Imposed pressures can also originate from a formation. If formation pressure exceeds hydrostatic pressure, and the well is shut-in, the pressure differential between the hydrostatic of the drilling fluid and the formation pressure, will be imposed throughout the system. This pressure can be read at the surface. At the surface, two different readings will be noted. These will be the drillpipe (pump) pressure and the casing (choke) pressure.
• If no influx of formation fluid occurs, then the hydrostatic pressure in the drill string, and in the annulus, will be the same; resulting in equal drillpipe and casing pressures.
• Usually, any formation fluid influx will have a density less than the drilling fluid, and will only go into the annulus. In this case, the total hydrostatic pressure in the annulus will be less than the hydrostatic pressure in the drill string. Since the formation pressure is constant for the bottom of the hole (both under the drill string and the annulus) the resulting pressures on the drill pipe and casing will differ. The surface drillpipe pressure will be less than the annular pressure since its hydrostatic is greater.
Depending on the situation, one or more of these types of pressures may exist in the well at any given time. If a type of pressure exists in the well bore, it exists everywhere in the system. However, it’s magnitude may vary throughout the system.
Pascal's Law
“The pressure at any point in a static fluid is the same in all directions. Any pressure applied to a fluid is transmitted undiminished throughout the fluid.”
The consequences of this law when applied to drilling practices are important. When a well is shut in during a kick, the pressure is exerted throughout the fluid column. Which means formations uphole experience the same pressures as those downhole.
