DRILLING MUD LABORATORY

JAMES A. CRAIG

TABLE OF CONTENTS Types of Drilling Fluids Composition of Drilling Muds Properties of Drilling Muds Functions of Drilling Muds Laboratory/Field Testing

§1. TYPES OF DRILLING FLUIDS Liquids

• Water-base muds (WBM)• Oil-base muds (OBM)

Gases• Air• Natural gas

Gas-Liquid mixtures• Foam (mostly gas)• Aerated water (mostly water)

A broad classification of drilling fluids

§ 2. COMPOSITION OF DRILLING MUDS

Liquid drilling fluids consist of:• The liquid (continuous phase) – drilling muds

are classified according to their base: water or oil.

• Solids reactive solids inert/inactive solids

• Soluble chemicals

Continuous PhaseWater base muds (WBM) Solid particles are suspended in water. Any oil added to WBM is emulsified into the

water phase and is maintained as small, discontinuous droplets.

It is called oil-in-water emulsion or emulsion mud.

The continuous phase can be:• fresh water • brackish water • sea water• saturated salt water• another type of brine fluid

Fresh water WBM – Usually available only on land locations.

Advantages• Commercial clays hydrate more• Most chemicals are more soluble

Disadvantages• Formation clays hydrate more, which can result in

borehole instability

Brackish water WBM• Usually in a marine environment• Slightly salty• Higher calcium and magnesium

concentration than fresh water

Seawater WBM• Chlorides and hardness varies• Chlorides in GoM: 15,000 – 30,000 mg/l• Calcium in GoM: ±400 mg/l• Magnesium in GoM: ±1200 mg/l• Hardness in North Sea is much higher

Saturated salt water WBM• Used primarily to drill through large salt

formations.• Salt must be added to achieve saturation.• Prevents hole enlargement due to leaching

or dissolving salt from the formation.• Leaching could result in hole problems and

expensive mud and cement costs.

Brine WBM• Usually used for clay (shale) inhibition.• Potassium chloride (KCl), calcium chloride

(CaCl2), formates (Na+, K+), bromides.

Oil base muds (OBM) Diesel or synthetic-base oil is the continuous

phase. Organophilic clay, and trace amounts of water

as the dispersed phase which acts as a polar activator for the organophilic clay.

If the amounts of water are more than 5%, then it becomes water-in-oil emulsion (invert emulsion).

All solids are in OBM are considered inactive because they do not react with oil.

Advantages of OBM Good rheological properties at high temperature. More inhibitive than inhibitive WBM. Effective against all types of corrosion. Superior lubricating characteristics. Permits mud densities as low as 7.5 ppg.

Disadvantages of OBM• Higher initial cost.• Requires more stringent pollution-control procedures.• Reduced effectiveness of some logging tools• Remedial treatment for lost circulation is more

difficult.• Detection of gas kick is more difficult because of gas

solubility in diesel oil.

Solids

Reactive – solids that can react with the water phase and dissolved chemicals.

Reactive commercial clay solids• Sodium montmorillonite or bentonite• Attapulgite

Reactive formation solids• Montmorillonite (swelling clay)• Kaolinite (non-swelling clay)• Chlorite (non-swelling clay)• Gumbo shale (combination of above clays)

Inert/Inactive – solids that do not react with the water phase and dissolved chemicals to a significant degree.

Inert commercial solids• Barite (barium sulphate) – used to increase mud

density up to maximum of ±22 ppg.• Hermatite (iron oxide) – used to increase mud density

up to maximum of ±25 ppg.• Calcium carbonate – used to increase mud density up

to maximum of ±14 ppg. Also as bridging agent in drill-in, oil and synthetic fluids.

• Lost circulation material (LCM) – Used to bridge off (seal) formations where whole mud is being lost to the formation e.g. nut shells (mostly pecan and walnut), mica, fiber (wood, paper, plastic, etc.).

Inert formation solids• Sand• Limestone• Dolomite

Chemicals

Chemicals are added to “fine tune” drilling fluids for specific purposes. Examples are:• Caustic Soda (NaOH)• Caustic Potash (KOH)• Lime (Ca(OH)2)• Chemical de-flocculant (mud thinner)• Lignosulfonates (organic acid)• Soda Ash (Na2CO3)• Starch

§3. PROPERTIES OF DRILLING MUDS

Density – weighing materials are used to increase mud weights. Examples are: barite, hematite.

Flocculation – thickening of the mud due to edge-to-edge association of clay platelets. Examples of flocculants are: hydrated lime, gypsum,

and sodium tetraphosphates.

Deflocculation – reducing the tendency of a mud to flocculate. • Examples of deflocculants (thinners and dispersants)

are: tannins (quebracho) lignitic materials, and various polyphosphates.

Viscosity – resistance to flow. • Examples of viscosifiers are: Attapulgite clays, asbestos

fibers, sodium carboxymethylcellulose (CMC).

pH – the pH of most muds is maintained between 9.5 and 10.5.

High mud pH is desirable to suppress corrosion rate, hydrogen embattlement, and the solubility of Ca2+ and Mg2+. High pH is also favourable for many organic viscosity control additives. • Examples are: lime, caustic soda, and bicarbonate

soda.

Filtration – tendency of the liquid phase of a drilling fluid to pass into the formation. • Filter loss additives include: pregelatinized starch,

CMC, and sodium polyacrylate.

Emulsion – creating a heterogeneous mixture of two liquids. • Emulsifiers include: modified lignosulfonates, certain

surface-active agents, anionic (negatively charged) and non-ionic (noncharged) products.

§4. FUNCTIONS OF DRILLING MUDSSome of the functions are:

Hole cleaning Pressure control Solids suspension Cooling & lubrication Power downhole tools Support part of drillstring

Hole Cleaning Where possible hole cleaning should be

achieved by Annular Velocity (AV). It should be 100 ft/min, higher in deviated

holes. In large hole sections the AV can be as low as

20 ft/min.

If high AV is not possible to achieve due to pump limitations or due to the risk of wellbore erosion, then viscosity must be increased.

Pump rate (bbls/min)AV (ft/min)

Annular volume (bbls/ft)

Slip velocity (ft/min) Cutting velocity AV

Drill pipe

Drill collars

Drill bit

Surface

Surface casing

Intermediate casing

Annular geometry

Pressure Control Static condition – the pressure balancing the

formation.

Circulation condition – effective pressure is increased by the pumping pressure. It is given as the Effective Circulating Density (ECD).

P (psi) 0.052 Depth (ft) Density (ppg)

Annular Pressure Loss (psi)ECD (ppg) Density (ppg)

0.052 Depth (ft)

Solids Suspension Ability of muds to suspend drill cuttings when

the pumps are switched off Else 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 needs to be easily broken or pressure surges will result when the pumps are switched on.

This can fracture the formation.

Cooling & Lubrication The drilling fluid removes heat from the bit

which is then dispersed at the surface. Extra lubrication may be required between

the drill string and the casing or wellbore, especially in directional wells.• Liquid additives are used, or oil based mud.• Solid additives are sometimes used such as glass

beads, plastic beads, graphite or nut plug.• Drill pipe rubbers are sometimes added to reduce

wear between the casing and drill pipe.

Power Downhole Tools Run 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.

Turbine motor Positive displacement motor (PDM)

Drillstring Support Aids in supporting part of the weight of the

drillstring and casing. The degree of buoyancy is directly

proportional to the density of the fluid.

Effective weight lbm Weight in air lbm BF

Mud density ppgBuoyancy factor, BF 1

Steel density ppg

Other Functions Should be environmentally acceptable to the

area in which it is used. Should not cause corrosion of the drilling

equipment and subsurface tubulars. Should not damage the productive formations

that are penetrated (filtration property).

Filter cake

Mud flow

Formation fluids

Filtrate invasion

Bridging solids

Other fine solids

Sand matrix

Bentonite

Filtration process

§5. LABORATORY/FIELD TESTING

Density – mud balance measures density.

Mud Balance

Material Specific gravity

Density

lbm/gal lbm/bbl

Water 1.00 8.33 350

Diesel 0.86 7.20 300

Bentonite clay 2.60 21.7 910

Sand 2.63 21.9 920

Barite (API) 4.20 35.0 1,470

Densities of some additives.

Mud rheology – measures viscosity and gel strength of mud. The 2 types are: Marsh funnel viscometer and Rotary viscometer.

Marsh funnel kit It measures number of seconds for a quart of the sample to run out.

Rotary viscometerIt determines the flow characteristics (viscosity and gel strength) of muds in terms of shear rate and shear stress.

Mud filtration – filter press measures the filtration of drilling muds.

Standard API Filter PressUsed at ambient temperature.

Filter cakes

API FL = 560 cc’s 44 cc’s 15.6 cc’s

125.0 cc’s 22.0 cc’s 9.0 cc’s 1.5 min 7.5 min 10.0 min

16.8% 10.4% 3.0% Solids Solids Solids

HPHT Filter PressUsed at elevated temperature and pressure.

Sand contents – sand content kit determines the volume percent of sand-sized particles in the drilling fluid.

Sand Content KitAPI defines sand-sized particles as any material larger than 74 microns (200-mesh) in size.

Resistivity – provides a rapid means of detecting soluble salts in barite and in waters, such as makeup or produced waters.

Resistivity MeterResistivity of water muds, filtrates and filter cakes are routinely applied in electrical logging.

pH (Hydrogen ion concentration) – The term pH is used to express the concentration of hydrogen ions in aqueous solution. It can be determined either using the colorimetric method (pH paper) or the electrometric method (pH meter).

pH PaperThe pH paper is impregnated with dyes that exhibit different colours when exposed to solutions of varying pH.

pH MeterIt determines the pH of an aqueous solution by measuring the electropotential generated between a special glass electrode and a reference electrode.

Oil, water and solids content determination

Retort KitThe retort provides a means of separating and measuring the volumes of water, oil, and solids contained in a sample of drilling fluid.

Methylene blue test (MBT) – The methylene blue capacity of a drilling fluid is an indication of the amount of reactive clays (bentonite or drilled solids).

MBT KitThe methylene blue capacity gives an estimate of the total cation exchange capacity (CEC) of the solids in the drilling fluid.

Lubrication – lubricity tester is used to determine lubrication property of OBM.

Lubricity Tester

Aging – aging cells and roller oven are used to determine the “aging” effects on muds.

Roller Oven Aging Cell