A simple physical model was used to develop an equation that relates the electrical conductivity of a water-saturated shaly sand to the water conductivity and the cation- exchange capacity per unit pore volume of the rock. This equation fits both the experimental data of Hill and Milburn and data obtained recently on selected shaly sands with a wide range of cation-exchange capacities. This model was extended to cases where both oil and water are present in the shaly sand. This results in an additional expression, relating the resistivity ratio to water saturation, water conductivity and cation-exchange capacity per unit pore volume. The effect of shale content on the resistivity index- water saturation function is demonstrated by several numerical examples. INTRODUCTION A principal aim of well logging is to provide quantitative information concerning porosity and oil saturation of the permeable formations penetrated by the borehole. For clean sands, the relationships between measured physical quantities and porosity or saturation are well known. However, the presence of clay minerals greatly complicates log interpretation, particularly the electrical resistivity and SP logs, and considerably affects evaluation of hydrocarbon-bearing formations. The conductance and electrochemical behavior of shaly sands and their relation to log interpretation have been studied by many workers. Wyllie and Lynch reviewed this work in some detail. Virtually all laboratory measurements of electrical resistivity and electrochemical potential of shaly sands published to date are the work of Hill and Milburn.

Equations have been developed that relate induced polarization (IP) in shaly sands to measurable petrophysical parameters. The induced‐polarization process has been modeled in terms of two mechanisms: clay counterion displacement and membrane blockage. The resulting equations can be used to determine shaliness, brine conductivity, and oil saturation from in‐phase and out‐of‐phase conductivities. Laboratory measurements have confirmed the IP dependence on these variables, as well as on temperature.

Summary This paper reports results of asphaltene adsorption/desorption on clayminerals, silica, and carbonates. It also describes the effect of adsorbedasphaltenes on rock wettability and a screeningpyrolysis-flame-ionization-detection (P-FID) test to evaluate the abilitypyrolysis-flame-ionization-detection (P-FID) test to evaluate the ability of solvents to remove asphaltene from kaolin and formation core material. Introduction Reservoir wettability is a major factor controlling the location, fluiddistribution, and flow properties of the system. Wettability conditions affectformation capillary pressure and relative permeability behavior, electricalproperties, and residual oil saturations. The wet-tability of originallywater-wet mineral surfaces may be reverseby adsorption of polar organiccompounds in crude oils. The highest concentration of polar organic compoundsgenerally is found in the heavy ends of crudes, particularly in the asphalteneand resin fractions. Wettability alterations of oil-bearing formations, particularly those containing clay minerals, have been attributed toparticularly those containing clay minerals, have been attributed to adsorption of these compounds onto mineral surfaces. Because of the high molecular weightsand multifunctional character of asphaltenes and resins, their adsorption byspecific minerals is a major element in wettability changes and was thereforeselected for study. Significant factors that control adsorption of asphaltenes and resins onmineral surfaces are (1) the presence, thickness, and stability of water filmson the mineral surface; (2) the chemical and structural nature of the mineralsubstrate; (3) asphaltene and resin contents of the crude; (4) the presence of asphaltenes and resins in crude oils in the form of colloidal micelles oraggregates; and (5) the ability of the hydrocarbon fraction of the crude tostabilize these colloidal aggregates in the oil or even to dissolve them intotrue solution. Further, specific asphaltene/mineral interactions control thedegree to which such adsorption is irreversible with respect to various organicsolvents and, hence, may determine optimum laboratory corecleaningprocedures. A variety of physical measurements, including molecular-weightdeterminations in various solvents, have deduced that asphaltenes associate orform aggregates even in dilute solution. Moschopedis et al. showed that intermolecular hydrogen bonding is involvedin asphaltene association and is reflected in the observed molecular weights. Vapor pressure osmometry determinations in nitrobenzene (epsilonr=34.8) producemolecular weights of 1,650 to 2,100 compared with 5,000 to 6,700 in benzene(epsilonr= 2.3). These nitrobenzene-derived molecular weights may be themolecular weights of the individual asphaltene particles. The power of various solvents has been expressed in terms of Hildebrandsolubility parameters, delta. The relation between delta and asphaltenesolubility was confirmed by Mitchell and Speight. They compared the weight of asphaltenes separated from Athabasca bitumen with a series of solvents(solvent/bitumen volume ratio of 40: 1) with the maximum asphaltene precipitateobtained by addition of an excess of n-pentane. The weight percent of precipitated asphaltenes decreased linearly with increasing delta. Completeprecipitated asphaltenes decreased linearly with increasing delta. Completesolubilization of the asphaltenes in the bitumen was obtained for solvents withdelta greater than = 8.4 cal 1/2 × cm / . For our initial adsorption experiments, the effect of solvent variation forasphaltene adsorption on the clay mineral kaolin was examined. The solventseries toluene/n-dodecane at 1.75 to 1.00 wt/wt toluene and chloroform was chosen primarily because of its increasing delta. The toluene/n-dodecanemixture of aromatic/aliphatic solvents has a delta close to the limiting valuerequired for complete solubilization of our asphaltene sample. Chloroform is aproton donor in hydrogen-bond formation and has a somewhat higher dielectricconstant and dipole moment than the other two solvents. Asphaltene adsorptionstudies were also carried out with a wide variety of other mineral and clayadsorbents from toluene solution. Collins and Melrose, Clementz, and Cuiec, indicated that the presence of a thin film of water on the mineral surfacereduces presence of a thin film of water on the mineral surface reducesasphaltene adsorption and can affect the kinetics of adsorption. To establishbase cases for further work, the presence of water, as well as resins, wasexcluded from all systems. The effects of various solvents on the desorption process were also examinedto evaluate their effectiveness in core-cleaning operations. Solvents werechosen on the basis of their delta values, polar character, andhydrogen-bonding capabilities. Experimental Asphaltene Adsorption on Kaolin From Solution. Adsorption studies werecarried out with a tar-sand-derived asphaltene (npentane insolubles) sample indifferent solvents and kaolin clay mineral (from J.T. Baker Chemical Co.) asthe adsorbent. Table 1 gives the asphaltene elemental analysis. Elementalanalysis and X-ray diffraction (XRD) indicate that the clay is predominantly inthe sodium form, consisting of 15% illite. The Brunauer-Emmett-Teller (BET)surface area, with N gas, was 11.9+/-0.4 m/g (five determinations). Thecation-exchange capacity, measured by Ba/Mg conductimetric titration, was 4.572meq/100 g. The solvents chloroform and toluene were analytical reagent grade(Mallinckrodt Co.) and contained less than 400 and 200 ppm water, re-spectively, according to Karl Fisher titration. n-Dodecane (Aldric Chemical Co.) was 99% pure (with less than 10 ppm water). Analytical reagent gradenitrobenzene (Baker Chemical Co.) had appx. 300 ppm water. Solvents were dried by storing over 0. 4-nm molecular sieves for at least 48hours before use. Kaolin mineral was dried at 110 deg.C for 14 hours and cooledin a desiccator. Glassware was oven-dried; solvents and asphaltene solutionswere manipulated under dry nitrogen atmosphere. Initial asphaltene concentrations were varied from 300 to 2.500 ppm. Mineral/asphaltene solutions in the ratio of 1 : 100 were shaken ppm. Mineral/asphaltene solutions in the ratio of 1 : 100 were shaken mechanicallyfor 48 hours at ambient temperatures; liquid was separated from solid bycentrifugation. Both initial and equilibrium asphaltene solution concentrations weredetermined spectrophotometrically with a Bausch and Lomb Spectronic 1001 TMspectrophotometer. Asphaltene concentrations were determined from calibrationcurves of absorbance vs. concentration at 450 nm for all solvents exceptnitrobenzene, for which 600 nm was used. Practical delta values for the solvent/asphaltene systems were obtained by avariant of the Bichard test and compared with literature values. Asphaltenesolutions in the respective solvents at various concentrations were eachtitrated against n-dodecane, a reference precipitant for asphaltenes. Titrationendpoints were marked by the first appearance of asphaltene precipitate, determined by microscopic examination. Plots of solvent/asphaltene ratio(milliliters per gram) vs. the n-dodecane/asphaltene ratio (milliliters pergram) produced a series of straight lines. The cotangents of per gram) produceda series of straight lines. The cotangents of the angles, theta, formed bythese lines with the x-axis have been used as practical solubility parametersfor the individual solvents.

Fig. 16-Equivalent counterion conductance (B) vs. resistivity of equilibrating brine at various temperatures.

A simple physical model describing shaly sand conductivity was previously described by Waxman and Smits. The electrical conductivity of a water-saturated shaly sand was expressed as a function of a geometry factor, the brine conductivity, an effective concentration of clay exchange cations, and their equivalent ionic conductance. Experimental data support this relation. The model was extended to cases where the sands were oil-bearing. An expression was obtained relating the resistivity index to water saturation, brine conductivity, and the clay parameters mentioned above. The assumptions involved in the model for oil-bearing sands have now been confirmed by additional laboratory conductivity measurements. We conclude that the effective concentration of clay exchange cations is increased by a quantity proportional to the decrease in water saturation. Twelve different rock samples from seven fields were utilized in these studies, incorporating a wide range of brine conductivities, oil saturations, and cation exchange capacities. Further, the temperature coefficients of electrical conductivity were measured for a set of shaly sands equilibrated with salt solutions covering a wide range of concentrations. When compared at the same electrolyte concentration, these temperature coefficients increased systematically with increasing clay content of the sands.