**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. |