Hydrostatic and Applied Pressure
Both hydrostatic and applied pressure are measured by the number of pounds of force pushing upon one square inch. In other words, “how many psi?” Applied pressure is that applied to the system with a pump. Hydrostatic pressure is created by the weight of a column of fluid. Please remember, they are still measured in the number of pounds per square inch.
Applied pressure, which is put into the system with a pump, will be found throughout the system. If you apply pump pressure to a well, there will be pressure not only at the top, but also at the bottom of the well. Since it is measured in psi, it can be added to the hydrostatic pressure at any depth.
The mud weight information commonly used in the field is #/gal. To figure hydrostatic pressure, take the number .052 x #/gal. x depth. The figure .052 is the psi/ft. for one #/gal. of fluid.
When this figure is multiplied by #/gal., the result is the psi/ft. for that weight fluid. This is sometimes called Fluid Gradient. Multiplying psi/ft. times depth gives the hydrostatic pressure for that depth.
If you have a Fluid Gradient Table look up the psi/ft. for the weight fluid and multiply this by depth to obtain the hydrostatic pressure. Since hydrostatic pressure is governed by vertical depth, it cannot be at the surface.
Cubic Foot Method
To determine the hydrostatic pressure — if the weight of fluid is in “how many pounds per cubic foot?” or more simply “#/cu. ft.”, follow this procedure.
Multiply .007 x #/cu. ft. x depth. The figure .007 is the psi/ft. for one #/cu. ft. of fluid. When multiplied by the number of #/cu. ft. of fluid, you have the psi/ft. for that weight fluid.
Multiplying psi/ft. by depth gives the hydrostatic pressure for that weight fluid and that depth.
You need to refer to a Fluid Gradient Table in Baker tech book or Halliburton tech book : look up the psi/ft. for the weight fluid and multiply this by depth to obtain the hydrostatic pressure. Since hydrostatic pressure is governed by vertical depth, it cannot be at the surface.