Differential Temperature Log

Key Takeaways

• Production and injection logging is the primary application for differential temperature logs in producing wells, where the thermal signature of fluid entry or injection determines which perforation intervals are contributing to production and at what relative rate — a flowing gas well will show Joule-Thomson cooling at gas entry intervals because gas expands as it enters the lower-pressure wellbore from the higher-pressure formation, and the differential temperature log transforms this cooling signature from a gradual temperature decline (visible on the absolute temperature log) to a sharp negative spike at the depth of gas entry (visible on the differential log); oil and water entry intervals typically show less dramatic Joule-Thomson cooling (liquids expand less than gases and therefore cool less on expansion) but may show as inflection points in the temperature gradient that the differential log identifies; the differential temperature log is run as a companion to the production spinner log (which measures mechanical flow velocity in the wellbore) and the temperature log in production logging combinations that together identify both the depth and the relative rate of each producing interval.
• Cement evaluation through temperature logging exploits the exothermic nature of the Portland cement hydration reaction — when cement sets, it releases heat (the heat of hydration), warming the cemented annular interval above the temperature of the surrounding formation; the magnitude of the temperature increase depends on the cement slurry composition, slurry weight, and the time elapsed since cement placement, peaking a few hours after placement and dissipating over several days; temperature logs run within 8-12 hours of cement placement show good-quality cemented intervals as warm relative to background and uncemented (fluid-filled) channels as cooler, allowing the cement evaluation engineer to identify the top and extent of the cement column; the differential temperature log sharpens the top-of-cement signature and identifies the boundaries of channels more precisely than the absolute temperature log; the temperature cement evaluation method is relatively crude compared to cement bond logs (CBL) and ultrasonic imaging tools (USIT, CAST-V) but provides valuable information in wells where mechanical damage to the casing prevents acoustic cement evaluation, and it is one of the few cement evaluation options available in casing that is too small for standard acoustic tools.
• Crossflow detection between producing intervals in a shut-in well is another application of differential temperature logs — when a well is shut in and the wellbore fluid is allowed to equilibrate, intervals with higher pressure that are in communication with the wellbore will flow fluid upward into the wellbore at depth, while intervals with lower pressure will accept fluid flowing downward from the wellbore; this thermal fluid movement creates thermal anomalies (warm fluid entering from hot deep intervals, cool fluid entering from shallower cooler intervals) that the differential temperature log identifies as temperature inflection points at the entry depths; this application is particularly useful in wells with suspected crossflow between a gas cap and an oil leg (where high-pressure gas migrating downward at shut-in conditions indicates communication that could harm reservoir management) or in injection wells where multiple injection zones are accepting fluid at different pressures.
• Gas hydrate stability zone identification in deepwater wells uses temperature logs to map the depth interval where methane hydrate can form in the wellbore and in the formation — gas hydrates form and are stable when temperature and pressure conditions fall within the hydrate stability envelope, which typically includes the near-seafloor sediment interval in deepwater settings; drilling through the gas hydrate zone requires managing the risk of hydrate dissociation (caused by thermal disturbance from the warm drilling fluid) or hydrate formation in the wellbore (caused by high-pressure gas contact with cold water), both of which create drilling hazards; the differential temperature log run shortly after a well is drilled provides an indicator of the gas hydrate stability zone by showing the depth at which the geothermal gradient changes (the hydrate zone has a thermal buffering effect because hydrate formation and dissociation are phase-change processes that absorb or release latent heat at approximately constant temperature); identifying the gas hydrate zone depth from the temperature log is used to inform casing setting depth decisions and to design cementing programs that avoid thermal disturbance of the hydrate interval.
• Injection well integrity monitoring uses repeated temperature log surveys over the life of the well to detect changes in injection fluid distribution that may indicate developing wellbore integrity problems — a newly completed injection well shows a characteristic temperature profile (warm at shallow depth where the injection fluid has warmed to formation temperature, cool at the injection interval where the cold surface water enters the formation); over time, if a perforation interval plugs with scale or suspended solids, the injection fluid redistributes to other perforations, and the temperature log will show the cool injection signature migrating to different depth intervals; if the casing or cement develops a leak, injection fluid can escape to a shallow interval and the temperature log will show an anomalous cool signature at the leak depth rather than at the intended injection interval; repeated temperature logs (run annually or more frequently in fields with active injection monitoring programs) provide a long-term record of injection distribution changes that complement production logs and surface monitoring for wellbore integrity management.