Oil Drilling Manual- drilling Pressure Gradient fluid density

Basic Drilling Formulas for Pressure Gradient



Pressure Gradient

Calculation Pressure Gardien during drilling process using mud weight formula

Pressure gradient, psi/ft, using mud weight, ppg

psi/ft == mud weight, ppg x 0.052

Example:

12.0ppg fluid psi/ft = l2.0ppg x 0.052 psi/ft = 0.624

Pressure gradient, psi/ft, using mud weight,
tb/ff psilft == mud weight, lb/tt ' x 0.006944

Example:

100 Ib/ft] fluid psi/ft = 1001b/ft3 x 0.006944
psi/ft = 0.6944

OR

psi/ft = mud weight, Ib/ft3 ~ 144
Example. 100 Ib/ft3 fluid

psi/ft = 1001b/ft.l ~ 144
psi/ft == 0.6944



Pressure gradient, psi/ft, using mud weight, specific gravity (SG)
psi/ft = mud weight, SG x 0.433

Example:
1.0SG fluid psilft == I.OSG x 0.433 psi/ft=0.433

Metric calculations
Pressure gradient, bar/m = drilling fluid density kg/I x 0.0981
Pressure gradient, bar/10m = drilling fluid density kg/l x 0.981

S.I. units calculations
Pressure gradient, kPa/m = drilling fluid density, kg/m3 + 102

Convert pressure gradient, psi/ft, to mud weight, ppg

ppg = pressure gradient, psilft + 0.052

Example: 0.4992 psi/ft

ppg = 0.4992psilft + 0.052 ppg = 9.6

Convert pressure gradient, psi/ft, to mud weight, lb/fe

Ib/ft3 = pressure gradient, psilft + 0.006944

Example: 0.6944psi/ft

Ib/ft3 = 0.6944psilft + 0.006944 Ib/ft3 = 100

Convert pressure gradient, psi/ft, to mud weight, SG

SG = pressure gradient, psilft -s- 0.433

Example: 0.433 psi/ft

SG = 0.433psilft .... 0.433 SG = 1.0

Metric calculations
Drilling fluid density, kg/l = pressure gradient, bar/m .... 0.0981
Drilling fluid density, kg/I = pressure gradient, bar/10m .... 0.981

S.I. units calculations
Drilling fluid density, kg/m ' = pressure gradient, kPa/m x 102

Rig and Surface
The following gauges and recording instruments need to be checked-
- Stand pipe pressure
- RPM
- WoB
- Torque
- Geolograph (depth measurement)
- Rig Floor Parameter Display/Monitor
- SPM

Flow Rate
- Flow rate greatly effects hole cleaning. Generally high flow rates provide better hole cleaning than low flow rates as they are better able to return cuttings to surface due to increased annular velocity.

- Flow rate greatly effects bit cleaning. Generally high flow rates provide better bit cleaning than low flow rates by increasing hydraulic energy at the bit.
- If a motor is in the hole increasing flow rate will increase the rotary speed developed by the motor. of Increasing motor speed must be considered carefully as it can greatly effect drilling performance-
- In clean shale increasing motor speed increases bit speed that will generally increase penetration rate without damaging the bit cutting structure or other downside.
- In hard abrasive sandstone increasing motor speed increases bit speed that will generally increase penetration rate momentarily.
However the higher rotary speeds will tend to increase the wear rate of the bit cutting structure that will reduce penetration rate and ultimately bit life.
- Increasing motor and consequently bit speed can change the directional tendency of the bottom hole assembly with a bigger effect on building/dropping tendency than azimuth.

The compromise between instantaneous penetration rate and sliding corrections must be considered.
- High flow rates can cause formation damage especially in highly fractured formations so excessive flow rates must be avoided.

Weight

- As a drill bit cutting structure wears more weight will be required to achieve the same RoP in a homogenous formation.
- PDC wear flats, worn inserts and worn milled tooth teeth will make the bit drill less efficiently.
- Increase weight in increments of 2,000lbs approx. • In general, weight should be applied before excessive rotary speed so that the cutting structure maintains a significant depth of cut to stabilise the bit and prevent whirl.
- If downhole weight measurements are available they can be used in combination with surface measurements to gain a more accurate representation of what is happening in the well bore.

Note:
Numerical values and characteristics of the equipment and procedures
described on this page are for guidance purposes only .

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