Improving Temperature-Logging Accuracy in Steamfloods
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A robust reservoir surveillance program is key to managing a steamflooding operation successfully. Time-lapse temperature surveys are a primary data type collected from the observation wells to evaluate reservoir heating and to monitor the steamchest. The objective of this study was to look at factors that can affect a temperature log and steps that can be taken to improve temperature-measurement accuracy. Field data and analytical assessment show that several factors can affect the accuracy of a temperature log, which can subsequently affect interpretation and operational decisions.
Temperature-logging tools are among the most common used in thermal projects. These tools are accurate, reliable, and relatively inexpensive. Fig. 1 shows a schematic of a typical temperature tool. It includes a casing-collar locator (CCL), a temperature sensor, and tool electronics. Temperature logs can also be obtained in conjunction with data from other logs such as the cement-bond, carbon/oxygen, pulse-neutron, and openhole logs. However, it is recommended that temperature surveys from these tools be used for quantitative analysis because the other logs are recorded normally by first lowering the logging tool to the bottom of the well and then logging up to the surface. These log measurements may also be taken at a higher logging speed. But temperature tools, by comparison, are logged downward, therefore avoiding wellbore-fluid-churning issues. Additionally, depth control is maintained by monitoring tension on the line and by correlating to the CCL.
Temperature sensors in the logging tool measure the temperature of the fluid in the wellbore, which is in thermal equilibrium with the adjacent formations. A resistance temperature detector (RTD) is the preferred sensor for temperature measurements. An RTD is a device that senses temperature by measuring the change in resistance of a material in a known and repeatable manner. Platinum thermistors are also used as temperature sensors; however, an RTD has better repeatability, long-term stability, and accuracy, and features greater sensitivity to small temperature changes than does a platinum thermistor. Additionally, platinum thermistors have lower maximum temperature limits typically and require more-frequent calibration, although their response can be faster than that of an RTD.
The sensor requires a certain minimum time to reach thermal equilibrium with the measured fluid, which is a function of its response time, defined as the time unit required for a sensor to reach 63.2% of the total output signal when subjected to a step change in input. The step change can be an increase or decrease in the parameter being measured, such as temperature. Response time is affected by the type of sensor, the medium in the wellbore (typically water), and the change in temperature. Response time in air is much slower than in water, a state that exists even for newer RTD sensors. If the logging speeds are too fast, the sensor may not have sufficient time to equilibrate with the medium, resulting in an inaccurate temperature reading. However, higher or varying logging speeds often represent an operational decision to save time.
Factors Affecting Temperature Measurement
Factors that will affect temperature measurement include logging speed, sensor-response time, and steamchest temperature. In the complete paper, the authors present an analytical basis upon which the sensitivity of these parameters on temperature measurement may be evaluated. The purpose of the analytical calculations primarily was to gain directional insight on parameter sensitivity, and not for purposes of quantitative analysis.
Key observations from the analytical calculations include:
- The higher the logging speed, the greater the error in measured temperature.
- The higher the steamchest temperature, the greater the error in measured temperature.
- The longer the sensor-response time, the greater the error in measured temperature.
- The error in measured temperature typically reduces over time.
Four field examples are presented in the complete paper to illustrate errors introduced in temperature measurements caused by running at higher or nonuniform logging speeds, and the effect of steamchest temperature on measurement error. With respect to these field observations, running at higher logging speeds shows the following:
- Lower magnitude of peak temperature that can affect operational decisions
- Temperature profile delayed or shifted downward
- Higher error in peak temperature value at higher steamchest temperature
- Disturbance of wellbore fluid when speed is not held constant from the surface to bottom of the well
Temperature-Log Quality-Check (QC) Guidelines
Some key guidelines for performing QC of a temperature log include the following:
- Ensure that the log was run downward into a stable and equilibrated water-filled temperature-observation well (TOW) at a constant and optimal logging speed from the surface to bottom of the well.
- Compare the temperature-tool reading at the surface with the surface temperature to identify significant deviations from expected temperature behavior that could be caused by tool issues.
- Compare the temperature gradient from the log with the geothermal gradient in the nonsteamflooded zones. A significant difference from the geothermal gradient could indicate poor tool calibration, tool error, or out-of-zone heating.
- Depth QC is a necessary step in temperature-log QC. One way this can be achieved is by comparing the CCL from the temperature survey with the CCL from previous temperature surveys.
- Compare time-lapse temperature surveys to identify significant deviations from expected temperature behavior that could be caused by tool issues.
Several field examples are presented in the complete paper to illustrate the effects of tool malfunction and poor calibration. Guidance for the interpretation of temperature signature in air, identification of liquid level in a TOW, and the wellbore-reflux phenomenon are also discussed through field examples.
Acquiring Temperature Data in Steamflood TOWs
- Before surveying, identify any safety issues such as high temperature and pressure in the wellbore owing to thermal expansion or steam leak into the casing, which can be hazardous for those accessing the wellbore for logging.
- Establish that the fluid level in the well before surveying is close to the surface in order to measure temperature accurately throughout the length of the well.
- Leave the well undisturbed before taking a temperature survey to minimize temperature-smearing effects, and ensure that the wellbore fluid has equilibrated thermally with the adjacent formation.
- Log the temperature tool going downward from the surface to the bottom of the well at a constant optimal logging speed to achieve necessary temperature accuracy.
- Ensure that the temperature tool has a current calibration record.
- Verify the specification and type of temperature sensor used in the temperature tool; this has an effect on the accuracy and repeatability of the measurement
- Perform log QC to identify any anomalous temperature reading caused by tool malfunction, improper calibration, or unsuitable wellbore conditions.
- To prevent wellbore reflux, ensure that sufficient hydrostatic pressure in the water column exists to prevent boiling.
- The assessment of field data, as well as the analytical modeling study presented in the complete paper, suggests that the accuracy of a temperature log can be affected by factors such as the logging speed, sensor type, sensor-response time, steamchest temperature, and wellbore fluid.
- Running temperature logs in a downward direction at a constant, optimal logging speed, in an undisturbed, equilibrated water-filled wellbore provides the most favorable results.
- Temperature surveys in a water-filled TOW minimize measurement error and result in faster response time, as compared with an air-filled TOW.
- Field data and analytical calculations suggest that higher logging speeds introduce greater error in measured temperature data and that these errors are greater at elevated steamchest temperatures. Temperature tools with longer sensor-response times need to be run at slower logging speeds to obtain accurate measurements.
Improving Temperature-Logging Accuracy in Steamfloods
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