The term refers particularly to a mud sample that has experienced stagnant conditions at the bottom of the hole, including the temperature, pressure and other conditions at that depth.

The result shows that the main reasons are high bottom hole temperature, corrosion of drilling tool during storage and drilling process, high value of formation drillability, and multi-fault.

The results show that the test error of sample A with large angle coefficient of hole bottom is under 5%, and will become smaller with the increase of furnace temperature;