Geothermal Well Temperature Measurement and Logging Services

Geothermal well temperature measurement (well temperature survey) is a core geophysical logging technique used to determine the vertical temperature distribution, geothermal gradient, and reservoir temperature of geothermal wells under static, flowing, or injection conditions. Accurate downhole temperature data are essential for geothermal resource assessment, reservoir management, well completion evaluation, and the safe design of production and reinjection facilities. This article outlines the technical principles, measurement items, methods, applicable standards, and practical applications of geothermal well temperature surveys.

Technical Overview

Geothermal well temperature measurement refers to the systematic acquisition of temperature data along the wellbore of a geothermal well, typically by running calibrated downhole sensors on a wireline, by deploying fiber-optic distributed sensing systems, or by installing permanent monitoring instruments. Because temperature is the single most important physical parameter governing geothermal resource quality, well temperature surveys form the technical foundation of geothermal exploration, reserves estimation, and field development.

A temperature survey produces a temperature-versus-depth profile (the well temperature curve) from which key parameters are derived, including the geothermal gradient, the static formation temperature, the bottom-hole temperature, and any thermal anomalies associated with aquifers, fracture zones, lost-circulation intervals, or wellbore leakage. Measurements are performed under clearly defined well conditions, principally static (shut-in), flowing (production), or injection states, since the thermal regime of the wellbore differs significantly among these states.

Professionally executed temperature surveys follow recognized standards such as GB/T 11615 (geothermal resources geological exploration), DZ/T 0075 (geophysical well logging specifications), and DZ/T 0260 (technical requirements for geothermal well drilling engineering), and are commonly carried out by accredited third-party laboratories equipped with traceably calibrated downhole instrumentation.

Testing Samples

Geothermal Production Wells

Completed geothermal production wells supplying heating, spa, agricultural, or power-generation loads. Both shut-in profiles and flowing profiles are measured to characterize reservoir temperature and to identify the main producing intervals during fluid withdrawal.

Reinjection (Injection) Wells

Wells used to return spent geothermal fluid to the reservoir. Temperature logs under injection conditions reveal the depth of fluid entry, injection profile distribution, and potential cooling anomalies within the reservoir.

Exploration and Appraisal Wells

Newly drilled wells in which bottom-hole temperature (BHT) measurements are taken shortly after drilling stops. Because circulating drilling mud suppresses the true formation temperature, these data require correction, such as Horner-type extrapolation, to estimate the undisturbed static formation temperature.

Observation and Monitoring Wells

Wells dedicated to long-term reservoir surveillance. Repeated temperature profiling over months or years tracks thermal depletion, inter-well interference, and reinjection breakthrough in producing geothermal fields.

Shallow Boreholes for Ground-Source Heat Pumps

Shallow vertical borehole heat exchangers for ground-coupled heat pump systems, where undisturbed formation temperature and initial ground temperature profiles are required inputs for thermal response testing and system design.

Testing Items

Typical measurement and interpretation items included in a geothermal well temperature survey comprise:

• Static (shut-in) wellbore temperature profile, producing the complete temperature-depth curve after thermal stabilization.
• Flowing temperature profile under stable production conditions, used to locate producing zones and evaluate drawdown behavior.
• Injection temperature profile under stable reinjection conditions.
• Bottom-hole temperature (BHT) measurement at total depth.
• True static formation temperature determination, using extrapolation methods such as the Horner plot to remove drilling-induced thermal disturbance.
• Geothermal gradient calculation, conventionally expressed in °C per 100 m of depth.
• Terrestrial heat flow estimation, combining gradient data with measured or inferred thermal conductivity.
• Identification of temperature anomalies, including gradient reversals, isothermal intervals, and localized departures indicative of lost circulation zones, cross-flow behind casing, channeling, or casing leakage.
• Cement top indication, based on the abrupt thermal anomaly produced by cement hydration heat after well completion.
• Long-term time-series temperature monitoring data for reservoir management and reinjection performance evaluation.

Testing Methods

Wireline Downhole Temperature Logging

A calibrated temperature probe, typically based on a platinum resistance sensor (PT100/PT1000) or a precision thermistor, is lowered on an armored wireline and either logged continuously at a controlled speed or stopped at fixed depth stations until sensor stabilization. Standard-grade tools achieve accuracy on the order of ±0.5 °C, while high-precision tools reach ±0.05 °C or better with resolutions of 0.01 °C. Depth control is maintained through calibrated measuring wheels and magnetic markers, with depth error typically held below 0.2% of measured depth.

Point-by-Point Static Measurement

For high-accuracy static surveys, the probe is halted at selected depth stations and readings are recorded only after the sensor has fully equilibrated. This stop-and-go method minimizes the lag error caused by sensor thermal inertia and fluid movement in the wellbore.

Distributed Temperature Sensing (DTS)

Fiber-optic DTS systems exploit temperature-dependent Raman backscattering in the optical fiber to deliver a continuous, meter-scale-resolution temperature profile along the entire wellbore in real time. DTS is particularly valuable for continuous monitoring of production, injection, and well-integrity behavior because it captures the full profile simultaneously rather than sequentially.

Static Formation Temperature Extrapolation

For newly drilled wells, a series of bottom-hole temperature readings acquired at increasing shut-in times is extrapolated using the Horner method or analogous time-temperature extrapolation techniques to reconstruct the undisturbed formation temperature that the drilling process temporarily suppressed.

Data Quality Assurance

All sensors are verified before and after each survey against certified reference thermometers or temperature baths with traceable calibration. Repeat runs are performed over key intervals to confirm repeatability, and documented quality-control procedures consistent with the general requirements of ISO/IEC 17025 are applied by accredited third-party laboratories.

Applications

Geothermal well temperature measurement data directly support a wide range of engineering and resource-management activities:

• Geothermal resource assessment and reserves classification, where verified reservoir temperature and gradient determine resource grade and exploitable potential.
• Reservoir characterization, including the identification of productive fracture and permeability intervals indicated by thermal anomalies.
• Well completion evaluation, using cement-top identification and post-grouting hydration anomalies to verify annulus sealing quality.
• Well integrity diagnostics, detecting casing leaks, micro-annulus channeling, and cross-flow between formations behind casing.
• Reinjection management, monitoring cold-water plume migration and thermal breakthrough between injection and production wells.
• Thermal facility design, providing design temperatures for downhole pumps, wellhead equipment, piping, heat exchangers, and corrosion-resistant material selection.
• Ground-source heat pump engineering, supplying undisturbed ground temperatures and initial conditions for thermal response tests and borehole field design.
• Regulatory compliance and environmental reporting, documenting reservoir conditions for resource administration authorities.

FAQ

How long should a well be shut in before a static temperature survey?

The shut-in period must be long enough for the wellbore to thermally re-equilibrate with the formation. As a general guideline, at least several days of shut-in are recommended for shallow-to-medium wells, and longer periods for deep wells or wells with heavy drilling-fluid invasion. When full stabilization is impractical, the stabilized temperature is estimated by extrapolation from serial bottom-hole measurements.

What accuracy is required for geothermal well temperature measurement?

For reservoir temperature and gradient determination, an accuracy of ±0.5 °C is generally acceptable, while resource-grade verification, heat-flow studies, and monitoring applications typically demand precision instrumentation with accuracy of ±0.05 °C to ±0.1 °C. All instruments should hold valid traceable calibration certificates.

Can temperature logs detect problems behind the casing?

Yes. Abnormal temperature behavior, such as localized cooling or warming against the normal geothermal gradient, frequently reveals cross-flow, channeling behind casing, casing leaks, or lost-circulation zones. Cement hydration heat is also used to confirm the top of cement after completion.

What is the geothermal gradient and why does it matter?

The geothermal gradient is the rate of temperature increase with depth, conventionally reported in °C per 100 m. It is calculated from the linear portion of the static temperature-depth curve and is a primary indicator of geothermal resource potential, a key input to reserves estimation, and a design basis for well depth and completion planning.

What are the advantages of fiber-optic DTS over conventional probes?

Distributed Temperature Sensing provides a continuous profile along the entire wellbore simultaneously, with no depth-sequencing lag, enabling real-time observation of transient thermal events such as injection front movement, leaking intervals, and production zone behavior during flow testing. Conventional high-precision probes remain preferred for certified point measurements and absolute temperature verification.