In some oil well when the temperature increased,caused decrease collapse resistance of casing. Why?
Without more context (are you addressing a specific example) it's only possible to give pretty basic answers.
In general, with increased temperatures, materials get weaker. It's why metal workers frequently heat their work pieces in a furnace before hitting them with (cool) hammers to reshape the work piece.
Specifically in oil wells, a problem that has happened repeatedly is when a liner is set with fluid below the seal assembly, then cement is pumped and it fills the annulus up to intersect with the previous casing (liner) shoe, trapping the fluid between seal and cement. As the fluid heats up, it's pressure will increase and stress the casing more than hydrostatic pressure. Then, if you lose annular density (unload the well ; displace to lighter wellbore fluid), you're closer to the casing's collapse pressure.
As water temperature increases, the motion of a bubble tends to weaken owing to the increase in saturated vapor pressure of water, and the surface configuration of a bubble becomes highly irregular because of thermal instability. The impulsive pressure depends not only on the bubble size and its distance from a solid wall but also on the water temperature. When the water temperature approaches the boiling point of water, the impulsive pressure abruptly decreases with increasing water temperature. The evidence obtained seems to be associated with the known temperature effect on cavitation damage at high water temperature.
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