Running continuously above 175 °C

In a hot well the electronics, seals, batteries and greases each have their own failure temperature, and most of them fail progressively: the readings drift first, the function is lost later. What has to be established for the job is this — after twenty hours close to its 175 °C rating, is the tool still measuring within spec?

How tools fail at high temperature

• Bias drift — Accelerometer and fluxgate bias shifts with temperature. Without full-range calibration and temperature compensation, the inclination and azimuth measured at bottomhole are already off, and nothing in the data tells surface that.
• Battery fade — Lithium thionyl chloride cells lose capacity and discharge voltage together at high temperature, so the run time quoted at ambient falls well short in a hot well.
• Seal ageing — Compression set builds up in FKM under sustained heat, and the thermal cycling of tripping does more damage to a seal than steady high temperature does.

What we do about it

• Probes are calibrated across the full temperature range on a temperature-controlled three-axis calibration bench, and the compensation coefficients are written into each individual probe rather than shared across a model.
• High-temperature battery packs are supplied graded by temperature class, quoted in usable hours at the rated temperature rather than at ambient.
• Seal replacement intervals are set by temperature class and number of thermal cycles, and written into the maintenance manual.