In offshore drilling operations, it is necessary to correct the calculated fracture gradient for the effect of water depth and flow-line height (air gap) above mean sea level. The following procedure can be used:

Example: Air gap = 100 ft Density of seawater = 8.9 ppg

Water depth = 2000 ft Feet of casing below mud-line = 4000 ft

Procedure:

1. Convert water to equivalent land area, ft:
2. Determine the hydrostatic pressure of the seawater: HPsw = 8.9 ppg x 0.052 x 2000 ft

HPsw = 926 psi b) From Eaton's Overburden Stress Chart, determine the overburden stress gradient from mean sea level to casing setting depth:

From chart: Enter chart at 6000 ft on left; intersect curved line and read overburden gradient at bottom of chart:

Overburden stress gradient = 0.92 psi/ft c) Determine equivalent land area, ft: Equivalent feet = 926 psi

0.92 psi/ft

Figure 5-2. Eaton's overburden stress chart.

2. Determine depth for fracture gradient determination: Depth, ft = 4000 ft + 1006 ft

Depth = 5006 ft

3. Using Eaton's Fracture Gradient Chart, determine the fracture gradient at a depth of 5006 ft:

From chart: Enter chart at a depth of 5006 ft; intersect the 9.0 ppg line; then proceed up and read the fracture gradient at the top of the chart:

1. Determine the fracture pressure: psi = 14.7 ppg x 0.052 x 5006 ft psi = 3827
2. Convert the fracture gradient relative to the flow-line: Fc = 3827 psi 0.052 * 6100 ft

Fc = 12.06 ppg where Fc is the fracture gradient, corrected for water depth, and air gap.

1S.WM 16j0w 17.KM

1S.WM 16j0w 17.KM

Figure 5-3 Eaton's Fracture gradient chart

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Responses

• Landolfo Napolitano
How do we calculate fracture gradient from overburden?
8 years ago
• Naomi Wilson