TEMPERATURE LOGGING BASICS
Temperature logging began around 1846 when William Thomson (Lord Kelvin) made measurements of temperature in water wells in England. His first technical paper on the subject was "Age of the Earth and its Limitations as Determined by the Distribution and Measurement of Heat within It". Kelvin's calculated age was 20 - 40 million years. Since radioactivity had not been discovered yet, Kelvin was unaware of the heat generated internally from this source, so he can be excused for a 100-fold error in his estimate of the Earth's age. Controversy, debate, and a slew of additional papers ensued for another 50 years.

The first wireline temperature log was run in 1933. Many modern logging tools have a temperature sensor built-in, so a separate device is not always needed. These are not highly accurate but sufficient for most petrophysical analysis models. High resolution and rapid response temperature logs are needed for some reservoir engineering tasks, such as gas leak and crossflow detection.

The temperature sensor is a thermistor exposed to the borehole fluid. A platinum element is usually used, as the electrical resistance varies linearly with temperature over a wide range and is stable over time.

According to PetroWiki, "The temperature tool is most effective when located at the bottom of a tool string. In a production well, the tool should always be logged downward so as to enter undisturbed fluid. The log should be recorded at a constant logging speed, not to exceed 30 ft/min."
 

Typical temperature log in a flowing gas well shows cooling at the perforations due to gas expansion into the casing, and a very slight warming as the gas enters the tubing. The log returns to geothermal gradient below the perfs. A derivative curve helps locate small changes in temperature. A casing collar locator (CCL) is shown in Track 1 for depth control - a gamma ray curve is often displayed as well.

APPLICATIONS OF TEMPERATURE LOGS
Temperature logs are used to establish local and regional formation temperature, temperature gradients in oil, gas, and geothermal wells, and gas inflow in open and cased hole, Other applications are (or were) location of cement top after setting casing, assessment of perforation efficiency in production and injection wells, as well as detection of crossflows and gas flows behind casing.

The temperature log is an integral part of all production logging operations as it is essential in assessing multi-phase flow rates, especially in deviated and horizontal wells. 
 

They have been widely used to assess completions in producing and injection wells. Here are two examples.

 
Temperature logs in oil production well (left) and water injection well (right). Production carries heat uphole so log is above but parallel to geothermal gradient curve. Injection carries cooler temperatures downhole so log is below geothermal gradient curve. The amount of heating or cooling depends on flow rate and specific heat of the produced or injected fluid. In both cases, the log merges with the geothermal gradient curve below the perfs. If a well is shut in, it gradually returns to the local formation temperature. (Images courtesy Western Atlas)
 

The "forgotten" log, the temperature survey in open hole, might be useful if some gas has evolved into the wellbore prior to logging. There is a temperature sensor on most modern logging tool strings - just ask for it to be displayed.

<== Temperature log recorded with density neutron log.  ln open hole over a gas shale shows cooling (highlighted in red) due to gas inflow. Vertical black bar indicates interval to be completed.
 

Temperature logs in geothermal wells are essential for both exploration and development.

Temperature logs from a Canadian geothermal prospect in the Rocky
Mountains of BC show that some exploration wells do not reach geothermal criteria. (GSC image) ==>

Back in the “good old days” before the invention of sonic logs, there was no genuine cement integrity log. However, the location of the cement top was often required, either to satisfy regulations or for general knowledge. Since cement gives off heat as it cures, the temperature log was used to provide evidence that the well was actually cemented to a level that met expectations.

<== In this example, the top of cement is located where the temperature returns to geothermal gradient. The log must be run during the cement curing period as the temperature anomaly will fade with time.