Thermal Data Trends are Useful for Welltest Diagnostics (#PTA). Downhole temperature data provide a useful diagnostic tool for understanding #welltest behaviour. Often thermal data offer the only indication of anomalies. Thermal data are also indicative of wellbore storage or afterflow effects and can help validate the start of radial flow conditions. One common effect is warming spikes indicating liquid (#Oil) slugging, clean-up, or influx. These effects may not be apparent from production data, if volumes are too-small-to-measure, or from pressure data, if the magnitude of the anomaly is minor relative to the drawdown. Liquids have a much lower thermal diffusivity than gas (#LNG), so liquids stay warm longer as they are produced up-hole.