Pressure Control – Subsurface Pressures

•Many different pressures are involved in drilling and controlling oil and gas wells.
•It is important to understand these pressures and how they are used to detect and control formation pressures.
•Pressure is defined as force per unit area:
Pressure (psi) = force (lb) /area (in.2)
•Hydrostatic pressure (PHYD) is the pressure caused by the density or Mud Weight (MW) and True Vertical Depth (TVD) of a column of fluid.
–The hole size and shape of the fluid column have no effect on hydrostatic pressure since, at a given depth, pressure is equal in all directions.
•PHYDis calculated by:
–PHYD(psi) =0.052 x MW (lb/gal) x TVD (ft)
–0.052 = The units conversion factor equal to:
12 in./ft/231 in.3/gal
or 0.052 gal/(in.2x ft)
•What is the hydrostatic pressure of a fluid column for the following conditions?
–MW = 12.8 lb/gal
–MD (Measured Depth) = 14,300 ft
–TVD = 13,200 ft
•The hydrostatic pressure is always calculated using the TVD.
PHYD= 0.052 x 12.8 x 13,200
= 8,786 psi at TVD
•Hydrostatic pressure gradient is the pressure increase per unit of vertical depth.
–PHYDG(psi/ft) = 0.052 x MW (lb/gal)
•What is the pressure gradient of a 12.0 lb/gal mud?
PHYDG(psi) = 0.052 x 12.0
=0.624 psi/ft
•Typical pressure gradients are:
–Freshwater 0.433 psi/ft
–Seawater 0.444 psi/ft
–Marine formation water
–(100,000 mg/l salt) 0.465 psi/ft
–Saturated saltwater
–(10 lb/gal) 0.520 psi/ft
–16-lb/gal mud 0.832 psi/ft
–19.2-lb/gal mud 1.0 psi/ft
Formation pressure (Pform) is the fluid pressure exerted within the pore spaces of any oil, water or gas formation, and is commonly called pore pressure.
•Normal pressure is the hydrostatic pressure exerted by a column of fluid equal to the density of the native fluid that existed in the geological environment when the solids were deposited.
–Since more wells are drilled in sediments characterized by marine formation water with about 100,000 mg/l salt, a gradient of 0.465 psi/ft will be used as the normal gradient for purposes of this discussion.
–Deviations from normal hydrostatic pressures are referred to as being abnormal —sur-pressures (high) and sub-pressures (low).
Indicators of Increasing Pressure