Acquiring wireline temperature measurements in open-hole.

Covered in this article:

❔Is a precise temperature measurement essential?

❔How is temperature data utilised in well log interpretation?

❔Which wireline services provide reliable temperature data?

Temperature is a critical parameter in petrophysics and well log data analysis, affecting various aspects essential for reservoir characterisation and management. Temperature influences the viscosity, density, and phase behaviour of reservoir fluids, which are vital for evaluating hydrocarbon volumes and designing effective production strategies. Additionally, temperature significantly impacts the electrical resistivity of formation fluids and rocks, making it crucial for accurate interpretation of resistivity logs used to infer hydrocarbon presence and saturation (ref). Temperature significantly impacts rock properties, affecting well log data interpretation. Elevated temperatures cause reversible changes in bulk and pore compressibility’s, and can lead to errors in estimating porosities from neutron and sonic transit-time logs (ref).

Wireline log temperature data is crucial for various aspects of reservoir characterization and management. Here are some of the reasons why temperature data is important:

1. Porosity and Permeability: Accurate temperature data allows for corrections in porosity and permeability measurements, leading to more reliable assessments of these petrophysical properties.
2. Cementation Factor: The cementation factor in Archie’s equation, used to determine water saturation, is temperature-dependent. Accurate temperature readings ensure the correct application of this factor, improving the reliability of saturation estimates.
3. Borehole Stability: Temperature gradients can cause thermal stresses in the borehole walls, affecting stability. Knowledge of temperature variations helps in planning and mitigating risks to maintain the integrity of the borehole and logging tools.
4. Chemical Reactions: Temperature influences geochemical reactions such as mineral dissolution and precipitation, which can alter reservoir characteristics like porosity and permeability over time. Accurate temperature data is essential for predicting and managing these changes.
5. Gas Hydrates: Gas hydrates are stable only within certain temperature and pressure conditions. Understanding the temperature profile is essential for identifying hydrate-bearing formations and managing their associated risks.
6. Well Construction and Completion Material Selection: Temperature data is critical for selecting appropriate materials for well construction and completion that can withstand thermal stresses and maintain integrity over the well’s lifespan.
7. Reservoir Production Management: Temperature affects fluid flow properties and phase behavior in the reservoir. Accurate temperature data helps optimize production strategies and enhance recovery.
8. Cement Design: Temperature influences the setting and bonding properties of cement used in wellbore construction. Accurate temperature profiles are necessary for designing cement slurries that ensure proper zonal isolation and well integrity.
9. Evaluation of Fractures: Temperature data helps in understanding the thermal effects on fracture propagation and closure. Accurate temperature readings are essential for modeling and managing fractures effectively.
10. Basin Model Calibration: Temperature data is used to calibrate basin models, which predict the thermal maturity and evolution of hydrocarbons. Accurate temperature profiles improve the reliability of these models.
11. Geothermal Energy Estimation: Temperature data is fundamental for assessing geothermal energy potential. Accurate temperature measurements help estimate the heat flow and thermal gradient, crucial for evaluating geothermal resources.
12. Basin Hydrodynamics: Temperature influences the movement and distribution of fluids within a basin. Accurate temperature data helps in understanding basin hydrodynamics, essential for effective reservoir management and exploration.
13. Hydrocarbon Maturation: The maturity of hydrocarbons depends upon the maximum temperature the organic remains have been subjected to, as well as time and pressure (ref)

By incorporating accurate temperature data from wireline logs, these processes and analyses become more precise, leading to better decision-making and optimized reservoir management.

Correcting for these temperature effects ensures more reliable petrophysical property estimates.However, to make effective corrections, requires an accurate understanding of the welbore temperature as an input.

A quick review of SLB’s log interpretation chart book (ref) emphasises the critical role of temperature plays in various wireline logs by accounting for its effects.

• Resistivity of NaCl Concentrations: Temperature affects the resistivity values of NaCl solutions, which can be referenced in Chart Gen-6.
• Gas Density: Temperature influences the density and hydrogen index of natural gas, detailed in Chart Gen-8.
• Sound Velocity of Hydrocarbons: Temperature affects the sound velocity in hydrocarbons, covered in Chart Gen-9.
• NMR Resonance Relaxation Times of Fluids: Temperature impacts the NMR relaxation times of water and hydrocarbons, as shown in Charts Gen-10, Gen-11a, and Gen-11b.
• Sigma (Capture Cross Section) of Water/Hydrocarbon Solutions: Temperature influences the sigma value of water and hydrocarbons, which is essential for calculating water saturation, found in Charts Gen-12, Gen-13, and Gen-14.
• Propagation Time of NaCl Water Solutions: Temperature affects the propagation time of NaCl solutions, detailed in Chart Gen-15.
• Attenuation of NaCl Water Solutions: Temperature impacts the attenuation values of NaCl solutions, as referenced in Chart Gen-16.
• Rw(eq) Determination from Static SP: Temperature data is crucial for determining the equivalent water resistivity from spontaneous potential logs, covered in Chart SP-1.
• Environmental Corrections of Compensated Neutron / APS (Porosity) Log: Temperature corrections are necessary for accurate porosity measurements in Charts Neu-10 and Neu-11.
• Laterolog Resistivity Correction: Temperature is a key factor in correcting resistivity logs, detailed in Chart RLl-1.
• Density Neutron Crossplot (Porosity Estimation): Temperature affects the cross-plot corrections used in porosity estimation, as shown in Charts Por-24 and Por-25.
• Saturation Determination (Archie’s Equation): Temperature influences the resistivity values used in Archie’s equation for saturation determination, referenced in Charts SatOH-3 and SatOH-4.

Without labouring the point, the critical importance of temperature is clear, yet somehow, over time, this measurement seems to have been overlooked by some. In many ways it seems that the development for tools and sensors available for measuring temperature have been sidelined and there are limited options available for obtaining an accurate borehole temperature measurement in open-hole.

How do we measure formation temperature with wireline logs?

In open-hole wireline logging we do not measure the static formation temperature directly. Sensors are used to measure the temperature of the wellbore fluid, which is assumed to be in thermal equilibrium with the surrounding formation, thereby providing an indirect measurement of the formation temperature. Of course there are a number of assumptions here which can lead to error if not accounted for in any computation:

• Thermal Equilibrium: Assumes the wellbore fluid and formation are in thermal equilibrium.
• Fluid Movement: Assumes no significant fluid movement that could alter the wellbore temperature.
• Heat Transfer: Assumes consistent and adequate heat transfer between the formation and the wellbore fluid.
• Tool Accuracy: Assumes the temperature sensors are properly calibrated and accurate.
• Undisturbed Conditions: Assumes the wellbore conditions are undisturbed prior to measurement.
• Wellbore Stability: Assumes no thermal effects from wellbore instability or drilling operations.

Therefore, to ensure accurate temperature measurements in open-hole wireline logging, several measures can be taken to minimize errors and correct for assumptions:

1. Ensure Thermal Equilibrium: Allow sufficient time for the wellbore fluid to reach thermal equilibrium with the surrounding formation before taking temperature measurements
2. Minimize Fluid Movement: Minimize operations that could cause significant fluid movement prior to logging, such as fluid injections or withdrawals. Implement a shut-in period to allow wellbore fluids to stabilize before logging.
3. Ensure Consistent Heat Transfer: Evaluate the thermal conductivity of the wellbore fluid and formation to ensure adequate heat transfer.
4. Maintain Tool Accuracy: Regularly calibrate temperature sensors to maintain accuracy and ensure they are functioning correctly.
5. Ensure Undisturbed Conditions: Assess and ensure that wellbore conditions are stable and undisturbed prior to taking temperature measurements. Avoid recent drilling or completion operations that might disturb the thermal equilibrium.
6. Account for Wellbore Stability: Continuously monitor wellbore conditions for signs of instability or thermal effects that could impact temperature measurements.

Naturally, not all of these factors are within an operator’s control, and there isn’t always sufficient time available before the start of an open-hole logging operation. However, considering these factors can improve temperature measurements.

Horner plots can be particularly helpful by providing a method to estimate formation temperature from buildup data over multiple runs, thereby enhancing the accuracy of temperature readings during the logging process.

Horner Plot for calculating static formation temperature

A tried and tested methodology to determine static formation temperature is to perform a Horner estimate by taking temperature measurements in the wellbore at different times after drilling (circulation) stops (ref). The Horner method, plots the measured temperature (at a given depth) from each of several logging runs, against log(T/(t+T)), where T is the time since circulation of the drilling fluid was stopped, and t is the length of time of circulation of drilling fluid prior to this. The goal is to estimate the static formation temperature, which is the temperature of the undisturbed formation (Note: keeping accurate records before and during logging is critical for this method to be effective).

Key Steps in Horner Plot Analysis

Data Collection:

• Shut-in Temperature Data: Record the bottom-hole temperature in the well at various times after the well is shut in.
• Circulation Time (𝘵𝘱): Note the total time the well was circulating fluid before being shut in.
• Buildup Time (Δt): The time elapsed since the well was shut in/circulation stopped.

Horner Time (tH) Calculation:

Calculate the Horner time for each temperature measurement using the formula:

Here, 𝘵𝘱 is the total production/circulation time before shut-in, and Δt is the time since shut-in.

Plotting:

• Plot the shut-in temperature (T(t)) on the y-axis.
• Plot the logarithm of Horner time (log(tH)) on the x-axis.
• This should yield a straight line if the temperature recovery follows a logarithmic trend.

Interpretation

Static Formation Temperature (Tf): Extrapolate the straight line to log(tH)=0 (i.e., tH=1). This intercept represents the formation temperature.

A sample horner plot is shown below (Ref)

Example wireline tools available accurate downhole temperature measurement

Measurement accuracy in temperature logging for wireline is influenced by a variety of factors, each playing a crucial role in ensuring precise and reliable data collection (ref).

• Logging Speed: One of the primary factors is the logging speed. Higher logging speeds can introduce significant errors in the measured temperature data because the sensor does not have adequate time to equilibrate with the wellbore fluid. This lack of equilibration results in inaccurate temperature readings, particularly in zones with large temperature differentials, such as near steamchests. Therefore, maintaining a constant and optimal logging speed is essential to minimize these errors and achieve accurate measurements.
• Sensor Type: The type and response time of the temperature sensor also significantly impact measurement accuracy. Resistance Temperature Detectors (RTDs) are generally preferred over platinum thermistors due to their superior repeatability, long-term stability, accuracy, and sensitivity to small temperature changes. However, the sensor’s response time—defined as the time required to reach 63.2% of the total output signal – can vary. Sensors with longer response times necessitate slower logging speeds to allow sufficient time for temperature equilibration. Consequently, faster logging speeds can result in delayed and inaccurate temperature responses, especially in dynamic thermal environments.
• Calibration: Calibration and maintenance of the temperature logging tools are paramount. Tools that are not properly calibrated or are malfunctioning can produce erroneous data, making it essential to perform quality checks and calibration before and after logging operations. Comparing time-lapse temperature surveys and ensuring consistency in logging procedures can help identify and mitigate these issues, thereby enhancing the reliability of the temperature data collected. Through careful management of these factors, the accuracy of temperature logging can be significantly improved, providing more reliable data for reservoir management and operational decision-making.

Typically, temperature logs are measured while the sensor is being lowered into the well. This is to reduce any temperature perturbations caused by the logging tool itself (ref).

Internal Cartridge Temperature

The accuracy of an internal cartridge temperature sensor for obtaining borehole temperature can vary based on several factors including the design and placement of the sensor, the thermal properties of the tool’s housing, and the thermal environment of the borehole. However, internal cartridge temperature sensors are generally less accurate than external sensors for a few reasons:

• Thermal Lag: The internal sensor is insulated by the tool’s housing, which can cause a delay in the sensor reaching the actual temperature of the borehole fluid. This thermal lag can result in less responsive and less accurate temperature readings, especially in dynamic environments where the temperature changes rapidly.
• Heat Dissipation: The tool’s electronics generate heat, which can influence the internal temperature reading. This self-heating effect can cause the internal sensor to register a higher temperature than the actual borehole fluid temperature, especially if the tool is operating under high power for extended periods.
• Positioning: The internal sensor measures the temperature at the tool’s location within its housing, which might not be in direct contact with the borehole fluid. This indirect measurement can lead to discrepancies compared to the true borehole temperature.

It is uncommon with most wireline tools for an internal electronics cartridge temperature to be calibrated and due to thermal lag, they are at best, useful only for:

• Relative Temperature Changes: Monitoring changes in temperature rather than absolute temperature values.
• Operational Simplicity: Simplifying the design and operation of temperature logging tools by eliminating the need for external sensors.
• Data Redundancy: Providing an additional temperature data point that can be compared against external sensors to verify readings.

For applications requiring high precision and accuracy, external temperature sensors are generally preferred. These sensors are in direct contact with the borehole fluid and can provide more immediate and accurate readings of the borehole temperature.

Flowline based RTD sensors in formtion fluid testing tools

Several major oilfield services companies offer wireline formation testing services that include fluid temperature measurement capabilities. Here are some examples of these services:

1. Schlumberger: Modular Formation Dynamics Tester (MDT / ORA / XPT) (note if you can explain why F is missing in the acronymn you will get a one&zero t-shirt)
2. Halliburton: Reservoir Description Tool (RDT)
3. Baker Hughes: Reservoir Characterization Instrument (RCI / RCX) / FTeX
4. Weatherford: Formation Pressure Tester (FPT)

The position at which the temperature measurements within these tools are located vary, but commonly all measure the flowline temperature of fluid whilst withi the tool, as opposed to borehole fluid temperature per se. The accuracy varies by provide but are generally very accurate to within 0.5 degC.

The main issue with this measurement in relation to estimating static formation temeprature is that typically the a formation testing tool will only be run once (perhaps twice) in the well and may not be taken to the bottom of the well. Hence the measurement is not consistent between runs for use with Horner type plots.

Wireline tools with external temperature sensors

Baker Hughes (TTRM) – The Baker Hughes TTRM (Tension Temperature and Mud Resistivity Module) is a wireline logging module that  used for measuring temperature, tension, and mud resistivity in wellbores. The TTRM tool includes a high-precision RTD temperature sensor that can operate between -55 degC and 245 degC providing real-time, continuous temperature readings. The resolution of the temperature measurement is 2 degC +/- 5%.

Halliburton (BHPT) – Similar to the Baker TTRM, Halliburton’s Borehole Properties Tool (BHPT) is a versatile wireline logging tool used by Halliburton for measuring various wellbore parameters, including temperature. This tool is designed to provide real-time data on pressure, temperature, and mud resistivity, which are essential for comprehensive wellbore evaluation and management. The temperature for the BHPT is provided by a Quartz Crytsal and is calibrated using a heat box. The resolution is reported as 0.05 degC with an accuracy of 1%. The temperature sensor can measure from 0 -177 degc.

SLB (EMS & LEH-MT / F) – SLB provide two main options for obtaining external temperatures as part of a wireline string. Either an external RTD sensor as part of the logging head in the LEH-MT or F with the MT having The resolution at 0degC is 0.3 degC, 100 degC is 0.35 degC, 0.55 at 200degC and 0.75degC at 300degC.

The EMS (ref – ems-br (slb.com)) is modular piece of equipment that contains a mud resistivity, temperetaure and 6 arm caliper measurement. The tool can be run in the toolstring without the caliper providing mud resistivity and temperature sensors, along with an electronics cardtridge. This tool is rated to 175degC and has a resolution of 0.1 degC and an accurance of +/-1 degC or 1%.

Maximum recording thermometers

Traditionally, the inputs used for measuring borehole temperature and delivering accurate Horner Plot method were provided using mercury based maximum reading thermometers. All of the vendors have slots within their toolstrings to house this thermometers, either in the wireline head in the case of Baker Hughes and SLB or at the foot of the toolstring in a dedicated housing in the case of Halliburton. Alternative options also exist when running hole finders such as Petromac’s HF bottom nose (ref) However, in many jurisdictions, the mercury based option for a thermometer is no longer a viable due to the HSE risks that Mercury in these thermometers present (ref), resulting in the banning of sale and export of such devices in key global markets. When using mercury thermometers, the thermal lag of the thermometer within the housing is often overlooked and time at TD should also be considered.

Integrated third party temperature sensors

Due to a lack of development in the realms of temperature sensors from the main wireline vendors, and the increased health risks of mercury as aforementioned, third party providers have been developing alternative solutions to the temperature measurement issue, an example of which is the AccSensum T2 thermometer (ref)

The AccSensum T2 thermometer is a memory-based temperature logging device designed for wireline operations. The T2 is engineered to provide accurate and continuous temperature measurements, enhancing the sensitivity and responsiveness of temperature logging over its predecessor the T-1000 (ref). The T2 thermometer is characterized by its innovative modular configuration, allowing seamless integration with various tool strings from different vendors.

Some of the benefits of using this type of technology over traditional mercury or alternatives include:

1. Small Footprint: The T2 can be mounted within standard wireline thermometer slots, minimizing the need for additional equipment.

2. Tool String Independence: It provides consistent, calibrated, temperature measurements over multiple wireline runs, irrespective of the toolstring, or vendor, used which improves time based computations such as Horner plots.

3. Continuous Logging: The T2 logs temperature continuously, even when the wireline tool is switched off, ensuring comprehensive data collection from the time the tool is inserted at surface to its download in the wireline unit.

4. Calibrated Measurements: The T2 delivers accurate bottom hole temperatures with a calibrated response with an accuracy of 0.1 degC.

5. Multiple sensors: Up to 3 T2 thermometers can be run in a wireline logging head/carrier (dependent on vendor) enhancing confidence in the data sets delivered.

6. User-Friendly Interface: Simple and reliable programming and data download via USB ensure ease of use.

7. Dedicated software for processing and interpretation – Data can be downloaded each run or at the end of the job and data is time-to-depth converted to provide a fast turnaround continuous log of depth versus time.

7. The only alternative to mercury thermometers in wireline cable heads – Due to the known HSE risks of mercury regions are gradually moving to an outright ban on the sale of these types of thermometers. Due to the risk of breakage at the wellsite the risk of mercury exposure is high.

5. Long Battery Life: The high-temperature battery can run the thermometer for up to 1000 hours, covering extensive logging operations.

Technical Advisory

This article has highlighted that wireline temperature is a key input for multiple applications within petrophysics as well as other disciplines. In summary temperature significantly influences reservoir characterisation, fluid properties, resistivity logs, and well log data interpretation.

When planning open-hole wireline operations which includes temperature data acquistion it advisable to:

• Review of Available Tools: Evaluate the accuracy, response time, and calibration needs of temperature sensors.
• Review Operational Conditions: Plan logging operations to ensure minimal thermal disturbance and accurate data acquisition.
• Data Analysis: Utilise methodologies like Horner plots and modern tools to ensure precise temperature readings.

The types of sensor and its position within the wireline string will be critical to the accuracy of any subsequent analysis of data, . Different types of available snesor include:

• Internal Cartridge Sensors: Generally less accurate due to thermal lag and heat dissipation issues.
• Flowline-Based RTD Sensors: Accurate fluid temperature measurements during formation testing.
• Wireline Tools with External Sensors: Real-time, continuous temperature readings (e.g., Baker Hughes TTRM, Halliburton BHPT, SLB EMS & LEH-MT/F).

• AccSensum T2 Thermometer: providing multiple memory based calibrated temperature measurements indepenent of the wireline string.

There are a number of considerations to consider which may imporve the accuracy of temperature measurements principally:

• Thermal Equilibrium: Allow wellbore fluid to stabilize as much as possible (where possible)
• Minimize Fluid Movement: Avoid operations causing significant fluid movement before logging (consider logging down)
• Tool Calibration: Regular calibration and maintenance of temperature sensors.
• Stable Conditions: Ensure undisturbed wellbore conditions before logging.

Horner Plots have proven a useful technique for estimating static formation temperature using temperature data over time after circulation stops. It is important that measurements are calibrated and consistent between runs to ensure consistency of measurement. Are logging speeds consistent, is tool movement consistent, is time on bottom (stationary) consistsent. All these variables need to be as consistent as possible to improve accuracy.

If you would like to discuss anything that has been presented in this article our experts have initimate knowledge of the design and production of temperature sensors and are experts in the planning, preparation and execution of wireline logging operations.

Get in touch [email protected].