Fixman’s approach (D‐model). By comparing the electric potential in such microscopic models with the external potentials derived for the equivalent homogeneous sphere using a Maxwell‐Wagner approach, we obtain the total current conductivity functions for these two models. The theory, therefore, provides explicit expressions relating the total conductivity functions to the model parameters. The behavior of the S‐model is described by a complex conductivity exhibiting a ...

A grain conductivity approach to shaly sandstones

The effects of clay conduction on the electrical behavior of shaly sandstones at low frequencies are analyzed by considering self-similar mixtures of conductive grains, as representative of these porous rocks. An essential step in this model is the representation of the surface conductivities of the clay platelets by an equivalent volume conductivity. Based on their pattern of occurrence, the clays are treated either as a continuous coating over the sand grains or as individual detrital grains mixed with a nonconductive mineral matrix. In the first case, the solids are modeled as layered conductive particles, taken as spheres to a first approximation. In the second case, we generalize from the results of a three-component self-similar mixture of spherical particles. For linear approximations, the models are described by expressions quite similar to the frequently used Waxman-Smits and dual-water equations, but dependent upon the clay type, concentration, and its distribution in the rock. The models are written in terms of geometric parameters readily available from well-log measurements. The derived analytical expressions are successfully applied to describe a set of available experimental core and geophysical log data. The results clearly demonstrate the applicability of the equations to well-log interpretation, as a practical procedure for computing formation factors and shaliness of argillaceous sandstones.

A volumetric approach for the resistivity response of freshwater shaly sandstones

The dc electrical response of freshwater-saturated shaly sandstone is analyzed on the basis of effective volume conductivity concepts for a concentrated mixture of “solid grains” in a continuous electrolyte. The bulk conductivity of this model is physically controlled by (1) the effective porosity and the average electrical tortuosity of its free-pore electrolyte, and (2) the amount and concentration of bound water that coats charged solid surfaces (mainly of clays), and a corresponding averaged electrical surface tortuosity. The latter is combined in an equivalent volume conductivity that is mainly due to the electrical double layers of charges developed at the clay-electrolyte interfaces. Analytical expressions, based on effective medium and general mixture theories, are developed to describe both the whole rock conductivity and the specific conductivities of its constituent elements. The derived equations for the bulk conductivity of the system describe, with sufficient precision, experimental core dat...

Imaging industrial contaminant plumes with resistivity techniques

A systematic geophysical procedure has been developed and applied to image groundwater contamination caused by industrial activities in Bahia, Brazil. The procedure combines the use of conventional Schlumberger sounding with a regular scheme of sampling the resistivity stratification in depth. This is achieved by traversing an area with multiple profiles measured at selected electrode spacing. By multiple profiles we mean closely spaced partial soundings made using only six electrode spacings. Partial and complete soundings are correlated and inverted assuming horizontally stratified models within the limits covered by each electrode array. Pseudo-resistivity sections constructed from these data are inverted and adjusted using a two-dimensional finite difference algorithm. Electrical and lithological well logs are used to constrain this interpretation. The procedure was successfully applied to investigate the groundwater conditions and to outline contaminant plumes within industrial areas of the Camacari Petrochemical Center, Reconcavo basin, Bahia. The study includes cases of electrically conductive plumes generated by infiltration of inorganic aqueous effluents and a resistive plume containing hydrocarbon contaminants.

Water saturation and permeability from resistivity, dielectric, and porosity logs

An analytical approach previously proposed to model the electrical properties of shaly sands has been adapted to obtain the cementation exponent (m), the water saturation (Sw), and the shaliness distribution from electromagnetic and porosity log measurements in an oil well. Shaliness is described by the clay volume fraction (p) and by the clay type parameter (β1/a) in the sand. For a shale‐coating structure these parameters can be related to the exchange cation density (Qv). Based on an equivalent flow regime inside this granular model, a Kozeny‐Carman type equation has been derived, expressing the intrinsic permeability (k) of the medium in terms of a porosity‐tortuosity factor ϕ(m-0.5) and of the parameter Qv. The power‐law derived expression shows that k decreases with the amount of clay, not only because a high Qv implies a narrowing of the pore channels, but also because it modifies the hydraulic tortuosity of the medium. This new equation has been statistically tested with extensive petrophysical la...

Nonlinear electrical response of saturated shaley sand reservoir and its asymptotic approximations

The quantitative interpretation of the electrical-resistivity log for reservoir characterization largely depends on a good understanding of the accurate relation between formation conductivity and rock constituents. This requires a mathematical model based on sound physical considerations. The models for the laws of physics are developed for electric conduction in linear conductors, so they can be extended easily to a homogeneous continuum. However, for further extension of these laws to a heterogeneous, porous medium, some approximations are needed. Using field equations designed for interpreting a homogeneous continuum to interpret a porous medium provides only a phenomenological description of the relations among macroscopic parameters of the porous medium.

GEOPHYSICAL EVALUATION OF SANDSTONE AQUIFERS IN THE RECONCAVO-TUCANO BASIN, BAHIA-BRAZIL

The upper clastic sediments in the Reconcavo‐Tucano basin comprise a multilayer aquifer system of Jurassic age. Its groundwater is normally fresh down to depths of more than 1000 m. Locally, however, there are zones producing high salinity or sulfur geothermal waters. In other areas, the system is being exploited heavily to support a petrochemical center and related industries. Analysis of electrical logs of more than 150 wells enabled the identification of the most typical sedimentary structures and the gross geometries for the sandstone units in selected areas of the basin. Based on this information, the thick sands are interpreted as coalescent point bars and the shales as flood plain deposits of a large fluvial environment. The resistivity logs and core laboratory data are combined to develop empirical equations relating aquifer porosity and permeability to log‐derived parameters such as formation factor and cementation exponent. Temperature logs of 15 wells were useful to quantify the water leakage t...

Multi‐frequency EM method for hydrocarbon detection and for monitoring fluid invasion during enhanced oil recovery

In this work we discuss the results of an experimental study performed with a multi-frequency electromagnetic method over a mature oil field in Reconcavo basin, Bahia – Brazil. Five 1.8 km transects 200m apart and extending over a block of the oil reservoir were surveyed. The processed EM data are represented as cross-sections of apparent resistivity and induced polarization parameter, using a consistent plotting procedure developed by Dias and Sato (1981). All the sections, controlled by seismic and well log data, although showing some distortions in the IP-resistivity configurations, allow to recognize the following geological features: (i) the oil sandstone horizons and their trapping shales; (ii) the oil-water interface and some zones of steam invasion; and (iii) lateral electric contrasts representing fault zones. These results suggest the real possibility of the use of the spectral EM method in the direct detection of hydrocarbons, as well as for monitoring the efficiency of the artificial fluid injection used for secondary recovery.

Estruturas geoelétrica e hidroquímica do sistema aquífero cristalino da bacia do alto rio Curaçá, semi-árido da Bahia

In this work we discuss a geophysical-hydrogeological strategy proposed to explore rationally the groundwater stored in the semi-arid terrains, such as those of the upper Curaca river basin, Bahia. Three main subjects were considered and solved in this study. The first one was to develop a conceptual geohydrologic model to represent the water storage and transmission within the aquifer zones of the area. Based on surface and aero-geophysical information and on geological data obtained in deep cored mining-wells we propose the model to consist of: (i) a thick layer of alluvial deposits and weathered rocks (saprolites) having up to 30 m thickness, covering almost continuously the Curaca river plain and those of their tributaries; and (ii) large and wide belts of densely fractured rocks, laterally and vertically alternating with zones of impervious crystalline basement. These belts of open, subvertical fracturing may reach more than 500 m in width and depths larger than 300 m. The second was to optimize a geoelectrical procedure capable of imaging, with good spatial resolution, these complexes underground reservoirs. Thus, a two-dimensional scheme of IP-resistivity imaging was performed with a closely spaced association of deep vertical electrical soundings. The third was to search and explain the mechanisms and processes responsible for the high water salinization in the wells drilled in this system. Based on a simplified model for a steady-state, two-dimensional flow of water in a water-table homogeneous and isotropic aquifer, we suggest this salt concentration process is mainly due to the intense and almost continuous evapo-transpiration, that characterizes the semi-arid lands. This process acts over a hydraulic circulating system controlled by the terrain topography. The circulation cells have different sizes and depth of propagation, the salt accumulation being concentrated in the smaller cells.

Geofísica elétrica na caracterização da hidrologia subterrânea na região do Aterro Metropolitano Centro, Salvador, Bahia

Resistivity and induced polarization (IP) vertical electrical sounding data, and superficial self-potential (SP) survey, allow to evaluate the underground hydrological conditions around the new Salvadors sanitary landfill. The inversion of the combined IP-resistivity data allows to differentiate zones of large shaliness within the sandstones of Barreiras Formation. They also allow to map the aquifer substratum topography and the water table configuration, as well as to infer the general pattern of the underground natural flow of water. The SP data, acquired around the first two cells already constructed for waste disposal confirm the general underground flow and identified areas of recharge and discharge around the cells. The resulting hydrological-petrophysical picture may be a useful reference frame to check eventual environmental impacts in the future, caused by the waste disposal in the area.