An arrow plot. A plot of dipmeter or drift results where the dip angle is plotted versus depth as the displacement of a dot. A line segment points from the dot in the direction of dip using the usual map convention of north being at the top. Sometimes called a vector plot.

Earth currents. The term usually refers to natural earth currents originating as a result of variations in the earth's magnetic field. Sometimes the term is applied also to earth currents resulting from artificial electric or magnetic fields.

As applied to well logging, a component of telluric current along the borehole may produce an undesired variable voltage between measure and ground electrodes which is superimposed on the SP. Such interference is easily detected by holding the sonde stationary in the hole and observing whether the SP voltage continues to vary. The greatest interference is likely to occur in regions of high magnetic activity and for large formation-resistivity to mud-resistivity ratios. In some instances, moving the ground electrode improves the situation. When interference is severe, it may be necessary to forego recording the usual SP curve and record, instead, a differential SP between two electrodes on the downhole configuration.

Telluric currents may create problems on resistivity measurements with large spacings and at very low frequencies (e.g., the ultra-long-spaced electrical log).

Recovery methods which increase ultimate oil production beyond that achievable with primary and secondary methods. These methods are usually employed in the latter stages of conventional secondary flooding applications, but may be implemented early in the secondary application or in combination with the secondary method. These oil recovery methods enhance the production of oil by increasing the proportion of the reservoir affected, reducing the amount of residual oil in the swept zones and reducing the viscosity of thick oils.

The tertiary methods usually are divided into three broad groups: thermal, miscible and chemical. See also primary and secondary recovery.

TDT. The Thermal Decay Time Log is a record of the rate of capture of thermal neutrons in a portion of formation after it is bombarded with a burst of 14-MeV neutrons. An electronic neutron generator in the tool produces pulses of neutrons which spread into the borehole and formation.

The neutrons are quickly slowed down to thermal energies by successive collisions with atomic nuclei of elements in the surrounding media. The thermalized neutrons are gradually captured by elements within the neutron cloud, and, with each capture, gamma rays are emitted. The rate at which these neutrons are captured depends on the nuclear capture cross sections which are characteristic of the elements making up the formation and occupying its pore volume. The gamma rays of capture which are emitted are counted at one or more detectors in the sonde during different time gates following the burst, and from these counts the rate of neutron decay is automatically computed. One of the results displayed is the thermal decay time, t, which is related to the macroscopic capture cross section of the formation, S, which is also displayed.

Because chlorine is by far the strongest neutron absorber of the common earth elements, the response of the tool is determined primarily by the chlorine present (as sodium chloride) in the formation water. Like the resistivity log, therefore, the measured response is sensitive to the salinity and amount of formation water present in the pore volume. The response is relatively unaffected by the usual borehole and casing sizes encountered over pay zones. Consequently, when formation water salinity permits, Thermal Decay Time logging provides a means to recognize the presence of hydrocarbons in formations which have been cased, and to detect changes in water saturation during the production life of the well. The TDT log is useful for the evaluation of oil wells, for diagnosing production problems, and for monitoring reservoir performance.

The TDT-K system utilizes two detectors and two variable time gates (plus a background gate) to sample the capture gamma radiation decay following the neutron burst. The width and positions of the time gates. as well as the neutron burst width and burst repetition rate, are varied in response to signals that are related to S (or more precisely, related to the formation decay rate, t, where t = 4550/S).

The TDT-M system utilizes sixteen time gates and one of four possible neutron burst widths and burst repetition rates. Counts from the sixteen gates are combined to form two "sum" gates (plus a background gate) from which S is computed. As in the TDT-K system, the combination of gates used to form the "sum" gates, as well as the burst width and repetition rate, are selected according to S (or t) of the formation.

The ratio of counts (R) in the near-spaced to far-spaced detector is recorded and used as an estimate of formation porosity. TDT is a mark of Schlumberger.

TMD. A record of the macroscopic thermal neutron cross sections of the formation (S_F) and the borehole (S_B).

An electronic neutron generator produces bursts of pulses of 14-MeV neutrons which spread into the formation and borehole. Following each burst, the neutrons are quickly slowed to thermal energies by successive collisions with nuclei in the surrounding media. The thermalized neutrons are then captured by elements within the formation and borehole, producing gamma radiation. Gamma radiation intensity is sampled in two detectors at six time intervals (time gates) following each burst. These data are used to compute S_F and S_B Also, the ratio (R) of counts in the near-spaced to far-spaced detector is recorded and used to estimate formation porosity.

Of the common earth elements, chlorine is by far the strongest neutron absorber and is found mainly in the formation water (as sodium chloride) rather than in the formation matrix. S_F primarily a function of the salinity and amounts of water present in the pore volume. Therefore, S_F is used with porosity from R to compute formation hydrocarbon saturation if the formation water is saline. The S_F curve is relatively unaffected by casing and tubing. S_B is used with S_F to obtain a very accurate true (intrinsic) formation cross section and an improved porosity estimate. In addition, S_B can indicate a variety of well bore conditions such as gas between the tubing and casing, and oil-water contacts. Thermal Multigate Decay Log (TMD) is a Welex trademark.

(1) A well about which information is restricted and passed only to those authorized, for security and competitive reasons.

(2) A small-diameter part of the borehole which causes downhole logging tools to hang up.

An empirical expression used for calculating porosity from interval transit time determined from acoustic logs:

where t = observed interval transit time, t_f = transit time in the pore fluid, and t_ma = transit time in the rock matrix. This relation works well in clean consolidated formations with uniformly distributed pores. In vuggy formations, the sonic log may not reflect the secondary porosity; and in unconsolidated formations, this relationship may overestimate porosity. In such cases, the formula may be empirically modified to give better values.

A specially designed system for use in highly deviated and bad holes which will mechanically position a stack of conventional-type logging devices opposite the zones or formations of interest. The Toolpusher uses drill pipe to push the logging devices to the desired depths in the well.

Once the tools have been assembled, they are lowered into the well on drill pipe until the top of the zone of interest is reached. At this point, a side entry sub is made up into the drill string through which the wireline is inserted and lowered to the latch assembly at the top of the tool stack. Its positive connection is ensured by applying tension to the wireline. The side entry sub is sealed. the hook and rotary table locked, (to minimize pipe rotation) and the drill string lowered until the tools are positioned at the bottom of the zone of interest. The wireline can be pumped down if necessary. Once in logging position. the compensated density tool is positioned with the rotary table such that an extendable pad on the tool contacts the high or low side of the hole.

The zone of interest is logged as the pipe is pulled out of the hole in single or multiple joints. Log data is multiplexed through a single conductor to the surface. When the zone of interest has been logged, tension is removed from the wireline releasing it from the latch. The wireline is recovered leaving the drilling crew a conventional trip out of the hole.

The logging devices available for use with the Toolpusher are: dual induction laterolog, compensated density, gamma ray, compensated neutron, and directional survey package. The Toolpusher is a Gearhart Industries trademark.

TD. (1) The total depth reached by drilling tools.

(2) The total depth in the well reached by a specific logging tool.

(1) A curve on a well log.

(2) To follow, locate, and monitor the behavior of a slug of radioactive material. See radioactive-tracer log.

(1) Well log track on the API log grid. Holding the well log vertically so that the top of the well appears at the top of the log,

See scale for illustration.

(2) Follow parallel or nearly parallel.