The average results of air- and water-permeability determinations are given for petroleum-reservoir sands in three Wyoming fields. The average amounts of materials of clay size in the sands and the types of clays present, as identified by X-ray diffraction methods, are also presented and discussed.

The sands are shown to be more permeable to air than to brines and more permeable to brines than to fresh water. Each of the sands exhibited different behavior when wetted by waters, and the percentage loss of permeability to waters, as compared to air, varied from sand to sand. The sand containing kaolins, illites, and mixed-layer clay (illite-montmorillonite) was found to be the most sensitive to water, and the sand containing only small amounts of kaolins and illites was the least sensitive. The sand that contained the most kaolins and illites was intermediate in water sensitivity. The water-permeability behavior of the sands and the dependence of this behavior on the clays present and the salinity of the water are discussed.

This paper deals with the application of methods involving the scattering of neutrons and gamma rays to the measurement of moisture content and density of granular materials. The measurement of moisture content is based on the principle that when fast neutrons emitted from a radioactive source collide with hydrogen atoms they are slowed down to a much greater extent than by collisions with other atoms. The number of slow neutrons thus produced is a measure of the number of hydrogen atoms present in the vicinity of the source. Water is the principal contributor of hydrogen atoms in a soil medium. A probe, containing a source of fast neutrons and a slow neutron detector, is inserted into the soil. The probe is connected by a cable to a suitably calibrated scaling instrument, and the moisture content determined from the count rate.

The density of a soil is measured with a probe that is similar but provided with a source and a detector of gamma radiation. The gamma rays emitted from this source are scattered by collisions with electrons of atoms in their path. The higher the density of the surrounding medium, the greater the scattering. In the range of densities normally occurring in soils, greater scattering results in fewer gamma rays returning to the detector. Thus, the density of the medium can also be related to the count rate obtained with the scaling instrument.

The advantages of the radioactivity methods are that continuous or repeated measurements of moisture content and density at any desired depth can be made, and that measurements are integrated over a large volume of soil, so that representative values are obtained. In addition, water may be detected in the solid or vapor states as well as in the liquid state. The soil undergoes a minimum of disturbance because the probe is lowered into an access tube slightly greater than 1 inch in diameter. Measurements can be made in a short time; once the access tube has been placed, it takes an average of about six minutes to determine both moisture content and density at a given depth. The instruments are not influenced by ordinary temperature changes. The method appears to be relatively independent of soil type, so that a single calibration curve for moisture content, and one for density, may be applicable to a wide range of materials.

A description of the theory of scattering of neutrons and gamma rays is included in this report as well as a discussion of the various types of sources and detectors which can be employed in the procedure. Field and laboratory tests utilizing these instruments are described, and the accuracy of test results discussed.

It is concluded that application of this method will provide a rapid, simple and accurate means for measuring moisture content and density of soils or similar granular materials. On this basis, recommendations for future research and more extensive applications of the method are reviewed.

For the last 20 years, the Ceramic Division at the Massachusetts Institute of Technology has been carrying on basic work with the kaolinite-water system. The results have been published from time to time in the Journal of the American Ceramic Society under the title of “Fundamental Study of Clay.”

At the start, it was realized that natutal clays were so complex that it would be impossible to consider them as a basis for quantitative calculations. Therefore, most of our research has been carried out on specially prepared samples taken from the natural clay. These kaolinite samples had a known particle size, were free from organic matter, and had controlled adsorbed ions. In this way, the effect of anyone variable on the rheological properties could be studied in a logical way.

The tension phenomenon described herein occurs in a wide variety of porous materials, including sands, clays, agricultural soils and porous rocks. When some of the water is removed from a water-saturated porous system, the residual water evidently remains physically interconnected, judging from the fact that water can be transmitted through the system at reduced water content by suction.

The removal of water may result in contraction of the system, as in the case of clay, or in the entry of air, as in the case of sand. The liquid phase and the solid phase in contact with it comprise a closely linked force system. Equilibrium can be established between the system at reduced water content and a separate mass of water at reduced pressure through a porous membrane in contact with both.

The equilibrium tension required in the external water phase is considered an attribute of the moist, porous, system itself. From this point of view, the tension originates through the combined action of the internal forces of the system in a virtual displacement of water. It follows from this and from the principle of virtual work that the tension is numerically equal to the differential work done by the internal forces per unit volume of water absorbed.

The movement of water, under tension, through porous systems represents a special class of flow phenomena in which tensiometers or equivalent devices are required for measuring the hydraulic potential. Flow patterns can be determined in much the same way as in systems characterized by positive hydrostatic pressures, but special attention must be paid to the Darcy coefficient (the capillary conductivity) which varies with the tension.

The theoretical conditions for the equilibrium of water in the soil and for emergence from the soil have been developed in terms of the tension and certain applications have been indicated.

The phenomenon referred to in the soil science literature as moisture tension has been recognized for almost forty years and has been used as a means of explaining the absorption and movement of water in the soil. It is closely related to osmotic pressure but its mechanism cannot in general be identified with the traditional mechanisms of osmotic pressure. Moisture tension has been observed in wet clay soils and in other finely divided porous systems containing interstitial water, but the phenomenon is not confined to colloidal systems, since moist sand and moist porous rock, such as pumice, show similar effects.