A flowing temperature gradient survey runs a temperature sensor slowly up or down a producing wellbore and records temperature against depth while the well is flowing. Undisturbed rock follows a smooth geothermal gradient, so any bend or step in the flowing profile marks a place where fluid is entering, leaving, or moving behind pipe. Because it needs only a temperature reading, it is one of the simplest and most robust diagnostic passes a production logging string can make.
Temperature Gradient Survey in one line: A flowing temperature gradient survey is a wellbore temperature log recorded during production, compared against the natural geothermal gradient. Departures from that gradient, such as sudden warming or cooling at a depth, reveal fluid entry points, crossflow between zones, or leaks that need investigation.
In a shut-in well left alone long enough, temperature increases with depth along a steady geothermal gradient set by the local heat flow of the earth. When the well is put on production, moving fluid carries heat with it and disturbs that clean line. The flowing temperature gradient survey captures the disturbed profile, and the interpreter looks for the depths where the curve deviates from the expected geothermal trend rather than at the absolute temperature itself.
Two physical effects dominate what the log shows. Liquid entering the wellbore tends to arrive close to the formation temperature at that depth, so a producing liquid zone often warms the profile toward the geothermal line. Gas behaves differently: as it expands across a restriction or into the wellbore it cools by the Joule-Thomson effect, so a gas entry frequently shows up as a cooling anomaly. Reading warming versus cooling helps distinguish what is entering and where.
Because the flowing profile blends contributions from every active zone above the sensor, interpreters usually run the survey together with a shut-in temperature pass and other production logging measurements. The shut-in log establishes the baseline geothermal gradient for that specific well, and the difference between the flowing and shut-in curves isolates where and how strongly fluid is moving.
The classic use is locating inflow entry points along a completion. A perforated interval that is actually contributing bends the flowing curve at its depth, while a perforation that is not producing leaves the curve undisturbed. This lets an operator confirm which zones are alive and which are dead without relying solely on a flowmeter, which can be unreliable in low-rate or multiphase conditions.
Temperature is also sensitive to flow that a spinner never sees. Crossflow, where fluid exits one zone and re-enters another behind pipe or through the wellbore, leaves a temperature signature even when net surface production is small. The same is true of a casing or tubing leak and of channeling behind cement: fluid moving where it should not still carries heat, so the temperature anomaly appears at the leak or channel depth even though nothing shows at surface.
This makes the flowing temperature gradient survey a distinct diagnostic rather than a substitute for other tools. A distributed temperature sensing fiber gives a continuous, permanent thermal picture, and a surface tubing head temperature reading gives one point at the wellhead. The gradient survey sits between them as a purpose-run wireline pass that produces a detailed depth profile on demand, useful when there is no fiber installed and a single surface reading cannot tell you which zone is responsible.
A temperature gradient survey is a snapshot taken during a logging job, but its value grows when it is filed against the continuous data the field already collects. When a well trends toward higher water cut or falling rate on the SCADA, the surface symptoms rarely name the culprit zone. Scheduling a flowing temperature survey turns that surface trend into a depth-resolved answer, and storing the interpreted result alongside the tag history gives the next engineer the context for why a zone was shut off or a workover was ordered.
In a cloud SCADA environment, the survey result becomes a reference layer on the well record. Operators comparing the surveyed entry points against ongoing wellhead pressure and temperature trends can watch for the profile changing over time, for instance a new cooling anomaly appearing months later that suggests a fresh gas breakthrough or a developing leak. The continuous surface data flags that something moved; the periodic gradient survey explains where.
For field teams, the practical workflow is to trigger a survey when remote monitoring shows an unexplained change, run the log during stable flow, and then annotate the well in the monitoring platform with the interpreted entry points, crossflow, or leak depth. That keeps the diagnostic and the live data in one place, so the decision to remediate is backed by both the real-time trend and the physical evidence from the wellbore.
A shut-in survey is run after the well has been static long enough for the wellbore to return toward the natural geothermal gradient, giving a clean baseline. A flowing survey is run during production, when moving fluid distorts that baseline. Comparing the two isolates exactly where fluid enters or moves, because the flowing curve departs from the shut-in line at those depths.
Gas expanding as it enters the lower-pressure wellbore cools by the Joule-Thomson effect, so the flowing temperature dips at the entry depth. Liquid entries usually do the opposite and warm the profile toward the formation temperature. Reading whether an anomaly is a cooling or a warming feature helps identify the phase and the mechanism of the inflow.
Yes. Fluid moving through a channel behind casing or through a casing leak carries heat, so it leaves a temperature anomaly at the leak or channel depth even when little or nothing shows at surface. This sensitivity to flow that a spinner cannot see is a major reason temperature logs are run for integrity diagnosis and not just for inflow profiling.
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