Petroleum eng

DRILLING FLUIDS

JAMES A. CRAIG

TABLE OF CONTENTSTypes of Drilling FluidsComposition of Drilling MudsProperties of Drilling MudsFunctions of Drilling MudsLaboratory/Field Testing

1. TYPES OF DRILLING FLUIDSLiquidsWater-base muds (WBM)Oil-base muds (OBM)GasesAirNatural gasGas-Liquid mixturesFoam (mostly gas)Aerated water (mostly water)

A broad classification of drilling fluids

2. COMPOSITION OF DRILLING MUDSLiquid drilling fluids consist of:The liquid (continuous phase) drilling muds are classified according to their base: water or oil. Solidsreactive solids inert/inactive solidsSoluble 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 watersaturated salt wateranother type of brine fluid

Fresh water WBM Usually available only on land locations.

AdvantagesCommercial clays hydrate moreMost chemicals are more solubleDisadvantagesFormation clays hydrate more, which can result in borehole instability

Brackish water WBMUsually in a marine environmentSlightly saltyHigher calcium and magnesium concentration than fresh water

Seawater WBMChlorides and hardness variesChlorides in GoM: 15,000 30,000 mg/lCalcium in GoM: 400 mg/lMagnesium in GoM: 1200 mg/lHardness in North Sea is much higher

Saturated salt water WBMUsed 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 WBMUsually 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 OBMGood 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 OBMHigher initial cost.Requires more stringent pollution-control procedures.Reduced effectiveness of some logging toolsRemedial treatment for lost circulation is more difficult.Detection of gas kick is more difficult because of gas solubility in diesel oil.

SolidsReactive solids that can react with the water phase and dissolved chemicals.Reactive commercial clay solidsSodium montmorillonite or bentoniteAttapulgiteReactive formation solidsMontmorillonite (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 solidsBarite (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 solidsSandLimestoneDolomite

ChemicalsChemicals 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 MUDSDensity 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.

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4. FUNCTIONS OF DRILLING MUDSSome of the functions are:

Hole cleaningPressure controlSolids suspensionCooling & lubricationPower downhole toolsSupport 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.

Drill pipeDrill collarsDrill bitSurfaceSurface casingIntermediate casingAnnular geometry

Pressure ControlStatic 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).

Solids SuspensionAbility of muds to suspend drill cuttings when the pumps are switched offElse solids will start to settle. This can result in: Bridging off of the wellboreStuck pipeHole fillLoss 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 & LubricationThe 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 ToolsRun 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 SupportAids in supporting part of the weight of the drillstring and casing.The degree of buoyancy is directly proportional to the density of the fluid.

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 cakeMud flow

Formation fluidsFiltrate invasionBridging solidsOther fine solids

Sand matrix

Bentonite

Filtration process

5. LABORATORY/FIELD TESTINGDensity mud balance measures density.

Mud Balance

MaterialSpecific gravityDensitylbm/gallbm/bblWater 1.008.33350Diesel 0.867.20300Bentonite clay2.6021.7910Sand 2.6321.9920Barite (API)4.2035.01,470

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 ccs 44 ccs 15.6 ccs

125.0 ccs 22.0 ccs 9.0 ccs 1.5 min7.5 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 OvenAging Cell