Properties of Drilling Fluids

Density of mud:

Density is defined as weight per unit volume. It is expressed either in pounds per gallon (lb/gal) or pounds per cubic foot (lb/ft"), or in kilograms per cubic meter (kg/m³), or compared to the weight of an equal volume of water, as specific gravity (SG). The pressure exerted by a static mud column depends on both the density and the depth; therefore, it is convenient to express density in terms of pounds per square inch per foot (psi/ft), or kilograms per square centimeter per meter (kg/cm/m).
In order to prevent the inflow of formation fluids and to lay down a thin, low-permeability filter cake on the walls of the hole, the pressure of the mud column must exceed the pore pressure-the pressure exerted by the fluids in the pores of the formation-by at least 200 psi (14 kg/ern'). The pore pressure depends on the depth of the porous formation, the density of the formation fluids, and the geological conditions.

Normally pressured formations, which have a self-supporting structure of solid particles (so the pore pressure depends only on the weight of the overlying pore fluids), and abnormally pressured or geopressured formations, which are not fully compacted into a self-supporting structure (so the pore fluids must bear the weight of some of the overlying sediments as well as the weight of the overlying fluids). The hydrostatic pressure gradient of formation fluids varies from 0.43 psi/ft to over 0.52 psi/ft (0.1 to 0.12 kg/cm/m), depending on the salinity of the water.

The bulk density of partially compacted sediments increases with depth, but an average (SG) of 2.3 is usually accepted, so that the overburden (or geostatic or litholostatic) pressure gradient is about 1 psi/ft (0.23 kg/cm2/m), and the pore pressure of geopressured formations is somewhere between the normal and the overburden pressure gradients, depending on the degree of compaction.
Besides controlling pore fluids, the pressure of the mud column on the walls of the hole helps maintain borehole stability. In the case of plastic formations, such as rock salt and unconsolidated clays, the pressure of the mud is crucial.
The buoyant effect of the mud on the drill cuttings increases with its density, helping transport them in the annulus, but retarding settling at the surface. Very rarely is an increase in mud density justified as a means of improving cutting-carrying capacity.

In the interest of well safety, there is a natural tendency to carry a mud density well above that actually needed to control the formation fluids, but this policy has several major disadvantages. In the first place, excessive mud density may increase the pressure on the borehole walls so much that the hole fails in tension. This failure is known as induced fracturing.

In induced fracturing, mud is lost into the fracture so formed, and the level in the annulus falls until equilibrium conditions are reached. The problem of maintaining mud density high enough to control formation fluids, but not so high as to induce a fracture becomes acute when normally pressured and geopressured formations are exposed at the same time. Under these circumstances, it i generally necessary to set a string of casing to separate the two zones. Several methods have been developed for predicting the occurrence of geopressures.' Knowledge of the expected pore pressure and fracture gradients.

Another disadvantage of excessive mud densities is their influence on drilling rate (rate of penetration R.O.P). Laboratory experiments and field experience have shown that the rate of penetration is reduced by mud overbalance pressure (the differential between the mud pressure and the pore pressure when drilling in permeable rocks) and by the absolute pressure of the mud column when drilling rocks of very low permeability. A high overbalance pressure also increases the risk of sticking the drill pipe.

Lastly, excessive mud densities are a disadvantage because they unnecessarily increase mud costs. Mud costs are not a very important consideration when drilling in normally pressured formations, because adequate densities are automatically obtained from the formation solids that are dispersed into the mud by the action of the bit. Mud densities greater than about I lb/gal (1.32 SG) cannot be obtained with formation solids because the increase in viscosity is too great. Higher densities are obtained with barite which has a specific gravity of about 4.25, as compared to about (2.6) for formation solids, so that much less of solids by volume is required to obtain a given density. Mud costs are increased not only by the initial cost of the barite, but also, and to a greater extent, by the increased cost of maintaining suitable properties, particularly flow properties. Because of the incorporation of drilled solids, the viscosity continuously increases as drilling proceeds, and must be reduced from time to time by the addition of water and more barite to restore the density.

Flow Properties:
The flow properties of the drilling fluid playa vital role in the success of the drilling operation. These properties are primarily responsible for removal of the drill cuttings, but influence drilling progress in many other ways. Unsatisfactory performance can lead to such serious problems as bridging the hole, filling the bottom of the hole with drill cuttings, reduced penetration rate, hole enlargement, stuck pipe, loss of circulation, and even a blowout.

The flow behavior of fluids is governed by flow regimes, the relationships between pressure and velocity. There are two such flow regimes, namely laminar flow, which prevails at low flow velocities and is a function of the viscous properties of the fluid, and turbulent flow, which is governed by the inertial properties of the fluid and is only indirectly influenced by the viscosity. Pressure increases with velocity increase much more rapidly when flow is turbulent than when it is laminar.

Laminar Flow:

Laminar flow in a round pipe may be visualized as infinitely thin cylinders sliding over each other. The velocity of the cylinders increases from zero at the pipe wall to a maximum at the axis of the pipe. The difference in velocity between any two such cylinders, divided by the distance between them. Defines the shear rate. The axial force divided by the surface area. Of a cylinder defines the shear stress. The ratio of shear stress to shear rate is called the viscosity. And is a measure of the resistance to flow of the fluid. The unit of viscosity is the (poise) the shear stress in dynes/m- divided by the shear rate in reciprocal seconds gives the viscosity in poises. The unit employed in mud viscometry is the centipoises (cp), which is one hundredth of a poise.

Turbulent Flow:

Flow in a pipe changes from laminar to turbulent when the flow velocity exceeds a certain critical value. Instead of layers of water sliding smoothly over each other, flow changes locally in velocity and direction, while maintaining an overall direction parallel to the axis of the pipe. Laminar flow may be compared to a river flowing smoothly over a plain, and turbulent flow to flow over rapids where interaction with irregularities on the bottom causes vortices and eddies.
Friction factor, which is a function of the Reynolds number and the roughness of the pipe wall.

PH:

The relative acidity or alkalinity of a liquid is conveniently expressed as pH.
Defined as the negative logarithm (to the base 10) of the hydrogen-ion concentration, pH units decrease with increasing acidity by a factor of 10. For example, the hydrogen ion concentration of a solution having a pH of 3 is ten times that of a solution of pH 4. At pH of 7, the hydrogen-ion concentration is equal to the hydroxyl-ion concentration and the liquid is neutral, as with pure water. Above pH 7, the hydroxyl-ion concentration increases by a factor of 10 with each pH unit; thus, the hydroxyl-ion concentration at pH 11 is ten times that at pH 10 (hydrogen ion concentration is one tenth).

The optimum control of some mud systems is based on pH, as is the detection and treatment of certain contaminants. A mud made with bentonite and fresh water, for example, will have a pH of 8 to 9. Contamination by cement will raise the pH to 10 to 11, and treatment with an acidic poly phosphate will bring the pH back to 8 or 9, other reasons for pH control include maintenance of lime-treated mud's, mitigation of corrosion, and effective use of thinners.

Measurement of pH is routinely made by comparing the color developed on immersing a paper strip impregnated with certain dyes (indicators) with the color of reference standards. If the liquid has a high concentration of dissolved salts, or is deeply colored (such as by tannins and lignite), the colorimetric method is not satisfactory, but an electrometric method employing the glass electrode can be used to give reliable results in most mud's. If the sodium-ion concentration is very high, a special glass electrode may be needed.

Alkalinity:
Alkalinity measurements are made to determine the amount of lime in lime" treated mud's. The mud is titrated to determine the total amount of lime, soluble and insoluble, in the system (Pm) The filtrate is titrated to determine the amount of lime in solution (Pt). The amount of undissolved lime is calculated from Pm Pt. Measurements of the alkalinity of water samples, and of filtrates of very lightly chemically treated mud's, can be used to calculate the concentration of hydroxyl (OH), carbonate (C03), and bicarbonate (HC03) ions in solution.

Cation Exchange Capacity:

Methylene Blue Test.

The methylene blue test serves to indicate the amount of active clay in a mud system or a sample of shale. The test measures the total cation exchange capacity of the clays present and is useful in conjunction with the determination of solids content as an indication of the colloidal characteristics of the clay minerals. Similarly, shale cuttings can be characterized and some estimations can be made regarding mud-making properties and possible effects on hole stability. Organic materials, if present in the sample, are destroyed by oxidation with hydrogen peroxide. The sample is titrated with standard methylene blue solution until the adsorptive capacity is satisfied, as shown by the appearance of a blue color in the water in which the sample is suspended. If other adsorptive materials are not present in significant amounts, the bentonite content can be estimated, based on an exchange capacity of 75 mill equivalents per 100 grams of dry bentonite.

Viscosity:

Although calculated from measurements at relatively low shear rates, the plastic viscosity is an indicator of high shear rate viscosities. Consequently, it tells us something about the expected behavior of the mud at the bit. One of our design criteria was to minimize the high shear rate viscosity. To accomplish this, we should minimize the plastic viscosity. A decrease in plastic viscosity should signal a corresponding decrease in the viscosity at the bit, resulting in higher penetration rate.

Increasing the plastic viscosity is not a desirable means of increasing the hole-cleaning ability of a mud. In fact, the increase in pressure drop down the drill string, caused by an increase in PV, would reduce the available flow rate and tend to offset any increase in lifting ability. In general, high plastic viscosity is never desirable and should be maintained as low as practical.
The plastic viscosity is primarily a function of the viscosity of the liquid phase and the volume of solids contained in a mud. The viscosity of the liquid phase is increased by addition of any soluble material. Many of the water-soluble polymers used for fluid-loss control are quite effective in increasing the plastic viscosity. Saturated salt water has twice the viscosity of fresh water. Diesel oil, which is commonly used as the liquid phase of oil-base mud's, has three times the viscosity of fresh water. Both salt water mud's and oil mud's tend to have high plastic viscosities.

The volume of solids in a mud, is the dry volume of solids plus the increase in volume due to hydration. The water of hydration actually becomes a part of the solid so far as its effect on viscosity is concerned. In other words, the plastic viscosity is increased by addition of any type of solid; but solids such as clays, which hydrate, will further increase the plastic viscosity as their volume is increased by hydration. This makes the hydration and dispersion of shale particles particularly detrimental.

As long as these particles are large and relatively unhydrated, their effect on viscosity is small. However, time, temperature, and agitation tend to disperse and allow hydration of the individual clay platelets, which results in increased viscosities. In order to combat the tendency of shale particles to disperse and hydrate, the "inhibitive" mud's were designed. Materials such as lime, gypsum, lignosulfonate, and polymers are added to inhibit the rate of dispersion and hydration. These materials do cause inhibition, but if the inhibited particles are not removed from the system, the solids content will continue to build. In time, the plastic viscosity will be as high or higher than before and other mud properties such as filter cake thickness will suffer.

Minimum plastic viscosities can be achieved only to the degree that the mud is kept free of drilled solids. (Figure) shows guidelines for plastic viscosity of water-base mud's at various mud weights. The lower curve represents mud's that contain only barite and sufficient bentonite to suspend the barite. This curve should represent minimum plastic viscosities for good mud performance. The upper curve is an average for many field mud's that have been checked.
Plastic viscosity decreases with increasing temperature, due to thinning of water. If the mud is checked at 130°F, the PV will be about 10 percent lower than at 120°F; if it is checked at 110°F, it will be about 10 percent higher. For this reason, all mud tests should be made at the same temperature, 120°F.

The desired viscosity of a mud is influenced by several factors, including:
I. Mud density;
II. Hole size;
III. Pump rate;
IV. Drilling rate;
V. pressure;
VI. Hole condition.

The viscosity of a mud is a function of three components:
I. Viscosity of the base liquid or continuous phase;
II. The size shapes and number of solids particles in the mud (plastic viscosity);
III. Inter-particle forces (yield point).

Plastic viscosity:

Is that part of the resistance to flow in mud caused by the friction between suspended particles and the viscosity of the base liquid. The plastic viscosity is a measure of the internal resistance to flow due to the amount, type and size of solids present in the mud. It is due to mechanical friction of the solids in the mud as they come in contact with Each other and with the liquid phase of the mud. The plastic viscosity depends on the concentration and size of solids present. The solids present in the mud can be considered either active or inactive. An example of an inactive solid would be drilled solids incorporated in the mud while drilling. Increasing the percentage by volume of solids in the mud can increase the plastic viscosity. If the volume percent solids remain constant, then reducing the size of the solid would also increase the plastic viscosity due to the increased surface area exposed. This increased surface area allows for more frictional contact. To reduce the plastic viscosity, either the solid concentration can be reduced or a flocculant can be added to increase the size of the particles thereby reducing the available surface area. In the field the reduction is usually made by dilution with water or separation with mechanical solids removal.

Funnel viscosity:
Routine field measurements of drilling mud viscosity are made with a Marsh Funnel, which measures a timed rate of flow. The values obtained are called “apparent viscosity”.