Composition of Drilling Fluids

Drilling fluids are classified according to their base:

Water Base Muds. Solid particles are suspended in water or brine. Oil may be emulsified in the water, in which case water is termed the continuous phase.

Oil Base Muds. Solid particles are suspended in oil. Water or brine is emulsi fied in the oil, i.e., oil is the continuous phase.

Gas. Drill cuttings are removed by a high velocity stream of air or natural gas. Foaming agents are added to remove minor inflows of water.

In water base muds the solids consist of clays and organic colloids added to provide the necessary viscous and filtration properties, heavy minerals (usually barite, added to increase the density when needed), and solids from the forma tion that become dispersed in the mud in the course of drilling. The water contains dissolved salts, either derived from contamination with formation water or added for various purposes. Further details are given in Table 1-1.

The solid particles may be conveniently divided into three groups according to size: (1) colloids, from about 0.005 to 1 micron (1 micron = 0.001 mm), which impart viscous and filtration properties; (2) silt and barite (sometimes called "inert solids"), 1 to 50 microns, which provide density as just discussed but are otherwise deleterious; and, (3) sand, 50 to 420 microns (assuming a 40-mesh screen on the shale shaker), which, apart from bridging large openings in certain very porous formations, is objectionable because of its abrasive qualities.

The activity of the colloidal fraction fundamentally is derived from the very small size of the particle (and consequent high surface area) relative to its weight. Because of this high specific surface, the behavior of the particles is governed primarily by the electrostatic charges on their surfaces, which give rise to attractive or repulsive interparticle forces. Clay minerals are particularly ac live colloids, partly because of their shape—tiny crystalline platelets or packets of platelets—and partly because of their molecular structure, which results in high negative charges on their basal surfaces, and positive charges on their edges. Interaction between these opposite charges profoundly influences the viscosity of clay muds at low flow velocities, and is responsible for the formation of a reversible gel structure when the mud is at rest.

Clays as they occur in nature are composed of various clay minerals, such as montmorillonite, illite, and kaolinite, of which montmorillonite is by far the most active. Other minerals, such as quartz, feldspar, calcite, etc., may also be present, in both the colloidal and silt-size ranges. When clays are mixed with water, the viscosity of the resulting mud per unit weight of clay added depends on the proportion of the various clay and other minerals present. Commercial clays that are used in drilling muds are rated by their yield, which is defined as the number of 42-gallon barrels (0.16 m3) of mud with an apparent viscosity of 15 centipoises produced by a ton (2,000 lb; 907 kg) of clay. Figure 1-1 shows that Wyoming bentonite, which contains about 85% montmorillonite, has by far the greatest yield. Similarly, in the drilling well, the rise in mud viscosity per foot of hole drilled is much greater when drilling in rich formations, such as the recent inontmorillonitic sediments of the Gulf Coast, than when drilling in lean formations, such as the silty shales of the Mid-Continent. In the former case, the viscosity must be kept within bounds by chemical treatment, dilution, or mechanical separation of drilled solids at the surface. In the latter case, the silts must be removed by mechanical separation, and the necessary rheological and filtration properties must be obtained by adding commercial clays.

Clay colloids are sometimes supplemented, or even entirely replaced, by organic colloids when required by particular problems. For instance, if clays are flocculated by soluble salts with consequent loss of rheological and filtration control, sail-resistant colloids (such as pre-gelatinized starch or cellulosic poly-

Table 1-1

Classification of Drilling Fluids According to Principal Constituent

Gas Water

Gas Water

Dry gas: Air, natural gas, exhaust gas, combustion gas Mist; Droplets of water or mud carried in the air stream

Foam: Air bubbles surrounded by a film of water containing a f o a m - s t a b i 1 i z i n g surfactant Stable Foam: Foam con-t a i n i n g fi 1 m -strengthening materials, such as organic polymers and bentonite

Solution: True and colloidal, i.e., solids do not separate from water on prolonged standing.

Solids in solution with water include:

1. Salts, e.g., sodium chloride, calcium chloride

2. Surfactants, e.g., detergents, flocculants

3. Organic colloids, e.g., cellulosic and acrylic polymers

Emulsion: An oily liquid maintained in small droplets in water by an emulsifying agent, e.g., diesel oil arid a film-stabilizing surfactant

Oil Mud: A stable oil-base drilling fluid contains:

1. Water-emulsifying agents

2. Suspending agents

3. Filtration-control agents

Contains cuttings from the formations drilled

May contain barite to raise density

Mud: A suspension of solids (e.g., clays, barite, small cuttings) in any of the above liquids, with chemical additives as required to modify properties.

Weight in Pounds per Cubic Foot

63 7 67.5 71.2 75.0 78.7 82.5 86.2 90 0 H---1-1---(-1-f-f—---1—

Weight in Pounds per Gallon

8.5 9.0 9.5 10.0 10.5 11.0 11.5 120 —I-«-f—-1-•---t «

Weight in Pounds per Gallon

8.5 9.0 9.5 10.0 10.5 11.0 11.5 120 —I-«-f—-1-•---t «

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