A log recorded in uncased boreholes which involves the use of electromagnetic induction principles for the measurement of formation conductivity or resistivity. The induction logging tool has advantages for use in nonconductive borehole fluids (air. oil gas) where other electrical resistivity logging tools cannot be easily used or should not be used. The induction log is widely used in electrically conductive drilling muds where it works well provided the formations are not too resistive and borehole effects are known and not too great (i.e., mud not too saline and hole diameter not too large).

Practical induction sondes include an array of several transmitter and receiver coils designed to provide focusing and deep investigation and to minimize borehole and adjacent-formation effects. A high-frequency alternating current, constant in magnitude, is passed through the transmitter coils. The resulting alternating magnetic field induces currents in the formation which flow in circular ground-loop paths coaxial with the sonde. Those ground-loop currents generate their own magnetic fields which induce in the receiver coils signals which at low conductivities are essentially proportional to formation conductivity. At high conductivities, the reading may be affected by skin effect. Receiver-coil signals produced by direct coupling with the transmitter coil are balanced out by the measuring circuits.

Induction tools can be run separately or can be combined with other devices to run combination services. Integrated tools, combining in one tool the devices necessary to perform different resistivity-measuring operations, are commonly used in the well-logging industry. Examples of such tools are the induction device with a deep depth of investigation in combination with: another induction device having a shallower depth of investigation, invaded zone investigative devices (e.g., short normal device, short laterolog or guard log, or Spherically Focused Logging device), long lateral, and SP.

(1) The covering on electrical wiring (or other conductive materials) which provides electrical isolation.

(2) The quality of electrical isolation, measurable with a test set, necessary for proper operation of logging instruments and calibrated measurement of downhole properties of formation and borehole.

An increase in interval transit time which occurs when attenuation of the acoustic energy is significantly greater at the far receiver (of a receiver pair) than at the near receiver The stretching of interval transit time is not related to skipping of a cycle. Compare cycle skip.

(1) The thermal-neutron decay time intrinsic to a particular material or medium. defined by (1/vS_abs)where v is neutron velocity and S_abs is the macroscopic capture cross section of the medium.

(2) The thermal-neutron decay time of a particular formation corrected for borehole and diffusion effects. See thermal decay time, Thermal Decay Time Log, and capture cross section.

The portion of formation surrounding a well bore into which drilling fluid has penetrated. displacing some of the formation fluids. This invasion takes place in porous, permeable zones when the pressure of the mud is greater than that of the formation fluids. A mud filter cake builds on the formation wall, limiting further invasion into the formation by mud filtrate. Directly behind the mud cake is a flushed zone from which almost all of the formation water and most of the hydrocarbons have been displaced by filtrate. The invasion process alters the distribution of saturations and other properties and, consequently, alters the values which are recorded on logs. The depth of invasion is the equivalent depth in an idealized model rather than the maximum depth reached by filtrate. In oil-bearing zones, the filtrate may push a bank of formation water ahead of it to produce what is referred to as an annulus.

Atoms, or groups of atoms, which have either taken on or given up one or more orbital electrons. As a result of the gain or loss of one or more electrons, each ion bears electrical charge. Positively charged ions are called cations and negatively charged ions are called anions.

In solutions, the ions interact with the solvent to become hydrated and the properties of the ion become modified by the solvent molecules attached in varying degrees to the ion. Other than in solutions, ions may exist in the solid state as in a crystal lattice; or in the gaseous state, where they are produced by the action of high-energy radiation on neutral atoms or molecules of the gas.

Base exchange. The reversible chemical reaction in which certain ions in a crystal lattice on particle surfaces or edges can become hydrated in the presence of aqueous solutions with the result that they can be replaced, equivalent for equivalent, by ions (usually cations) exhibiting similar charge from the solution without a loss in the crystal structure.

According to a widely held view, the cations do not occupy precise sites: some are located in a plane parallel to and within a few angstroms of the clay surface (Helmholtz layer); others are contained in a zone of diminishing concentration (called the diffuse layer) extending outward from the Helmholtz layer.

Materials, particularly clays, which exhibit this property support surface conductance with their exchangeable cations.

The process by which a neutral or uncharged atom or molecule (of gases, liquids, solids) acquire a charge, thus becoming an ion.

When a substance dissolved in a liquid (or the liquid itself) undergoes ionization, electrolytic ion pairs of opposite charge are produced at once. A substance in the gaseous state requires a source of energy for ionization. When a substance in gaseous state is ionized, usually by radiation, only positively charged molecules or ions are formed as the result of the loss of one or more electrons. Both the positive ions and electrons are free to cause further ionization of the gas and produce both positive and negative ions. In all cases, the charged particles and ions are subject to motion or deflection in electric fields. Their motion constitutes a flow of electric current.

(1) A type of gamma-ray detector. Consists of a gas-filled cylindrical metal shell containing a center rod (i.e., electrode) maintained at about 100 volts positive to the cylinder wall. An incident gamma ray interacts with the cylinder wall material or the gas maintained at high pressure in order to produce a high-speed electron. The high-speed electron, drawn to the positively charged center rod, produces additional electrons and ions in the collision with gas atoms. The electrons (along with some negative ions) moving to the center electrode constitute a minute flow of electrical cur rent, the size of which is proportional to the number of gamma-ray interactions. Long chamber lengths and high gas pressures are used in order to improve detector efficiency, but vertical resolution suffers with increased chamber size. See also pair production. Compare Geiger-Mueller counter, proportional counter, and scintillation counter.

(2) Has been used as a slow-neutron detector.