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Production logging began in the mid 1930's with the introduction of temperature measurement in oil and gas wells. The temperature log is the oldest production log, yet it continues to appear in production logging suites more often than any other log. This is true because the temperature survey has unique features unmatched by any other survey. Dr. R.M. "Mac" McKinley, arguably the world's leading expert on production logging, recommends that when interpreting a suite of production logs, the temperature log should be interpreted first, and the other logs interpreted in light of the temperature log.
Early thermometers were run on "slick line" and recorded downhole on oxide coated metal charts. Modern electric wireline thermometers (often called temperature tools) are capable of much faster response, and produce a continuous record of the wellbore temperature. Several different types of temperature sensors are used. Temperature tools produce pulses, the frequency of which is directly proportional to the measured temperature (some modern digital systems use telemetry). In analog recording trucks, the transmitted pulse rate is converted to a voltage and recorded on a strip chart recorder. In digital trucks, the pulses (or telemetry) are sampled and processed by a computer. Interestingly, in the not too distant past temperature log interpretation experts sometimes specified that a particular temperature log be run with the older analog technology because digital processing introduced at least some digital noise or "stair stepping" as an artifact of sampling error. The popular differential curve amplifies the tool response; it electronically simulates having two temperature sensors in the well, spaced a fixed distance apart, where the temperature differential between said sensors is greatly magnified.
When run and interpreted properly, a temperature survey becomes a high precision flow survey with the best vertical resolution available. This is true because while the typical temperature tool does not necessarily have particularly impressive absolute accuracy, it can resolve temperature changes as small as 0.05°F. Further, the differential trace is the most precise means of determining the depths of fluid entry or exit in a well bore. It is important that the temperature scale be sensitive enough to allow proper interpretation, 1.0°F/inch is an appropriate scale for many applications. Temperature surveys are best run going into the hole with the temperature sensor located as near the bottom of the tool string as possible. 20 to 30 feet per minute (fpm) has been suggested as a good line speed range for temperature logging.
The static temperature in a wellbore increases gradually with depth. In most of North America the increase or "gradient" will be between 0.5 and 2.5°F for each 100 feet of increase in depth, with a value of 1.7°F/100 feet (3°C/100 meters) being typical. The majority of this static gradient is attributable to heat production from the radiaoactive decay of minerals in the earth's crust; therefore, local variations in gradient are thought to primarily reflect the relative richness of shale in the subsurface. The detailed variations in static gradients are determined by variations in the effective thermal conductivity (k) of the rock and its pore fluids combined. While a detailed discussion of temperature behavior in wells is beyond the scope of this effort, suffice it to say that the interaction between well activity, principally fluid flow from production or injection, and the static gradient is what makes the temperature survey such a remarkable tool. A competent log analyst can detect fluid entries and exits from nothing more than tiny excursions from the static gradient.
Cement produces heat as it cures through the heat of hydration chemical reaction. As a result, cement tops can be located if a temperature survey is run before the setting cement cools, usually logging 12 hours or so after the cement is placed is a good choice. Squeezed cement can be frequently located, but a differential log may be necessary. A simple application of the temperature log in producing wells is the location of gas entry. The endothermic expansion (Joule-Thompson effect) of gas produces an easily identifiable cooling anomaly that readily identifies zones of gas production. The source of water or liquid hydrocarbons entering a producing well can often be identified, both the origin and the actual point of entry into the well. Fluid injection profiling can be done with a temperature survey. Lost circulation zones encountered during drilling operations can be located with temperature logging. The temperature survey has been used as a frac evaluation log, and as an acid placement evaluation log. These are but a few of the applications for temperature logging.
Temperature Logging for Injection Well Integrity
Under the US EPA underground injection control (UIC) program regulations at 40 CFR §146.8(c)(1), the temperature log is approved for demonstrating external mechanical integrity, defined as "...no significant fluid movement into an underground source of drinking water through vertical channels adjacent to the injection well bore." The temperature survey is a valuable tool for investigating well integrity; both injection related flows behind pipe, and non-injection related flows behind pipe (inter-formational flows) can be identified.
In deep injection wells, say below 2,000 feet where the downhole temperature generally exceeds 115°F, injection cools the wellbore in the vicinity of the injection zone. In shallow injection wells, say above 1,000 feet, injection may cool or warm the wellbore in the vicinity of the injection zone depending on seasonal injectant temperature. In shallow injection wells, contrast in temperatures is ordinarily reduced, and temperature logs should be recorded at high sensitivity as a matter of course. Once injection starts, the flowing temperatures stabilize quickly. When an injection well is shut in for logging, the wellbore fluid begins to revert toward static conditions; it is this change that helps identify problems in the injection well. Zones that have taken injected water, either by design or not, will exhibit a "storage" signature on shut in temperature surveys (storage signatures are normally cold anomalies in deeper wells, but may be cool or hot in shallow wells). Losses behind pipe from the injection zone, as well as casing and tubing leaks, can be detected on both flowing and shut in temperature surveys and exhibit a "loss" signature (under the EPA UIC regulations, the temperature log is not approved for casing, tubing, and packer integrity testing / leak detection). Behind pipe non-injection related inter-formational flow exhibits its own unique temperature signature. Several shut in temperature logging passes are often run sequentially; initial minimum shut in times are dependent on previous cumulative injection, and range from maybe 6-12 hours for one month of prior continuous injection, to 96-192 hours for 10 years of previous injection. A nifty trick sometimes used is the injection of hot water to provide temperature contrast in temperature surveys, especially for shallow wells.
Some Pointers
Always run temperature logs going into the well to minimize the smeared response caused by logging line and tool movement; if forced to log up, do so very slowly to minimize the disturbance. If possible, limit line speed to 20 fpm or less. Select the most sensitive log scaling practical. Stabilize injection or production for at least 48 hours prior to running routine surveys. Run both flowing and shut in temperature surveys; a succession of shut in surveys is even better. Surface leakage during temperature logging will adversely affect results; likewise do not allow excess grease to enter the well from the lubricator grease head, if used. Allow one hour or more between temperature surveys for temperature equilibrium to be restored. Hire Dr. R.M. "Mac" McKinley to tell you what the heck your temperature log means when you get done...
The foregoing is an oversimplified discussion of the temperature log.
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