- What is Drilling Fluid or Mud?
- “It is a mixture of liquids and chemicals that allow the drilling and completion of a well”.
- Drilling Fluid has to provide many functions in order that these objectives be achieved.
Primary Functions
- Lift and Carry Drilled Cuttings to Surface
- Control Formation Pressures
- Maintain a Stable “In Gauge” Hole
- Cool and Lubricate the Bit
- Lubricate the Drill String
- Secure Hole Information
- Power / Transmit signals from Downhole Tools
- Prevent fluid from entering the formation
- Permit separation of solids at surface
- Form a thin low permeability filter cake
Negative Functions
- Not injure people or be damaging to the environment.
- Not require unusual or expensive methods of completion
- Non damaging to the fluid bearing formation
- Not corrode or cause excessive wear of drilling equipment
- Ridiculously expensive
- The Drilling Fluid Company must be able to:
- Provide cost effective solutions to the operators drilling problems
- Maintain the mud properties
- Provide an adequate supply of products on site and at the base
- Provide adequate reporting
- Engineer the fluid in widely differing conditions and locations
- Provide back up testing facilities
- Avoid damaging the reservoir
Balancing Sub-Surface Pressures
lThe pore pressure depends on:
The density of the overlying rock
The density of the interstitial fluid
Whether the rock is self supporting or is
supported by the fluid.
Tectonic activity
Surface terrain
lIf the fluid hydrostatic pressure does not balance the
pore pressure the following may occur:
Influxes of formation fluid into the wellbore
Lost circulation
Hole Instability
Stuck pipe
1) Balancing Sub-Surface Pressures
lThe pressure balancing the formation pressure is composed from the hydrostatic pressure under static conditions:
nP = Depth (ft) x Density (ppg) x 0.052
lUnder circulating conditions the effective pressure is increased by the pumping pressure. This forms the Equivalent Circulating density (ECD):
The most important parameter is the Annular Velocity (A.V.)
Where possible the annular velocity should be 100 ft/min, higher in deviated holes.
In large hole sections the A.V. can be as low as 20 ft/min.
If the A.V. is insufficient to clean the hole the
viscosity must be increased
For top hole high viscosities must be used
Cuttings removal is harder in deviated and
horizontal holes as the vertical component of the
mud is reduced.
3) Suspension of Solids
Whenever the pumps are switched off solids will start
to settle. This can result in:
Bridging off of the wellbore
Stuck pipe
Hole fill
Loss of Hydrostatic
A gel structure is required to suspend the cuttings
under zero shear conditions:
The gel structure is caused by time dependant
attractive forces which develop in the fluid.
The longer the fluid is static the stronger these
forces become
The gel structure should be easily broken
The gel properties are especially important for
deviated and horizontal wells as the distance
solids have to settle is very small
4) Minimise Formation Damage
Damage to the formation while drilling to the
resevoir:
Formation swelling (Normally clay and
Salt formations)
Washouts (Clay and Salt formations or
any unconsolidated formation)
This can result in:
–Difficult directional control
–Poor zonal isolation
–Excess mud and cement costs
–Poor Hole Cleaning
–Stuck Pipe
–Difficult fishing jobs
Damage to the reservoir will result in loss of production or the need for remedial treatment. This can result from:
Solids blocking reservoir pores
Emulsion droplets blocking reservoir pores
Swelling clays
Ions from the formation and drilling fluid forming insoluble salts
5) Isolate the Fluid From the Formation
The differential pressure forces fluid into the wellbore, resulting in whole mud or filtrate entering the formation. Either, or both, of these is undesirable because:
The loss of whole mud into the wellbore is expensive and damaging
The loss of filtrate into the wellbore may cause formation damage
The flow of fluid is affected by the
formation of a filter cake
The filter cake reduces the flow of fluid
into the formation.
Special additives are added to improve
the cake quality:
–Bridging material
–Plate like material
–Plugging material
The filter cake should be thin with a low
permeability
This avoids reducing the effective
hole diameter
It also reduces the chance of
differential sticking
6) Cooling and Lubrication
The drilling fluid removes heat from the bit
which is then dispersed at the surface
Fluid formulations are not changed to
improve this function
Very occasionally the temperature of
the fluid exceeds the flash point. In this
case it is necessary to improve surface
cooling
Extra lubrication may be required between
the drill string and the casing or wellbore,
especially in directional wells
Liquid additives are used (IDLUBE), or
Oil based mud
Solid additives are sometimes used
such as glass beads or nut plug
Drill pipe rubbers are sometimes
added to reduce wear between the
casing and drill pipe
7) Support Part of the Tubular Weight
lAids in supporting part of the weight
of the drill string and casing
lThe degree of buoyancy is directly
proportional to the density of the
fluid.
The fluid density is never
changed to increase the buoyancy
8) Maximise Penetration Rates
The fluid properties greatly
influence penetration rates by:
Removing cuttings from
below the bit and wellbore
Reducing the cushioning
effect of solids between the
bit teeth and the formation
Reducing the hydrostatic
differential
Increasing the jet velocity
9) Control Corrosion
The fluid should be non corrosive to the:
Drill string
Casing
Surface equipment
Corrosion can lead to:
Wash outs
Twist offs
Pump failure
Surface Leaks
11) Other Functions
Power Down hole motors
Turbines to turn the bit or
power MWD / LWD equipment
Transfer information from
measurement equipment to the
surface
This is done with a pressure
pulse
