(1) Interconnected pore volume occupied by free fluids. Hydrodynamically effective pore volume. Compare total porosity. See porosity.

(2) Also electrically effective porosity. May differ from hydrodynamically effective pore volume.

Those properties that serve to describe the ability of a material to withstand stress without undergoing permanent deformation. All solid substances, including rocks, follow Hooke's law, that is, the proportionality relation between strain (or deformation) and stress (or force per unit area). The stress-strain ratio in simple linear compression or expansion is Young's modulus of elasticity (E),

where F/A is the force per unit area or stress, and DL/L is the strain or elongation or shortening per unit length under the application of expansion or compression.

The stress-strain ratio under hydrostatic compression or expansion is the bulk modulus of elasticity (K),

where DV/V is the volume expansion or shrinkage per unit volume under the application of expansion or compression.

The stress-strain ratio in shearing, or application of a force, tangential to the surface displaced, is the rigidity or shear modulus of elasticity (µ).

where F/A is the shearing stress and DL/L is the shearing strain or deformation without change in bulk volume.

Poisson's ratio (s) is a measure of the geometric change of shape

where W stands for width. It is always comprised between 0 and 1/2, its theoretical value being 1/4 for elastic bodies. The above-mentioned properties are responsible for the propagation of sound or acoustic waves through rocks.

Two types of body waves are propagated through elastic media: (a) longitudinal or compressional waves wherein the back and forth oscillations of particles are in the direction of propagation, their velocities being given by

where r_b is bulk density of rock. (b) Transverse or shear waves wherein the back and forth oscillations of particles are in a direction perpendicular to the direction of propagation, their velocities being given by

The longitudinal waves always arrive before the transverse waves, and to the present time, the former only have been used extensively in well logging. (From Pirson.)

(1) Resistivity of the rocks. By a lateral device.

(2) Delineation of porosity in rocks by electrofiltration potential measurements made with an SP electrode.

(3) Electrical anisotropy of the rocks. Determination of the direction of dip by an early form of dipmeter.

(4) Temperature measurements.

(5) Resistivity of the mud. Location of water flows.

(6) The electromagnetic teleclinometer. Survey of crooked holes.

A generic term used to refer to the specific resistivity well log which usually consists of short normal, long normal, lateral, and SP curves. Often used incorrectly to refer to borehole electric logs of other types. Compare electric log.

The electrical survey (i.e., normal and lateral formation-resistivity measuring systems) is suitable for use in wells drilled with relatively fresh mud.

In the early years of the development of electrical resistivity measuring well-logging tools. several different devices were employed utilizing a number of different electrode spacings. These devices were used in making the electrical surveys often referred to as "ancient" resistivity surveys, or sometimes ancient well logs. The curves on these resistivity logs were simply referred to as first curve, second curve, third curve, and fourth curve. The first, second, and third curves usually could be recorded simultaneously while logging upward during the depth-controlled survey. The fourth curve was recorded usually while the tool was being run into the hole. From time to time attempts were made to standardize the electrode spacings of the respective devices used within certain geographical regions. But, it was difficult to arrive at a common standard because of the diversity of rock types, bed thicknesses, environmental conditions, and customer (or user) preferences. It was not until 1947 that the API recommended spacings (for different devices) were adopted throughout the oil and gas industry. From that time on, the standard API spacings were offered to the industry, except where customers specifically asked for spacings tailored to meet their specific requirements. After 1947, the spacings were to appear on the log heading and the curve types and spacings were standardized (with few exceptions) as follows:

Between l932 and the late 1940s, before the API standards were adopted, the electrical survey could have consisted of the following curves and spacings for various geographic locations.

Quite often, today's user of the old or "ancient" electrical surveys will discover that the curve type (e.g., normal, lateral, or limestone curve) and the spacing used (e.g., 16", 64", 18'8", etc.) will be missing from the log heading. In that event, the user must determine what kind of devices and what spacings were used for the second, third. or fourth curves by a careful examination of the behavior of each curve through different bed thicknesses and as the curve approaches surface casing. The preceding table might provide some guidance.

The component of the SP comprised of the sum of the liquid-junction potential and the shale-membrane potential, both of which are related to the ratio of the activity of the formation water to that of the mud filtrate. The liquid junction potential is produced in the formation at the interface between the invading filtrate and the formation water, as a result of the differences in ion diffusion rates from the more concentrated to the more dilute solution. The negatively charged chloride ions have greater mobility than the positive sodium ions and an excess negative charge tends to cross the boundary, resulting in an emf. The shale membrane potential results because the shale bed acts as a cationic membrane, permitting the sodium cations to flow through it but not the chloride anions. That results in an excess of positive charges in the dilute solution and an excess of negative charges in the concentrated solution. The liquid junction potential and shale potential are additive. See SP and SSP.

Streaming potential, electrofiltration potential. A component of the SP produced as a result of movement of the invasion fluid through the mud cake or permeable formation. The electrokinetic potential is the potential difference which arises across a capillary when a stream of invading fluid is passed through it. The ions in the diffuse layer are swept along by the invading fluid so that a displacement of charge occurs. Opposite charges are built up at opposite ends of the capillary.

An electrofiltration potential also exists at the interface between the borehole and shale beds, a circumstance which tends to nullify somewhat the effect of the mud-cake electrofiltration potential in producing variations on the curve. In rare instances, in the case of a very low-permeability formation, little mud cake may be formed and a large electrofiltration potential may be generated across the formation itself.

(1) A material in which the flow of electric current is accompanied by a movement of ions.

(2) Any chemical compound which when dissolved in water dissociates into positive and negative ions, thus increasing its electrical conductivity. See dissociation.

EPT. The EPT is a device that measures the propagation time (TP1) and attenuation rate (A1) of a microwave frequency electromagnetic wave that is propagated through the formation near the borehole. These two measurements can be related to the (composite) dielectric constant of the formation close to the borehole. The EPT is a shallow investigation device that has a depth of investigation of 1 to 4 in., depending on the formation conductivity. As a result, the EPT responds primarily to the flushed or invaded zone of the formation. The utility of the EPT arises from two basic facts. First, the dielectric constant of earth formations is dominated by the amount of water contained in the rock pores. That results from the fact that the dielectric constant of water is an order of magnitude greater than that of the other constituents of reservoir rocks; namely, oil, gas, and the rock matrix. Second, at microwave frequencies, the dielectric constant of water saturated rocks is relatively independent of water salinity, except in ranges corresponding to very high salt concentrations. Those two facts imply that the dielectric constant inferred from the EPT measurements is effectively a salinity independent datum capable of distinguishing between water and oil in the zone of investigation. Also, these measurements can be used to derive values for formation porosity and water saturation that are essentially salinity independent. EPT is a mark of Schlumberger.

emf. (1) The force that drives electrons and thus produces an electric current.

(2) The voltage or electric pressure that causes an electric current to flow along a conductor.

A decrease in the neutron log apparent porosity reading below that expected on the basis of the hydrogen indices of the formation component. Excavation effect results from the presence of a second formation fluid with a hydrogen index lower than that of the water. Thus, for example, in the presence of gas saturation:

f_N = fS_gH_g + fS_wH_w – Df_Nex

where f_N is derived for the existing lithology type and where S_g and S_w are respectively gas and water saturation. H_g and H_w are respectively the hydrogen indices of gas and water, and Df_Nex is the excavation effect.

The term "excavation effect" originates from the comparison of a fully water-saturated formation with another one containing the same water content, but having a larger porosity, the additional pore space being filled with zero hydrogen-index gas. On the basis of hydrogen index. both formations should give the same neutron porosity response. However, the second formation differs from the first in that the additional pore space occupied by the gas has been provided by "excavating" some of the rock framework. The two formations give neutron log apparent-porosity responses which differ by the amount of the excavation effect for this case.

Excavation effect is greater for larger contrasts between the hydrogen indices of the second fluid and the formation water, for higher formation porosities, and for intermediate water saturations.