temperature log, Oil & Gas Glossary

What Is a Temperature Log?

A temperature log is a continuous wellbore measurement that records formation temperature as a function of depth using a sensitive resistance thermometer or thermocouple sensor on a wireline or LWD tool, used to determine the geothermal gradient for pore pressure prediction and drilling design, detect fluid movement behind casing (injection conformance, lost circulation zones, cement channelling), identify flowing perforations, and monitor steam injection profiles in thermal EOR operations.

Key Takeaways

Static temperature logs measure undisturbed formation temperature; run-in-hole logs may be disturbed by drilling fluid circulation.
Geothermal gradient averages 25-30°C per kilometre in most sedimentary basins; higher gradients indicate heat sources or thin crust.
Differential temperature anomalies (departures from smooth background gradient) indicate fluid movement, gas entry, or exothermic reactions.
Cement exothermic hydration creates a positive temperature anomaly detectable for 12-48 hours after cementing.
Gas expansion produces a cooling anomaly (Joule-Thomson effect) at the gas entry point in producing wells.

How Temperature Logs Measure Formation Heat Flow

A temperature logging tool uses a platinum resistance thermometer (PRT) or a thermistor with precision of ±0.01-0.1°C to measure fluid or formation temperature as the tool is pulled through the wellbore. In a static well (pumps off, no circulation), the temperature measured by the tool at each depth approaches the undisturbed formation temperature after sufficient equilibration time. In practice, the wellbore temperature is disturbed by drilling fluid circulation (which cools the bottom of the well and warms the upper section), meaning that a truly static temperature log requires the well to be shut in for several hours to days after drilling — a requirement called the static temperature survey.

The undisturbed formation temperature increases with depth following the geothermal gradient, typically 25-35°C per kilometre in sedimentary basins. This gradient is controlled by the heat flow from the Earth's interior, the thermal conductivity of the rock, and the presence of radioactive heat-producing elements in granites or shales. Local departures from the smooth geothermal gradient — positive anomalies (warmer than expected) or negative anomalies (cooler than expected) — indicate fluid movement, chemical reactions, or formation property changes. A positive anomaly at a cemented interval indicates exothermic hydration of Portland cement. A negative anomaly in a producing gas well indicates Joule-Thomson cooling as expanding gas absorbs heat from the formation. A positive anomaly at a known tight zone may indicate lost circulation during drilling. All of these diagnostic applications rely on the temperature log as a sensitive fluid movement and chemical activity indicator.

Temperature Log Applications Across International Jurisdictions

In Canada, temperature logs are run as standard practice in WCSB wells for geothermal gradient determination, which feeds into pore pressure prediction models for subsequent wells in the area. AER regulations for well completion require that bottomhole temperature be reported in the well completion report; the maximum temperature encountered during drilling is reported from the maximum reading thermometer on the drillstring, while the accurate formation temperature is determined from the temperature log. SAGD wells in Athabasca oil sands use continuous temperature surveys to monitor steam chamber growth: the advancing steam front creates a sharp temperature transition from ambient formation temperature (5-12°C at shallow depths) to steam temperature (160-200°C), and time-lapse temperature logging maps the steam chamber development over time.

In the United States, temperature logs are used for injection conformance verification in Gulf of Mexico water injection wells, where the temperature anomaly at the injected water entry points indicates which perforations are taking fluid. BSEE production logging requirements for OCS wells include temperature log data for monitoring injection profiles and verifying cement integrity. In Norway, Sodir regulations require maximum formation temperature data from exploration wells; temperature logs from NCS exploration wells contribute to the basin-wide thermal model used for petroleum maturation assessment and trap timing. In the Middle East, temperature surveys in Arab Formation producers at Ghawar record bottomhole temperatures of 85-100°C — important for designing completion materials, tubing metallurgy, and fluid properties models that must account for thermal effects at the producing depth.

Fast Facts

A temperature anomaly of as little as 0.1°C is detectable by precision temperature logging tools, making it possible to identify even minor fluid movements that cause only slight temperature departures from the geothermal trend. Gas entry into a producing wellbore causes a Joule-Thomson cooling effect of approximately 0.2-0.5°C per 100 kPa of pressure reduction, meaning a gas entry point with 1 MPa drawdown can produce a temperature anomaly of 2-5°C that is easily visible on the temperature log. This thermal fingerprinting of gas entry, water entry, and injection conformance makes the temperature log one of the most sensitive and versatile diagnostic tools in production logging, providing fluid movement information at a fraction of the cost of more complex multi-arm spinner or holdup tools.

Cement Evaluation Using Temperature Logs

The hydration of Portland cement is a strongly exothermic reaction — freshly placed cement generates 200-350 kJ per kilogram of cement as it hydrates. This heat causes the wellbore temperature to increase measurably (typically 10-30°C above ambient) in the cemented annular zone during the first 12-48 hours after cementing. A temperature log run within 24 hours of cementing provides a qualitative indication of where cement fill is present: zones with temperature elevation above the geothermal baseline indicate active cement hydration, while zones at or below ambient temperature indicate either no cement fill or channels in the cement where borehole fluids remain and no hydration is occurring. This temperature-based cement evaluation is qualitative and less precise than acoustic cement bond logs (CBL/USIT), but provides a rapid and inexpensive first check on cement coverage before the more expensive acoustic evaluation is run after cement has hardened.

Tip: When running a temperature survey to evaluate cement placement, plan to log within 12-24 hours after cement placement while the hydration reaction is still generating measurable heat. If the survey is run too soon (less than 4-6 hours), the cement exotherm may not yet be fully developed. If run too late (more than 48-72 hours), the heat has dissipated and the temperature has returned toward ambient, erasing the cement hydration signature. For SAGD wells or other thermal operations where cement quality is critical for steam containment, the temperature survey provides a preliminary cement coverage check that guides the decision of whether to wait for acoustic bond logging or to attempt a remedial cement squeeze immediately.

Temperature log is also referenced as:

Continuous temperature survey — used in well completion reports when the temperature measurement was made as a continuous log from total depth to surface; distinguished from a maximum reading thermometer which records only the single maximum temperature encountered
Geothermal survey — used when the primary purpose is formation temperature gradient determination rather than fluid movement diagnostics; emphasises the geothermal heat flow measurement objective
Temperature profile — the curve of temperature versus depth produced by the logging tool; used when referring to the log data display rather than the acquisition method

Frequently Asked Questions

Why do temperature logs need to be run after the well has equilibrated?

Drilling fluid circulation disturbs the natural geothermal temperature profile by transporting heat from depth to surface via the circulating mud. The rotating drillstring also generates frictional heat near the bit, further modifying the temperature profile. When circulation stops (at the end of a drilling section), the disturbed temperature profile begins to equilibrate back toward the undisturbed geothermal gradient, but this equilibration takes time proportional to the depth and degree of disturbance. A temperature log run immediately after stopping circulation will show a significantly cooler profile in the deep section (cooled by circulating fluid) and warmer in the upper section than the true formation temperature. In practice, waiting 8-24 hours after stopping circulation is typical for getting a reasonably accurate static temperature measurement at formation evaluation depths; for very precise geothermal gradient determination, waiting 3-7 days and applying Horner-type temperature equilibration correction (analogous to pressure buildup analysis) provides the most accurate formation temperatures.

How is the temperature log used for gas entry detection in producing wells?

In a producing gas well, gas enters the wellbore through perforations at reservoir pressure and then expands as it moves up the wellbore to lower pressures. Gas expansion is an endothermic (heat-absorbing) process described by the Joule-Thomson effect: expanding gas cools in proportion to the pressure drop and the gas's Joule-Thomson coefficient. For most natural gas compositions, the Joule-Thomson coefficient is approximately 4-7°C per 10 MPa of pressure drop, meaning a gas entry with 5 MPa drawdown will cool the wellbore by 2-3.5°C at the entry point. This cooling anomaly is localised to the gas entry perforations and is visible on a temperature log as a negative departure from the background geothermal trend. Multiple gas entry points at different depths can be identified as separate cooling anomalies. The magnitude of each anomaly is proportional to the gas flow rate at that depth, providing a qualitative allocation of production between multiple perforated intervals.

Why Temperature Logs Matter in Oil and Gas

Temperature is a fundamental physical parameter that influences every aspect of wellbore operation and reservoir behaviour: fluid viscosity and density, chemical reaction rates, cement hydration kinetics, gas dissolution in liquids, polymer degradation, corrosion rates, and thermodynamic phase equilibria all depend directly on temperature. The temperature log provides the downhole temperature measurement that underpins all of these engineering calculations. For pore pressure prediction, the temperature gradient feeds into velocity-pressure transforms. For well integrity, the cement exotherm temperature log validates cementing quality before the well is completed. For production optimisation, the temperature anomaly pattern reveals which perforations are producing and at what relative rates. At the cost of a simple resistivity measurement with no radioactive sources and no high-voltage current injection, the temperature log delivers fundamental diagnostics for wellbore integrity, cement quality, and fluid movement that would otherwise require much more expensive and operationally complex measurements.