Drilling Technology

Hydrostatic Pressure

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Hydrostatic pressure is defined as the pressure due to the unit weight and vertical height of a column of fluid.

Hydrostatic Pressure = Fluid Density x True Vertical Depth

Note: It is the vertical height/depth of the fluid column that matters, its shape is unimportant.

Figure 1.1 Different shaped vessels

Since the pressure is measured in psi and depth is measured in feet, it is convenient to convert mud weights from pounds per gallon ppg to a pressure gradient psi/ft. The conversion factor is 0.052.

Pressure Gradient psi/ft = Fluid Density in ppg X 0.052 Hydrostatic Pressure psi = Density in ppg X 0.052 X True Vert. Depth

The Conversion factor 0.052 psi/ft per lb/gal is derived as follows:

A cubic foot contains 7.48 US gallons.

A fluid weighing 1 ppg is therefore equivalent to 7.48 lbs/cu.ft

The pressure exerted by one foot of that fluid over the area of the base would be:

7.48 lbs

144 sq.ins

Figure 1.2

Area definition of a cubic foot

r

Figure 1.1 Different shaped vessels

Figure 1.2

Area definition of a cubic foot

SECTION 1 : FUNDAMENTAL PRINCIPLES OF WELL CONTROL

Example:

The Pressure Gradient of a 10 ppg mud

= 10 x 0.052 = 0.52 psi/ft

Conversion constants for other mud weight units are:

Specific Gravity x 0.433 = Pressure Gradient psi/ft

Pounds per Cubic Foot ^ 144 = Pressure Gradient psi/ft

1.3 FORMATION PRESSURE

Formation pressure or pore pressure is said to be normal when it is caused solely by the hydrostatic head of the subsurface water contained in the formations and there is pore to pore pressure communication with the atmosphere.

Dividing this pressure by the true vertical depth gives an average pressure gradient of the formation fluid, normally between 0.433 psi/ft and 0.465 psi/ft. The North Sea area pore pressure averages 0.452 psi/ft. In the absence of accurate data, 0.465 psi/ft which is the average pore pressure gradient in the Gulf of Mexico is often taken to be the "normal" pressure gradient.

Note: The point at which atmospheric contact is established may not necessarily be at sea-level or rig site level.

1.4 NORMAL FORMATION PRESSURE

Normal Formation Pressure is equal to the hydrostatic pressure of water extending from the surface to the subsurface formation. Thus, the normal formation pressure gradient in any area will be equal to the hydrostatic pressure gradient of the water occupying the pore spaces of the subspace formations in that area.

The magnitude of the hydrostatic pressure gradient is affected by the concentration of dissolved solids (salts) and gases in the formation water. Increasing the dissolved solids (higher salt concentration) increases the formation pressure gradient whilst an increase in the level of gases in solution will decrease the pressure gradient.

SECTION 1 : FUNDAMENTAL PRINCIPLES OF WELL CONTROL

For example, formation water with a salinity of 80,000 ppm sodium chloride (common salt) at a temperature of 25°C, has a pressure gradient of 0.465 psi/ft. Fresh water (zero salinity) has a pressure gradient of 0.433 psi/ft.

Temperature also has an effect as hydrostatic pressure gradients will decrease at higher temperatures due to fluid expansion.

In formations deposited in an offshore environment, formation water density may vary from slightly saline (0.44 psi/ft) to saturated saline (0.515 psi/ft). Salinity varies with depth and formation type. Therefore, the average value of normal formation pressure gradient may not be valid for all depths. For instance, it is possible that local normal pressure gradients as high as 0.515 psi/ft may exist in formations adjacent to salt formations where the formation water is completely salt-saturated.

The following table gives examples of the magnitude of the normal formation pressure gradient for various areas. However, in the absence of accurate data, 0.465 psi/ft is often taken to be the normal pressure gradient.

Figure 1.3 Average Normal Formation Pressure Gradients

Formation Water	Pressure psi/ft	Gradient (SG)	Example area
Fresh water	0.433	1.00	Rocky Mountains and Mid-continent, USA
Brackish water	0.438	1.01
Salt water	0.442	1.02	Most sedimentary basins worldwide
Salt water	0.452	1.04	North Sea, South China Sea
Salt water	0.465	1.07	Gulf of Mexico, USA
Salt water	0.478	1.10	Some area of Gulf of Mexico

SECTION 1 : FUNDAMENTAL PRINCIPLES OF WELL CONTROL

Figure 1.4

SECTION 1 : FUNDAMENTAL PRINCIPLES OF WELL CONTROL

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