Radial Differential Temperature Log

A radial differential temperature log is a wellbore measurement that records the difference in temperature between multiple points at the same depth but at different radial positions within the borehole — typically comparing the temperature of fluid at the borehole wall against fluid at the center of the wellbore, or comparing measurements at azimuthally offset sensor positions around the borehole circumference — used to detect localized heat sources or heat sinks within the formation adjacent to the wellbore, including behind-casing fluid migration, channeling through cement, formation fluid entry, and gas expansion effects that create temperature anomalies distinguishable from the ambient geothermal gradient.

Key Takeaways

The radial differential temperature log exploits the physics of convective heat transfer between formation fluids and the wellbore — fluid entering the borehole from the formation carries the temperature of its source zone, while fluid circulating from the surface or being displaced in the annulus carries a different thermal signature; by comparing temperatures at different radial positions at the same depth simultaneously, the tool isolates local heat exchange effects from the background geothermal trend that affects all sensors equally, making the differential measurement more sensitive to fluid movement than absolute temperature alone.
The primary application of radial differential temperature logs is cement integrity evaluation after primary cementing — if cement behind the casing has channeled or failed to seal an interval, formation fluids (gas, water, or oil at formation temperature) migrate through the channel and exchange heat with the casing, creating a temperature anomaly detectable as a differential between sensors at different radial positions or as a deviation from the expected thermal recovery curve; gas migration through cement channels is particularly well-detected because adiabatic expansion of gas as it moves from higher to lower pressure creates a strong cooling anomaly at the leak location.
Radial differential temperature logging is conducted after cementing operations during the cement curing period — typical logging windows are 12 to 48 hours after cementing, when the cement exothermic heat of hydration has begun to dissipate and formation fluid migration through any cement channels creates detectable anomalies against the cooling background; logging too early (within 6 hours) can mask channeling anomalies with the intense heat of cement hydration, while logging too late (beyond 72 hours) may miss early gas migration events that self-seal as the wellbore pressure equilibrates.
Modern differential temperature tools combine the radial differential measurement with absolute temperature sensors, caliper measurements, and gamma ray or casing collar locator (CCL) tracks to correlate temperature anomalies with specific depth intervals, formation tops, and casing joint positions — the integrated log display allows the cement engineer to identify whether an anomaly corresponds to a known permeable formation interval, a casing coupling point (which can simulate a temperature anomaly by affecting local heat conduction), or a genuine cement channel requiring remedial cementing action.
Beyond cement evaluation, radial differential temperature logs are used during production logging to identify behind-casing communication between producing and non-producing zones, to locate water or gas influx points in producing wells, and to evaluate the effectiveness of temporary and permanent well isolations — the differential measurement is less affected by wellbore flow conditions than absolute temperature, making it useful for identifying localized inflow events even when the borehole fluid is in turbulent flow from the production stream.

Fast Facts

Temperature logging in oil wells dates to the 1930s, when simple resistance thermometers were lowered into completed wells to detect temperature anomalies associated with gas entry and behind-casing flows. The differential temperature log — measuring temperature differences rather than absolute temperatures — was developed in the 1950s and 1960s as analog electronic improvements allowed the precise, simultaneous comparison of multiple sensor outputs. Modern digital differential temperature tools achieve temperature resolution of 0.01°C to 0.05°C and sampling rates of 1 to 5 samples per meter of depth, sufficient to detect small fluid entry events. Schlumberger (now SLB), Halliburton, and Baker Hughes each market integrated temperature-based cement evaluation tools that include differential and absolute temperature, ultrasonic cement bond, and natural gamma ray in a single combinable string.

What Is a Radial Differential Temperature Log?

Temperature in a wellbore is not uniform across the borehole cross-section — fluid at the formation wall may be exchanging heat with adjacent rock or migrating formation fluids, while fluid at the center of the borehole is insulated from that exchange by the intervening fluid column. By measuring temperature at multiple radial positions simultaneously, a differential temperature tool captures the spatial variation in heat exchange across the borehole diameter, creating a measurement that is highly sensitive to local heat sources and sinks at specific depths.

The key advantage of the differential measurement over absolute temperature is its insensitivity to broad-scale temperature trends. The geothermal gradient — the background increase in temperature with depth due to heat flow from the Earth's interior — affects all sensors equally. Subtracting one sensor reading from another eliminates the geothermal gradient from the result, leaving only the differential effects caused by local heat exchange. A fluid migrating from a hot zone, gas expanding through a channel, or exothermic cement hydration at a specific interval creates a differential signal at that depth that stands out clearly against the zero-differential baseline of undisturbed rock.

In practice, radial differential temperature logs are most valuable as part of an integrated wellbore evaluation program — the temperature data is interpreted alongside cement bond logs, ultrasonic measurements, casing inspection tools, and well history data to build a complete picture of wellbore integrity. A temperature anomaly alone may have multiple explanations; correlated with cement bond quality and formation permeability data, it becomes a definitive diagnosis of the fluid migration mechanism.

Radial Differential Temperature Log Operations and Interpretation

The logging tool is run on wireline or as part of a memory-logging string lowered through the casing on tubing or coiled tubing. Multiple temperature sensors — typically platinum resistance thermometers or thermistors — are positioned at different radial distances from the tool body, typically on extendable arms or pads that maintain contact with the casing wall while the central sensor measures the fluid temperature at the borehole center. The differential signal is computed in real time at surface or downloaded from memory at tool retrieval.

Interpretation begins with identification of the temperature baseline — the expected temperature profile based on known geothermal gradient, the thermal effects of the wellbore construction sequence, and any known circulation or production events that have altered the thermal state of the wellbore. Anomalies are classified by their thermal signature: warm anomalies (positive differential) indicate heat influx from hotter formation fluids or exothermic reactions; cool anomalies (negative differential) indicate influx of cooler fluids, gas expansion cooling, or heat withdrawal into cooler formations. The spatial extent of the anomaly — sharp peaks versus broad gradients — indicates whether the heat source is localized (a discrete fracture or casing leak) or distributed (a permeable interval accepting or contributing fluid along a significant length).

Gas channeling behind casing produces a characteristic signature: a strong negative anomaly (cooling) at the point of gas entry into the channel due to Joule-Thomson expansion, followed by a broad warm anomaly at shallower depths as the gas recompresses and transfers heat back to the wellbore. This paired cool-then-warm signature is diagnostic of behind-casing gas migration and cannot be easily confused with other wellbore heat exchange phenomena.

Radial Differential Temperature Log Across International Jurisdictions

Canada (AER / WCSB): Alberta Energy Regulator Directive 009 (Casing Cementing Minimum Requirements) requires cement evaluation logging for wells that penetrate certain formations, and radial differential temperature logs are among the accepted methods for demonstrating cement integrity behind surface casing and intermediate casing strings. WCSB operators in the Peace River and Montney play areas — where sour gas (H₂S) and CO₂-rich formations require confirmed cement isolation from shallow freshwater aquifers — use temperature logging combined with acoustic cement bond logs to satisfy AER Directive 009 cement evaluation requirements. CNRL and Tourmaline operating programs specify post-cementing temperature survey timing (typically 24-hour wait after casing cement) before running evaluation logs.

United States (API / BSEE): API Standard 65-2 (Isolating Potential Flow Zones During Well Construction) references temperature logging as one of several techniques for evaluating cement seal effectiveness in wellbores that penetrate potential flow zones. BSEE requirements for well integrity on the OCS (30 CFR 250, Subpart B) mandate cement integrity demonstrations for surface, intermediate, and production casing strings in offshore wells, and radial differential temperature logs are used by Gulf of Mexico operators as part of the cement evaluation suite for high-pressure wells, subsalt wells, and wells with known cement placement challenges (unstable boreholes, lost circulation zones, high-density mud columns). Halliburton's Radial Differential Temperature (RDT) tool and SLB's DIFT (Differential Temperature) service are widely used on the GOM shelf and deepwater.

Norway (Sodir / NORSOK): Norwegian Continental Shelf well integrity regulations under the Norwegian Petroleum Directorate's (Sodir) well integrity guidelines require cement evaluation for all production and injection wells, with temperature logging accepted alongside acoustic and ultrasonic cement bond methods. NCS operators including Equinor, Aker BP, and ConocoPhillips Norway use temperature log suites for evaluation of cement in the North Sea chalk and sandstone reservoirs, particularly for wells that penetrate the shallow gas hazard zone above the Ekofisk chalk where cement integrity is critical for well control and environmental protection. NORSOK D-010 Well Integrity in Drilling and Well Operations provides the regulatory framework for cement evaluation requirements.

Middle East (Saudi Aramco): Saudi Aramco's well integrity standards require cement evaluation for all wells in the Arab Formation gas cap and underlying aquifer zones, where behind-casing communication could compromise the reservoir seal maintained by the overlying evaporite cap. Aramco uses temperature logging — including radial differential temperature tools — as part of the cement evaluation suite for Arab Formation development wells, particularly in the Ghawar, Safaniya, and Shaybah fields where the vertical extent of the Arab D reservoir requires thorough confirmation of zonal isolation between the gas cap, oil column, and water leg. Aramco's Dhahran drilling technology group has published guidelines on temperature log interpretation for Arab Formation well conditions.

The radial differential temperature log is also called an RDT log, differential temperature survey, or temperature differential log in wireline logging service catalogs. Related terms include temperature log (geothermal survey), cement bond log (CBL), ultrasonic cement evaluation, behind-casing flow, cement channeling, geothermal gradient, Joule-Thomson cooling, production logging, and well integrity evaluation. The distinction between a radial differential temperature log (comparing temperatures at different radial positions at the same depth simultaneously) and a standard temperature log (measuring absolute temperature at a single point as a function of depth) is that the differential measurement eliminates the geothermal background trend and isolates local heat exchange effects, making it more sensitive to fluid migration anomalies than absolute temperature alone.