The difference between pressures outside and inside a casing string. This is normally referred at the casing shoe.
The maximum pressure differential during a cementing job occurs just before the lead cement slurry turns the corner. Since the hydrostatic pressure in the annulus is greater than the hydrostatic pressure inside the casing, the maximum pressure differential is negative.
The differential pressure after placement is the hydrostatic pressure in the annulus minus that of the inside casing after the cementing slurries are in place.
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