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QMax Technical Bulletin #6

Viscosity Modification in Oil Muds

All drilling fluids are typically described by whichphase is external, and it is not the percentage of thedominant product that determines the type. Thus,you have water-based drilling fluids, and oil-baseddrilling fluids. A direct emulsion contains typically 20to 30% oil phase (as high as 80% in Mexico)contained in an external water phase, and althoughit may have a greater volume of oil, it would still be a

water-based mud.There are two types of oil exterior phase drilling fluidsThe first type is called an all-oil mud which would onlycontain slight amounts of incidental water fromdrilling, or contamination from tanks and lines, and aninvert which intentionally contains water as the internaphase. This water phase is generally brine, formed bythe addition of specific salts, typically calciumchloride, with concentrations most often between20 and 35% salt.

For the sake of brevity this paper will discuss the most

common oil-based system, the invert.Typically, oil muds can have their viscosities altered byseveral means:

1. Addition of Viscosifiers

The most common method of viscosifying an invertis by the use of organophilic clays. These are clays,which have been treated with amines in order tomake them oil dispersible. They can be bentonite,attapulgite or hectorite; with bentonite typicallyused up to about 350o F, and hectorite used athigher temperatures. Attapulgite is more oftenused in packer fluids where it imparts suspensionproperties without undue increases in the viscosity.These products are added to increase the carryingcapacity, gel strengths, and suspension propertiesof the fluid. They will have the added effects ofimproved filter cake and filtration control.


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2. Addition of Viscosity ModifiersCertain products such as dimer fatty acids orpolyamides can be used as viscosity modifiers.These will impart low shear rate viscosities and gelstrengths to the inverts without much effect on theplasticviscosity. Typically these products requirehigh temperature or shear to fully yield. Care mustbe taken to allow sufficient mixing time at the blend-ing plant, but not to overtreat; otherwise the prop-erties will increase too much when the mud goesthrough the drill bit several times. It should also benoted

that the fatty acids require at least 10% of a polaradditive (e.g. water) to function properly, while thepolyamides need solids to function properly.

3. Changing the Oil-Water Ratio

Alternately, the oil-water ratio can be adjusted tochange the viscosity. Greater amounts of oil willdecrease the viscosity, whereas increasing thevolume of the internal water phase will increase theviscosity. Additionally, unforeseen increases in thewater content may cause the mud to become waterwetting, leading to shale hydration problems, well-

bore instability, water-wet solids etc. The addition ofappropriate wetting agents will reverse this problem

As with most percentage increases in fine solids,increasing the concentration of emulsified waterdroplets will generally affect the plastic viscos-ity, because the water droplets themselves act assolids.

4. Addition of Filtrate Control Additives

Polymeric additives, such as resin copolymer, whichare often added to lower the HTHP fluid loss, canoften have the effect of viscosifying inverts. Also,gilsonite or asphalt products, which are normallyused to impart filtration control, will, at highconcentration, impart some viscosity to the invert.These two types of products can impart viscosityeven to pure oil systems.


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5. Addition of Weighting AgentsAs in all mud systems, the addition of weightingmaterials, whether calcium carbonate, barite, orhematite, will increase the viscosity of the invert.Since the already entrained water droplets of anemulsion act as solids, it is normal that higherdensity muds will need a higher oil to water ratio.This high oil content is needed to prevent excessiveviscosities caused by the cumulative effects fromboth the water droplets and the added weightingmaterials. This is particularly noticeable with thelower density calcium carbonate, where more

volume of carbonate is required to obtain a desireddensity. Alternately, hematite is going to requiremore wetting agent because it is a more difficultsolid to oil-wet.

6. Fine Drill Solids

Although oil exterior drilling fluids generally have ahigher tolerance to solids than water-based drillingfluids, this is primarily due to the prevention ofinteraction of water with water-sensitive shales.However native clays or drill solids, which have be-come finely ground due to their being recirculated,

will have high surface areas. These oil-wetted solidswill help lower the fluid loss due to their smallparticle size, but the high amount of colloidal sizedfines causes an increase in plastic viscosity. Also,due to the huge surface area of these fines therewill be robbing of surfactants needed to keep theemulsion stable and this may cause erratic increas-es in viscosity. It is good operating practise to usefinest possible screens on shale shakers and torun centrifuges periodically, to rid the mud systemof these fine, non-wanted particles. Generally thecosts associated with extra surfactants or dilution to

regain desired properties, is more than the costsincurred by the use of adequate solids controlequipment. When drilling salt zones, particularlywith concentrations of brine greater than 35%, beextremely careful that upon cooling there is noprecipitation of dissolved salts. These colloidalparticles of salt will cause a tremendous need forwetting agents to prevent water-wet solids. Thesimplest method of control is the addition of a littlefresh water.


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7. Altering the Size of Water DropletsWater in inverts is generally required to be tightlyemulsified, unless a relaxed filtrate is wanted. Anyextra water in the mud system, caused by eitherwater flows, incorporation of water from drilling aparticular formation, or even from heavy rains, willeventually need to be emulsified. Generally thereare enough wetting agents and emulsifiers in aninvert to incorporate the water as it is added. How-ever, either through the incorrect type of emulsifieror insufficient amounts, it is possible that the waterdroplets may be large, (generally noted by poor

emulsion stability readings). Forming a better, morestable emulsion can solve the problem of low emul-sion stabilityreadings or poor HTHP performance (showing waterin the filtrate). The emulsion will generally be im-proved by the water droplets being more finely sizedby the high turbulent shearing action of the bit,and/or the addition of more emulsifier package. Thenumber of particles of water will increase as theirsize decreases. The exponential increase in thenumber of particles will cause a significant increasein the viscosity of the invert.

8. Type of Oil

Inverts can be formulated from many different baseoils or combinations of base oils. The list can in-clude crude oil, diesel (winter or summer grades),ethers, esters, internal olefins, linear alpha olefins,linear paraffins, mineral oils, or poly alpha olefins.It may be that a particular oil is used for resolvingsome particular problem, other than one that occursin the drilling of a well, e.g. disposal. It needs to beremembered however that generally the base oilused will mpart its properties to the invert.

Viscous base oils will be made more viscous whenformulating inverts from them, while low toxicityoils are inherently more difficult to emulsify due tothe low content of aromatics, and may requiredifferent emulsifiers or viscosifiers, or at leastincreased amounts of viscosifiers.


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9. Wetting Agents as ThinnersWater-wetted particles such as colloidal sizedsolids have been mentioned as causing high plasticviscosities and high overall viscosity if they existin large amounts. The effect is magnified wheninsufficient surfactants are available to both oil-wetthe solids and also to reside at the surface layers ofthe water droplets. Consequently, the addition of awetting agent or secondary emulsifier will providesufficient surfactant to resolve this problem. Adecrease in viscosity will occur as the particlesgo from a water-wet state to an oil-wet state. This

will be noted by the solids no longer piggybackingto each other on the shakers causing whole mudlosses, or sticking to metal surfaces, and theirappearance will generally be more angular and lessrounded. As well the mud will lose its grainyappearance and exhibit a shiny appearance.

10. Changes in Pressure

Unlike water-based fluids, which are insensitiveto the effect of pressure, oil-based fluids arecompressible, and consequently the viscosity isaffected by pressure changes, with an increase in

pressure leading to an increase in viscosity. Theseincreased pressures are caused by increases indepth, changes in the circulation rate, or increasesin density.

11. Changes in Temperature

Oils, which are heated, will decrease their viscositywhile oils that are cooled will increase theirviscosity. Starting pumping after long periods ofinactivity will cause cold invert to fill the drill stringand the annulus, and to slowly warm up as it isexposed to bottom hole temperatures. During thistransition period, there will be increased danger oflost circulation caused by the increased equivalentcirculating density in the well, until the circulatingsystem fully adjusts to the downhole temperature.Care must be taken to ensure that densitycontrol takes account of this effect. Also, there canbe losses over the shaker when mud is initiallycirculated, after sitting, due to having losttemperature in the upper part of the wellbore.


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Note that the effect of temperature is typicallyoffset somewhat by the increase in pressure, asthe effects of increased depth will lead to higherpressure which leads to increased viscosity, whilethe higher temperatures downhole would tend todecrease the viscosity.

12. Acid Gas Influx

Hydrogen Sulphide that is not treated out by a zincscavenger or amine product can rob the invert ofthe lime that is necessary in many cases to formthe calcium soap used as the primary emulsifier.In general, sufficient excess lime is carried toneutralise the gas prior to this problem occurring.However, in severe cases you may experienceerratic properties. Although carbon dioxide is nottreatable by a scavenger, it too will consume thelime of the invert. To regain control over theviscosity fluctuations caused by the effect of eithergas, the invert will need its density increased toprevent the gas invasion. This density increasewill need emulsifier additions as well as wettingagents to compensate for the extra solids, aswell as the replacement of the lime, which was

removed by the acid gases. In general, the controlof invert viscosity can be summarised in severalsimple rules.

a) Reduce plastic viscosity by removing the solids,adding wetting agents or secondary emulsifiers,and/or by dilution with the base oil.

b) Increase yield point by the addition of water,organophilic clays or polymeric additives, ordecrease it by the addition of wetting agents ordilution with the base oil.

c) Gel strengths are treated similarly to yield pointmentioned above.

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