Improving Temperature-Logging Accuracy in Steamfloods

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 191539, “Temperature-Logging Guidelines and Factors That Affect Measurement Accuracy,” by Gede Adnyana (retired, former Chevron), Jyotsna Sharma, SPE, Don•Mims, SPE, David Barnes, and Ron Behrens, SPE, Chevron, prepared for the 2018 SPE Annual Technical Conference and Exhibition, Dallas, 24–26. The paper has not been peer reviewed. 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-Tool Overview 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 down-ward, 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.