Drilling Fluid Functions and Properties

FUNCTION

RELEVANT PROPERTY

OF MUD AND RIG

SYSTEM

EFFECT OF PROPERTY

ON PENETRATION RATE

AND BOREHOLE

EFFECTIVE CONTROL

1)  Removal of drilled cuttings from the hole

a)   Annular Velocity

a)   If excessive may create washouts in unconsolidated formations.

a)   Pump rates must be determined by hole cleaning requirements whilst preventing discharge of whole mud across the shale shakers.

b)  Viscosity

b)  Increased viscosity may decrease penetration rate

a)   Raise by adding viscosifiers, e.g. Bentonite or Polymer

b)  Lower by watering back or adding dispersants, e.g. CFL.

c)   Reduce PV by removing solids.

Viscous slugs may be circulated for hole cleaning.

2)  Control of subsurface pressure

Mud Gradient

a)   Increased gradient, decreases penetration rate. Policy relating to static overbalance will be maintained at all times.

a)   Raise by adding weighting agents e.g. Barites - Never increase gradient using drilled solids.

b)  Lower by dilution and effective use of solids removal equipment.

c)   Centrifuge to remove Barites.

d)  Constant monitoring of all mud tank levels/flow checks.

3)  Prevent the wall from caving

a)   Fluid loss.

b)  Mud cake texture.

c)   Mud gradient

d)  Solids content.

e)   Proper mud system selection

a)   Relaxed fluid loss systems tend to provide faster penetration.

b)  Thin, hard mud cakes reduce drag and torque.

c)   Minimum overbalance on the pore pressure must be maintained at all times.

d)  Drilled solids produce softer and thicker filter cakes. Excessive levels may also increase the viscosity & reduce penetration rate.

e)   Improper mud selection may induce hole problems and reduce penetration rate

a)   Lower FL by adding Bentonite, starch, CFL , PAC, Resinex or plugging particles.

b)  For surface hole utilise the plastering effect of Bentonite. Deeper - thin by CFL addition, reduced Bentonite content, reduce viscosity. Use resin type fluid loss additive.

c)   Maintain required gradient by adding Barites or solids removal.

d)  Solids removal, thinners or dilution.

e)   Observe returns and tank levels, and monitor drill speed logs, torque, etc.

4)  Release the cuttings at the surface.

a)   Gel strengths.

b)  Viscosity.

c)   Pump rate

d)  Mud gradient.

e)   Penetration rate.

f)   Solids Removal Equipment

a)   High gel strengths increase break circulation pressure and do not allow efficient release of cuttings at surface.

b)  High viscosity may reduce penetration rate and solids removal efficiency.

c)   High pump rate reduces solids removal efficiency.

d)  Balance solids removal with mud gradient.

e)   Faster penetration generates more solids to be processed by solids removal equipment.

f)   The finest possible shaker screens must be used with due regard to viscosity and pump rate Operating pressure should be 35 psi - 50 psi (dependant on MW) for desander/desilter systems..

a)   Decrease by dilution and addition of dispersants.

b)  Use viscosifier / dispersant to raise / lower, as required.

c)   Pump rates must be determined by hole cleaning requirements whilst preventing discharge of whole mud across the shale shakers.

5)  Suspend cuttings and weighting material when circulation is stopped.

a)   Gel strengths

c)   Mud gradient.

a)   High gel strengths increase break circulation pressure and do not allow efficient release of cuttings at surface.

b)  Increased gradient, decreases penetration rate but will have to maintain effective overbalance at all times.      As above.

a)   Use viscosifier / dispersant to raise / lower Gel strength as required to maintain within specification. Before running casing, viscosity and gel strength must be reduced to the lowest practical level to reduce swab & surge pressures

b)  Mud gradient must be determined by formation pressures but a high mud gradient reduces settling velocity.

6)  Help suspend the weight of the drill string and casing.

a)   Mud Gradient

a)   Buoyancy decreases the weight that the surface equipment has to support.

a)   Mud gradient must be determined by formation pressures but higher mud gradients will exhibit increased buoyancy.

7)  To cool and lubricate the bit and drill pipe

a)   Depends on mud and bit type, temperature gradient & depth.

b)  Mud cake texture affects drill pipe friction.

a)   Inefficient cooling reduces bit life, especially with PDC bits.

a)    Refer to bit company literature for recommendations on flow rate.

b)    Mud cake should be thin and tough.

8.   To prevent damaging effects to the formation penetrated.

a)   Mud gradient.

b)  Fluid loss.

c)   Mud type.

a)   Minimum overbalance to reduce fluid loss.

b)  To reduce flushing of reservoirs.

c)   Prevent washout / emulsion or water blocking / changes in wettability/ incompatibility.

a)   Control by dilution or removal of weighting agent, as required.

b)  Lower by adding fluid loss additive or sized bridging agents.

c)   Proper mud selection with regard to formation damage.

d)  Control pump rates.

9.   To allow maximum information from the formations penetrated.

See No. 8.

Proper mud type selection to prevent excessive washouts will provide better wireline log results.

As above.