DEPTH CONTROL BASICS
Portions of this page are from Petroleum Engineer's Handbook (PEH) online version.

Depth is the most important measurement made in relation to well

log data acquisition. Accurate depth measurements allows for information to be compared to the wellbore and across multiple wells in a field. There are several types of data from geology, drilling measurements, completion and production operations that use depth as a normalizing tool. t allows all the phases of a wells life to be correlated to the reservoir or zone of interest. Small errors will have a great impact on the success of a well. Therefore, depth control during logging, completion, and work over is critical to make sure it is correct and consistent.
Correlating wireline depths in a shallow well or a vertical well are normally a simple task to perform. However, as the depth, deviation, and complexity of the well bore increases, it is more critical to have a good understanding of the principles of depth control. Understanding the principles will allow you to identify issues and corrective actions that can be taken to insure success.

The main consideration is appropriate logging speed to reduce variations in cable tension caused by tool drag. Depth control in a cased hole operatiom usually involves correlation between one or more cased hole logs with existing open hole logs. Hence, the speed must be slow enough that the resolution of events on the gamma ray (GR), collar locator (CCL), and neutron log (if run) is sufficient to identify the characteristic log response of the depth interval to be completed or serviced. Make sure the open hole base log is correct for the well you are working on.

Collar Locator Log
The two primary cased hole logging tools used for deoth control are the gamma ray (GR) and the collar locator (CCL). The GR can be correlated with previous open hole logs to tie into the reservoir zone of interest. The CCL, run in tandem with the GR, is used to tie in the drillers pipe tally to the logger’s depth measurements.

All collar locators work on the basis of Faraday's law. Each collar locator (CCL) has a coil with a magnet located at each end. The magnets create a fairly large magnetic flux which surrounds the coil. When a CCL is traveling down hole, the changes of pipe mass at the end of each pipe joint (the pipe collar) disturbs the flux of the CCL. As the flux changes, a voltage is created in the coil.

      Circuit diagram for a Collar Locator log ==>

At this point, what happens to the signal depends on what type of CCL is being used, either powered or non-powered. In the non-powered CCL, the voltage is placed on the wireline and travels up-hole to the logging unit. Diodes prevent the signal from travelling through the mud to the sueface.

In the powered CCL, the voltage is passed to an amplifier circuit in the CCL tool and then the amplified signal is placed in the wireline and sent to the surface computer to be processed.

Example of a Collar Locator log – large, sharp peaks are generated by changes in pipe mass, for example pipe joint collars or broken, heavily corroded or gaps in the pipe.

Gamma Ray Logging Tool
The gamma ray logging tool measures natural radiation emitted from rock formations traversed by the logging tool. Shales and clays have higher radioactive count rates than the usual reservoir rocks (quartz, calcite, dolomite) due to potassium and thorium. Many unconventional reservoirs and source rocks contain uranium as well as potassium and thorium.

On the log, low GR values arw toward the left side, usually in Track 1. A GR curve spanning Tracks 2 and 3, with low values toward the right, is sometimes presented to aid correlation to open hole porosity logs.

A cased hole GR log may be scaled in counts per second (cps) or APIgr Units. The APIgr scale may or may not be calibrated, and if calibrated, may or may not have casing and borehole size corrections applied. Read the log heading carefully.

A cased hole neutron log may also be run with a GR / CCL depth control log. It can be scaled in counts per second, APIn Units, or porosity units (% or decinal fraction), spanning Tracks 2 and 3. In all cases, low porosity is toward the right and high porosity and shale to the left.

The shape of the cased hole gamma ray log curve is used to correlate depths with the open hole logs. The depth recording system in the logging unit is manually adjusted so that cased hole depths match open hole logs. Then and only then can the planned completion or work over proceed.

Example of a gamma ray log (Track 1) with collar locator. A neutron log spans Tracks 2 and 3. The neutron near and far count rates are at the right hand side of Track 3.

Types of Cased Hole Gamma Ray Detectors
A Geoger-Mueller detector is an ionization chamber that contains a low pressure gas and has a high voltage applied to the electrodes. A gamma ray strikes an atom of gas and causes a positive and negative ion to be formed. Because of the high voltage that has been applied to the electrodes, the ions accelerate toward their respective electrodes and in the process strike other gas atoms, creating more ions. The effect is a multiplication takes place (Townsend Avalanche). Eventually the ions strike or reach the lectrodes producing a current or pulse which is roportional to the amount of ionization produced in the gas volume.

The current produces a voltage drop across a resister. The voltage drop is coupled as a negative pulse into an amplifier circuit, sent up the wireline, detected by the computer and counted over a short time interval. The computer translates the number of pulses into a curve on the log.

This type of tool is often called a Gun Gamma Ray as the tool is rugged enough to survive the detonation of perforating guns and other explosive devices.

Schematic diagram of Geiger-Mueller detector

The scintillation gamma ray detector is much more efficient and thus more sensitive than the Geiger-Mueller detector. The detector consists of a sodium iodide crystal and a photomultiplier. When a gamma ray enters the crystal, a photon (a tiny speck of light) is emitted. The photomultiplier amplifies this tiny pulse into a useable electrical signal that can be sent up the wireline to the computer. The crystal degrades with rough handling and shocks so, depending on tool design, it may not be suitable for all cased hole applcations.

Schematic diagram of NaI Scintillation detector

Some Definitions
Dead time is the interval of time where the detector is incapable of responding to a
second incoming gamma ray.

Resolve time is the time between two barely recordable pulses.

Variation of count rate due to the random disintegration of radio-active material are called statistical variations. It is reduced by averaging the raw output from the GR tool over a specific distance or time interval using an averaging formula. The longer the filter distance, the less defined the formation bed boundaries are on the log. Typical filters are 1 or 2 seconds long and logging speed is set so that this time represents 2 or 3 feet of travel. G-M detectors need longer time constants and slower logging speeds to achieve a good quality log.