2

DRILLING FLUID - TROUBLE-SHOOTING

All the drilling fluid problems can be attributed mainly to four physical properties:

Density (Mud Gravity)

Viscosity (Plastic viscosity & yield point)

Gel strength ( Gel0 and Gel

10)

Filtration (Fluid loss in down hole)

The adequate monitoring and control of the properties should be the primary

concern for maintaining drilling fluid in good condition. Sp. gr. of mud provides

hydrostatic pressure to the well bore. It is easily controlled by addition of weighing

materials like barite, CaCO3, Hematite, and Galena etc. or by dilution with water or

oil. Mud wt. can be increased by using the following formula -

Kgs of weighting material 1000 S.G.WM

(SG2-SG

1)

=

M3 of fluid (SGWM

-SG2)

Where,

SG2

- final specific gravity

SG1

- initial specific gravity

SGWM

- Sp. gravity of weighting material

2

Similarly desired weight reduction can be calculated by using the following

formula:

V1 (SG

1 - SG

2)

V2

=

SG2-SG

Where,

V2

- volume of diluting fluid

V1

- volume of original fluid to be diluted

SG1

- original specific gravity

SG2

- final specific gravity

SG - Sp. gravity of diluting fluid

Viscosity indicates the initial resistance of a drilling fluid. It comprises of two parts

i.e. plastic viscosity (PV) & yield point (YP). Using good solid control equipment,

by dilution or by selective flocculation where as YP can be controlled by addition of

chemicals can control PV. Addition of clays, bentonite extenders, polymers etc.,

increases the YP and addition of thinners and dispersants like CLS, cutch etc,

reduces the YP.

The control of filtration loss helps in minimizing the thickness of filter cake in

addition to less formation destabilisation, which minimises many down hole

problems.

The control of gelation helps in minimising the swabs and surge problems.

Solids in drilling fluid are to be maintained in correct range, as they are very vital

for many drilling fluid properties, ROP etc. In drilling fluid we have two types of

solids: -

2

Reactive (clays)

Low gravity solids

Non reactive (sand, silt, limestone)

High gravity solids weighted materials

If not controlled properly, they can lead to many problems viz, mud viscosity

& treatment related problems, gelation, differential sticking, mud loss, drag

and torque etc.,

Drilling Fluid Trouble Shooting:

Problems Symptoms Solution

2

ContaminationHigh Drilled Solids

Abrasion

Salt Rock

Salt Water

High viscosity and gel strength. Slow drilling rate. Reduced temperature stability. Mud does not respond to chemical treatment.

Premature bit failure and excessive wear of swabs, liners and valve seats.

High viscosity and high gel strengths, increase in water loss and salt content. Grainy appearance to mud.

Same as salt – rock except increase in pit volume and a reduction in mud weight.

Reduce drilled solids by water dilution, use of selective flocculants and mechanical separation.

Use Desander to hold sand content to a minimum.

Adjust mud properties with lime and organic thinners and water loss control agents, or convert to saturated salt system.

Raise weight to overcome salt water flow. Adjust mud properties with loss control agents. If only stringers are encountered dilute with water.

Problems Symptoms Solution

Abnormal Pressure Lost circulation

Low pressure

Partial to complete loss of returns

Pull up in casing and wait for fracture to close. Adjust flow properties to afford minimum flow resistance and equivalent circulating density. Reduce mud weight, if possible. For low weight muds add flake or fibrous materials to avoid increasing mud weight from material additions. In high weight muds, add fine flake or granular lost circulation material for partial loss of

2

Formation Gas or water influx

Gas Cutting

Differential sticking of drill pipe.

High Pressure

Increased pit volume possibly preceded or accompanied by gas or salt water cut mud. Mud continues to flow when pumps are shut off.

Normally shows up as gas-cut mud after trips but dissipates rapidly. If encountered while drilling, gas cutting or kicking will occur in heads. Possible change in chloride content.

Normally occurs when drill pipe is not in motion. Cause in excessive pressure differential between well bore and porous formation

Full or partial circulation while stuck. Probably high fluid loss and high solids content mud.

returns.

Shut in well. Record drill pipe and casing pressure. Circulate out gas or water influx and separate at surface. Using data obtained calculate necessary mud weight, mix mud and circulate to kill well.

Raise weight only as necessary. Keep gel strengths as low as possible. Continue to circulate and avoid use of blowout preventers if possible. Use degasser as necessary to clear gas from mud.

If possible reduce mud weight, Use spotting fluid. For prevention measure reduce fluid loss at static. Bottom hole temperature at 500 psi differential pressure and use minimum safe mud weight.

Problems Symptoms Solution

Corrosion

2

General, salt muds etc.

Aerated Mud

Sulphide stress corrosion

Formation

Bit Balling

Bentonite Swelling.

Running, sloughing, caving shales.

Plastic salt

Internal and external pitting, more pronounced internally in uncoated pipe

Severe pitting of drill pipe more pronounced internally.

Severe brittle, flat fractures of susceptible materials, either tool joints and / or pipes.

Little, or no progress in footage. Balled up bit and drill string. Swabbing on trips and connections. Bit generally comes out in good condition, showing little wear, but heavily packed with cuttings.

Increase in viscosity, gel strength and solids content. Tight places in hole, slow drilling. Mud difficult to control

Hole fill-up after trip. Excessive cuttings over shaker. Tight connections.

Tight connections. Even with saturated or oil based mud pipe may become stuck.

Add corrosion inhibitors, Use oxygen scavengers.

Extremely difficult to control. Use corrosion inhibitors and oxygen scavengers. Maintain sufficient weights to avoid influx of formation fluids. Add sulphide scavengers.

Add lubricants, surfactants, etc. or oil slowly at suction. Maintain low viscosity and gel strength to keep hole clean. Utilise available horsepower for most efficient hydraulics. Do not emulsify for best results.

Keep viscosity and gel strength low to prevent swabbing. Use an emulsion mud to keep bit and drill string clean. Use an inhibitive mud. Raise mud weight to hold bore hole open

Raise weight, increase, viscosity, decrease fluid loss and maintain mud in laminar flow. Use inhibitive mud.

Increase mud weight. Ream through tight spots. If stuck, spot fresh water then increase mud weight.

Problems Symptoms SolutionMud characteristics

2

Foam, surface.

Foam, internal

High fluid loss

High fluid loss

High viscosity

High viscosity

Increase of surface mud weight.

Unstable Mud.

Foam on surface of mud pits. (Not that while converting mud)

Reduction in mud weight, increased viscosity, fluffy appearance. Pumps run rough or hammer and pump pressure falls off.

Normal viscosity but high fluid loss.

Filter cake spongy, soft and too thick. There is sufficient fluid loss agent in system.

High funnel viscosity, plastic viscosity, yield point, gels and solids. Sometimes normal gel and / or solids.

High funnel viscosity, yield point and gels and normal plastic viscosity and solids.

High viscosity.

Barite settles out.

Not serious unless mud weight reduced by internal foam. Keep guns submerged. Use fine spray of water or oil to break foam. Use defoamer if foam persists. In salt or low solids mud, bentonite will be helpful.

Eliminate all mechanical causes of foam. Maintain low viscosity and gel strength. Use defoamers, if necessary.

Add fluid loss agent through hopper.

Add bentonite to system (run Methylene Blue test)

Run mechanical solids removal equipment to discard drill solids and fine barite particles. Water dilution also required. Thinner may be used later.

Add dispersant. Run mechanical solids removal equipment. Check mud for contamination.

Run mechanical solids removal equipment and add water.

Add viscosifier to increase viscosity.

Problems Symptoms Solution

2

Slow Drilling RateSee ‘Bit Balling’ under Formation and ‘High Drill Solids’ under Contamination.

High temperatureHigh temperature Gelation.

Bearing FailureLocked Cones

Difficult to break circulation. Inability to run tools to bottom. High viscosity and gel strengths of mud off bottom. Decrease in alkalinity, increased water loss and increase insoluble calcium.

Cones locked or bearings loose with teeth structure still on cones

Reduce solids concentration by water dilution and add bentonite. Increase thinner concentration. To any mud spot a slurry or mud treated with 1 – 2 lb/bbl sodium chromate in that portion of hole subjected to high temperature.

Reduce drilled solids by water dilution, mechanical separation and use selective flocculants. Use sealed bearing bits.

Abnormal Pressure Formations:

Occurrence Warning Signs Causes Action Of Take

2

When Drilling

When Making Trip

Increase in pump speed or decrease in pump pressure

Drill break

Hole fails to take mud when filling up

Drilling mud too light or hole in drill pipe

Drilled into porous formation

Swabbing

Observe mud pit level. If increase is observed close blowout preventor and increase mud weight. If no increase in mud pit level, check for hole in drill pipe.

Observe mud pit level. If increase is obtained. Close blowout preventer and increase mud weight. Circulate returns up from bottom mud and , if showing of gas, oil, or saltwater is of sufficient intensity, increase the mud weight.

Attempt to run drill pipe as near back to bottom as time will permit. Circulate and condition mud. Close blowout preventer and put through choke, if necessary. Increase mud weight only if necessary.

Occurrence Warning Signs Causes Action Of Take

When Making Trip Well begins to flow Swabbing and / or Attempt to get drill

2

When Out of Hole

back, increase in mud pit level.

Loss of circulation; unable to fill hole

Well begins to flow back; increase in mud pit level

failure to fill hole.

Running drill pipe is too fast, spudding drill pipe.

Swabbing and / or failure to fill hole

pipe as near back to bottom, as safety and time will permit. Close blowout preventor. Circulate and condition mud; increase mud weight only if necessary.

Attempt to keep annulus full using water, if necessary. Watch annulus and should well kick, close blowout preventer. Add lost circulation material to mud system and regain returns. Increase mud weight only if necessary.

Attempt to run drill pipe as near back to bottom as safety and time will permit. Circulate and condition mud. Close blowout preventer and put through choke, if necessary. Increase mud weight only if necessary.