Trouble Shooting Of Drilling Mud

The drilling fluid is a major factor in success of the drilling program. There are four mechanical or physical properties to control in the treatment of drilling mud:

  1. density
  2. viscosity
  3. gel strength
  4. filtration

Mud Density (or Weight)

The density or weight of a drilling mud is easily controlled by additions of barite to increase weight, or by water or oil to decrease weight.

See Table 1 and Table 2 to determine weight reduction by oil or water.

Table 1. Effect Of Water On Weight

 Water add BBL/100BBL mud. Weight of Resultings Lb./Gal
 0  10  11  12 13 14 15 16 17
 5  9.9  10.9  11.8  12.8  13.7  14.7  15.6 16.6
 10  9.8  10.8  11.7  12.6  13.5  14.4  15.3  16.2
 15  9.8  10.6  11.5 12.4  13.3  14.1  15  15.9
 20  9.7  10.6  11.4  12.2  13.1  13.7  14.7 15.6
 25  9.7  10.5  11.3  12.1  12.9  13.7  14.5  15.3
 30  9.6  10.4  11.1  11.9  12.7  13.5  14.2  15
 35  9.6  10.3  11  11.8  12.5  13.3  14  14.7
 40  9.5  10.2  10.9  11.7  12.4  13.1  13.8 14.5
 45  9.5  10.2  10.9  11.6  12.2  12.9  13.6  14.3
 50  9.4  10.1  10.8  11.4  12.1  12.8  13.4  14.1
 60  9.4  10  10.6  11.2  11.9  12.5  13.1  13.7
 70  9.3  9.9  10.5  11.1  11.7  12.2  12.8 13.4
 80  9.3  9.8  10.4  10.9  11.5  12  12.6  13.1
 90  9.2  9.7  10.3  10.8  11.3  11.8  12.4  12.9
 100  9.2  9.7  10.2  10.7  11.2 11.7  12.2  12.7

Table 2  Effect of Oil On Weight Mud

 Oil Add BBL/100BBL Mud  Oil In Resulting Mud, % By Volume Weight of Resulting Mud, Lbs/Gal
 0  0  10  11 12 13 14 15
 1  1  10  10.9  11.9  12.9  13.9  14.9
 5  4.8  9.8  10.8  11.7  12.7  13.7  14.6
 8  7.4  9.7  10.7  11.6  12.5  13.5  14.4
 10  9.1  9.7  10.6  11.5  12.4  13.3  14.3
 11  9.9  9.7  10.6  11.5  12.4  13.3  14.2
 13  11.5  9.6  10.5  11.4  12.3  13.2  14
 15  13.1  9.5  10.4  11.3  12.2  13  13.9
 18  15.3  9.5  10.3  11.2  12  12.9  13.7
 20  16.7  9.4  10.3  11.1  12  12.8  13.6
 25  20 9.3 10.1  11  11.7  12.5  13.3
* weight reduction calculated for addtion 40deg. API oil

Use Equation formula to determine the barite required to raise the density from one mud weight to another and the resulting volume increase.

solids Vs mud weights
Figure 1. solids Vs mud weights

Viscosity and Gel Strength

The control of viscosity and gel strength depends on the solids and the chemical environment. The primary concern is the amount and typo of low-gravity solids contained in the mud. A centrifuge can also be utilized to lower the low gravity solids content of a drilling mud. Water can be expected to decrease the plastic viscosity. The addition of a suitable chemical will decrease the yield point. When solids are in the correct range, most viscosity and gel strength problems can be corrected by proper chemical treatment.

Filtration

An API filtration test is a relative measure of performance. It does not necessarily follow that down-hole filtration is the same. Only high-pressure, high-temperature tests can give an indication of down-hole filtration. The condition of the mud, and particularly the type of solids present in the mud, greatly influence filtration.

Specific Problems

Contamination

Problem – High Drilled Solids

Symptoms

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

Solution

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

Problem – Abrasion

Symptoms

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

Solution

Use a desander to hold sand content to a minimum. Normally <2% by volume is ideal.

desander and desilter
Desilter and desander with vibrational shaker

Problem – Cement

Symptoms

High viscosity, high gel strengths, and increase Ph, water loss and filtrate calcium.

Solution

Pretreat if possible, or for low concentration remove chemically with sodium bicarbonate, add lignosulfonate and/or lignite to reduce viscosity gel strengths.

Problem – Gypsum or Anhydrite

Symptoms

High viscosity, high flat gel strengths, and increase water loss, filtrate calcium and sulfate.

Solution

Pretreat for small quantities or remove chemically with soda ash. For drilling massive anhydrite change to mud which will tolerate anhydrite.

Problem – Salt Rock

Symptoms

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

Solution

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

Problem – Salt Water

Symptoms

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

Solution

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

Abnormal Low Pressure

Problem – Lost Circulation

Symptoms

Partial to complete loss of returns. Complete losses may best be remedied by the use of soft plugs such as diesel oil-bentonite squeezes, or diesel oil-bentonite-cement squeezes.

Solution

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 returns.

Abnormal High Pressure

Problem – Formation Gas or Water Influx

Symptoms

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

Solution

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.

Problem – Gas Cutting.

Usually from gas bearing shale and or other high pressure, low volume formation.

Symptoms

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.

Solution

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.

Problem – Differential Sticking of Drill Pipe.

Symptoms

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.

Solution

If possible, reduce mud weight. Spot diesel or crude oil treated with surfactants. A non-oil spotting fluid may be preferred in the Gulf of Mexico, due to environmental constraints on oil spots. For prevention, measure fluid loss at static bottom hole temperature and 500 psi differential pressure and use minimum safe mud weight.

Corrosion

Problem – General, salt muds. etc.

Symptoms

Internal and external pitting, more pronounced internally in uncoated pipe.

Solution

Add corrosion inhibitors. Use oxygen scavengers.

Problem – Aerated Mud

Symptoms

Severe pitting of drill pipe more pronounced internally.

Solution

Extremely difficult to control. Use corrosion inhibitors and oxygen scavengers.
Problem – Sulfide Stress Corrosion

Symptoms

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

Solution

Maintain sufficient weight to avoid influx of formation fluids. Add sulfide scavengers.

Formation

Problem – Bit Balling

Symptoms

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

Solution

Use inhibitive mud systems containing lubricants, surfactants, etc. or oil slowly at suction. Utilize available horsepower efficient hydraulics. for results, do not emulsify. Control drill to avoid packing fraction of drilling fluid.

Problem – Bentonitic Swelling

Symptoms

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

Solution

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.

Problem – Running, Sloughing, Caving Shales

Symptoms

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

Solution

Raise weight, increase viscosity, decrease fluid loss, maintain mud in laminar flow. Use asphalt or gilsonite products to seal micro fractures.

Problem – Plastic Salt

Symptoms

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

Solution

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

Mud Characteristics

Problem – Foam at Surface

Symptoms

Foam on Surface of Mud Pits (not while Converting Mud).

Solution

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

Problem – Foam, Internal

Symptoms

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

Solution

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

Problem – High Fluid loss (1)

Symptoms

Normal viscosity but high fluid loss test.

Solution

Add fluid loss agent through hopper.

Mud hopper with premix chamber.
figure 2. Mud hopper with premix chamber.

Problem – High Fluid Loss (2)

Symptoms

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

Solution

Stabilize system with deflocculant addition.

Problem – High Viscosity (1)

Symptoms

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

Solution

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

Problem – High Viscosity (2)

Symptoms

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

Solution

Add deflocculant. Run mechanical solids removal equipment. Check mud for contaminants.

Problem – Increase Of Surface Mud Weight

Symptoms

High viscosity.

Solution

Run mechanical solids removal equipment and add water.

Problem – Unstable Mud

Symptoms

Barite settles out.

Solution

Add viscosifier to increase viscosity.

Slow Drilling Rate

Problem – Bit Balling

Symptoms

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

Solution

Use inhibitive mud systems containing lubricants, surfactants, etc. or oil slowly at suction. Utilize available horsepower efficient hydraulics. for results, do not emulsify. Control drill to avoid packing fraction of drilling fluid.

Problem – High Drilled Solids

Symptoms

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

Solution

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

High Temperature

Problem – High Temperature Gelation.

Symptoms

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 in soluble calcium.

Solution

Reduce solids concentration by water dilution. Increase thinner concentration. Spot high temperature treated mud on bottom.

Bearing Failure

Problem – Locked Cones

Symptoms

Cones locked or bearings loose with teeth structure still on cones.

Solution

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

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