0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 4 5 6 7 8 9 10 11 12 pH % Molar Fraction
H2CO3 HCO3- CO3--
This graph illustrates an important point, as you increase the pH with caustic the concentration of carbonic acid falls as the bicarbonate climbs to a maximum. As you further increase the pH the bicarbonate disappears and is replaced by carbonate ions. You can use this information to estimate the types and concentrations of bicarbonate, carbonates and hydroxides in your mud, this is called the Pf/Mf Method.
Pf/Mf alkalinity
If we took a pH reading of a mud sample, looking at the graph above, we could deduce the types of ions in solution.
If pH > 11.6 (excess OH-), the only species you could test for would be OH- & CO3 2-
.
If pH = 11.6 then the only ion present would be the CO32- (any OH- present would increase the pH).
If pH < 11.6, then there would be no OH- (as it is all used up to convert bicarbonate to carbonate): only HCO3- & CO32-.
If the pH < 8.3 there would be only HCO3- & H2CO3.
The fifth and final case would be if there were no other ions present except the hydroxide ions you added.
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All these ion concentrations can be estimate in the field by the Pf/Mf Method. (There are other methods that will be covered later in this manual but the theory is the same). The method involves taking a small filtrate sample, finding the pH, adding a pH color indicator (phenolphthalein indicator is pink above pH 8.5) and titrating (just as the pink color disappears) with 0.02N H2SO4. The volume of acid added to make the pink disappear equals the Pf. Another indicator is added to the filtrate sample (bromocresol green, apple green color below pH 4.5) and acid 0.02N H2SO4 is added just to the point where the liquid turns green and the volume recorded. That second volume of acid is equal to the Mf.
Table 2.13
Pf/Mf Method Comments
If Pf = Mf [OH-] = (2Pf – Mf) x 340 Only [OH-] ions, no contaminates
If Pf = 0 [HCO3-] = Mf x 1220 Only [HCO3-] ions, will have a low pH (< 8.3) If 2Pf = Mf [CO32-] = Pf x 1200
Only [CO32-] ions, two protons needed to neutralize CO32-
If 2Pf > Mf [OH-] = (2Pf – Mf) x 340
[CO32-] = (Mf – Pf) x 1200 Both ions present. pH is > 11.6 If 2Pf < Mf [CO32-] = Pf x 1200
[HCO3-] = (Mf – 2Pf) x 1220 Both ions present. pH is between 11.6 and 8.3
By knowing the concentration and quantity of acid required to neutralize an alkaline solution and by using pH dependant color indicators, the concentration of species may be calculated.
An excessive concentration of either HCO3- or CO32- can become, in essence a contaminant.
2.7 SURFACE CHEMISTRY – COLLOIDS REVISITED
The formal study of colloids began in the latter part of the 19th century with the studies of Thomas Graham. The first colloids studied were gelatins and glues, and so Graham used the Greek work “kolla”, meaning glue, as the root for his newly coined term.
Colloidal particles may be gaseous, liquid, or solid. They may occur in various types of suspensions, e.g. solid/gas (aerosol), solid/solid, liquid/liquid, liquid/solid (emulsion), gas/liquid (foam). It may be useful to observe that a suspension is any system in which small solid or liquid particles are more or less evenly dispersed in a second medium, typically a gas or a liquid.
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Colloid examples: a) Clay, b) Cement, c) Latex or blood, d) Polymers
In the size range of colloidal particles, the surface area of the particle is so much greater than its volume that some unusual behavior is observed, e.g. the particles do not settle out by gravity (i.e. they neither float nor sink). Many macromolecules are at the lower limit of this size range (a nanometer). The upper limit to colloidal particle size is commonly taken to be the size at which the particles become visible in an optical (i.e. light) microscope (about 1 micrometer). Natural colloidal systems include rubber latex, milk, blood, and egg-white.
Aerosols are suspensions of liquid or solid particles in a gas. The particles are often in the
colloidal size range, making many aerosols colloidal suspensions. Fog (water/air) and smoke (C/air) are common examples of natural aerosols. Fine sprays such as those used with perfumes, insecticides, inhalants, anti-perspirants, and paints are man-made aerosols.
An emulsion is a stable mixture of two or more immiscible liquids held in suspension by small amounts of substances called emulsifiers. Small carbohydrate polymers like starch (which are
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themselves colloidal in size) often act as emulsifiers by coating the surfaces of the dispersed phase and thus preventing coalescence. Such emulsifying agents are called protective colloids as they protect the dispersed phase from coalescence and subsequent separation. Long-chain alcohols and fatty acids can also act as emulsifiers by "solubilizing" the dispersed phase by virtue of the formers solubility in the dispersing medium (often water).
These emulsifying agents are called detergents. Commercial polymerization reactions are often carried out in emulsion form. Floor and glass waxes, many drugs, photographic coatings, and paints are all examples of emulsion systems.
Foams are dispersions of gases in liquids or solids. If the gas globules are of colloidal size, the
foam is colloidal foam. Yeast breads are examples of solid foams. Shaving cream and whipped cream are good examples of liquid foams. Useful foams for automobile seats and mattresses are made from natural and synthetic (e.g. polystyrene, polyurethane) latexes. Metal and concrete foams are also possible.
Any of these surfaces and interfaces can, and commonly do, occur in drilling fluids.
2.7.1 Surfaces
Surfaces can be very complex, and the majority of this science is beyond the scope of this chapter. Suffice it to say that there are two major properties; surface area and electronic charge. What do we mean by surface area? As explained above the smaller the particle gets the greater the surface area becomes. Surface area is also a function of the interior of the particle, if the material is porous (like a sponge) then liquid or gases can travel through the interior spaces. Clay is like a sponge; in fact with some clay a handful has as much surface area as a football field. Fully dispersed kaolinite clay can have a surface area of 15 m2/g, and a bentonite close to 800 m2/g.
The other property is electronic charges. Think about a copper wire, how does an electric current travel down a wire? At an atomic level there are “holes” where electrons can travel through the copper atoms and areas with electron density and deficiency. When a charge is applied to a wire, the electrons travel through these holes from a low electron density to a high electron density.
Most surfaces have both these properties in varying degrees. These properties can influence (catalyze) or be part of a chemical reaction. They can form a semi-permeable membrane and channel water. They can provide pores to “store” atoms and bind atoms. They can also bind together and form colloids and suspensions.
With drilling fluids these properties can influence viscosity, emulsified brine droplets, barite particles etc. Knowledge of the nature of a surface allows for a better understanding and control of drilling fluid properties. For example, the surface of steel usually has a net negative charge when in an aqueous environment. When a cationic surfactant is added to the fluid, its molecules bond to the steel, providing a defensive coating from a corrosive environment.
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2.7.2. Surface Tension
Surface tension is the force at the surface of a liquid due to adhesive forces of the liquid
molecules for the walls of the container and the attractive forces of the molecules of liquid for each other. When the adhesive forces of the molecules for the walls of the container are greater than the attractive forces between the liquid molecules, then the surface of a liquid confined to a narrow diameter container will curve downward forming a concave surface called a meniscus. Most important examples are water solutions. The water adheres to the surface of the container greater than the water molecules are attracted to each other. We do not see this downward curvature when the surface area is great, but if the liquid is confined to a small diameter tube such as a graduated cylinder, pipette, burette, or volumetric flask then the surface tension is great enough to noticeably distort the surface. In such cases when we are trying to read the liquid surface level such as measuring a liquid in a graduated cylinder, then one should make the reading at eye level and the lowest curvature of the meniscus should be read.
When the adhesive forces against the walls of the container are less than the intermolecular forces, then the surface of a confined liquid will bulge upward slightly forming a convexed surface. Again, such a surface should be read at eye level and the topmost part of the surface should be read. Surface tension helps to explain why the feathers of a duck can help the duck float on water.
Although molecules in a liquid are electrically neutral in nature, there are often small attractive forces between them. These attractive forces (called Van der Waals forces) are caused by the asymmetrical charge distribution inside the molecules. Within a body of a liquid, a molecule will not experience a net force because the forces by the neighboring molecules all cancel out (Figure 22). However for a molecule on the surface of the liquid, there will be a net inward force since there will be no attractive force acting from above the molecule (Figure 22). This inward net force causes the molecules on the surface to contract and to resist being stretched or broken. Thus the surface is under tension and has Surface tension.
Figure 22
Figure 23
mg
F
F
Due to the surface tension, small objects will "float" on the surface of a fluid. A needle will float on water! This can be seen in Figure 23. When an object is on the surface of the fluid, the surface under tension will behave like an elastic membrane. There will be a small depression on the surface of the water. The vertical components of the forces by the molecules on the object will balance out the weight of the object. Surface tension also occurs at the interface between a solid and gas, a solid and a liquid and between two immiscible liquids.
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This is why water forms beads and soap forms bubbles. The degree of polarization in a liquid, determines the degree of imbalanced attractive forces in it. This net force is called the fluid’s surface energy. Surface tension is measured in dynes/cm. At 20°C the surface tension of water is 72.7 dynes/cm, decreasing to 67.9 dynes/cm at 50°C.
2.7.3 Emulsion and Foam
An emulsion is a stable mixture of two or more immiscible liquids held in suspension by small concentrations of substances called emulsifiers. As a drilling fluid term, the word emulsion applies to small oil drops, the dispersed or discontinuous phase, in water the continuous phase. Invert emulsions employ oil as the continuous phase, while water is the dispersed phase. In an invert emulsion system, the emulsified water drops may at times be sub-micron size. This creates a proportionately large surface area.
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