Drilling Fluid Components

Previous chapters have dealt with the major drilling fluid ingredients from the standpoint of their effect on the properties and performance of the drilling fluid. In this chapter, the principal constituents of drilling fluids will be examined as commercial products. As such, some attention will be given to the composition, origin, processing and manufacture of these products, in the expectation that potential suppliers reading this chapter will find a market for new and improved materials. Estimations will be made of the current consumption in the United States of most of these products.

Only incidental attention will be given to their application. Several patents involving applications will be cited because they contain information on methods of preparation. In general, however, the index should be consulted for references to applications of the various products.

Drilling Fluids Industry Products

In the fifty years since the birth of the mud industry, the number of products has increased from half a dozen to nearly 1,400 trade-named additivies.1 While early additives were only used to make water muds heavier, thicker, or thinner, in the current listing,1 materials are offered to perform one or several of sixteen separate functions in ten different drilling fluid systems. One major supplier offers 75 trade-named products. Of the 81 suppliers listed, many sell several of the twenty-five generic materials in addition to their brand-named products.

The major suppliers have outlets in all active drilling areas. Many smaller dealers operate in limited areas or with only a few products, and are not included in the referenced list.

A sub-group of the API Committee on Standardization of Drilling Fluid Materials surveyed drilling fluid ingredients so that those regulatory agencies concerned with the environmental effects of chemical substances could become acquainted with the nature and the amounts of such materials used in drilling fluids. This survey2 furnished an estimate of quantities used in drilling fluids in the United States in 1977 (excluding completion, workover and packer fluid materials) and the normal range in concentration.

About 40 specific substances were listed. Additional substances were included in general composition groupings, such as proprietary oil muds and other specialty products not identified. The total tonnage estimate for 1977 appears to be lower than actual usage, largely due to underestimation of the quantities of barite and clays consumed. According to the U.S. Bureau of Mines,1 these products amounted to a total of 3,530.000 tons (3,210,000 tonnes).

in earlier surveys of the uses of chemicals in drilling fluids, 4 5 6 • - the effect of drilling technology developments on the composition of the drilling fluid was noted. Drilling industry emphasis in recent years has been on restraining the rise in drilling cost by designing drilling fluids which reduce the time required to drill the well (faster penetration and less hole trouble). This approach has met with considerable success, but it has resulted in a proliferation of specialty products, some of questionable value.

The practicality of reusing synthetic drilling fluids (as suggested over 20 years ago3) is being approached as improvements continue to be made in cuttings-removal equipment. At present, however, two natural substances, bentonite and barite, the two substances first offered as commercial mud materials continue to lead the market in tonnage and cost.

Water

Water is the most important single substance involved in drilling fluids technology. Water in the formations drilled is usually the limiting factor in air drilling, so very few wells are drilled with dry air. In all other instances, at some time in the course of drilling, water is the major component (by volume) of the drilling fluid. Even when the use of water-mud is discontinued in favor of an oil-mud or a foam, water continues to play an important role in the performance of the drilling fluid. The unusual characteristics of water affect each step in the drilling operation from spud-in to completion, and the availability and chemical content of the makeup water must be considered in the planning stage.

Water is a peculiar substance. Franks, in the introduction to a four volume treatise on the subject of water,9 wrote .. of all known liquids, water is probably the most studied and the least understood, although many of its properties are accepted as international standards ..0 Among the unusual properties of water in comparison with other liquids are the highest surface tension, dielectric constant, heat of fusion, and heat of vaporization, and the superior ability of water to dissolve a variety of substances. A most important property is that water, unlike most other substances, expands when it freezes under normal pressure.

The special molecular structure attributed to water serves to explain its peculiar properties. The simple formula, H20, does not indicate the nonlinear, unsymmetrical structure of the molecule, which can be regarded as a tetrahedron with the oxygen atom at the center and the corners being occupied by two hydrogen nuclei and two pairs of electrons. The polar nature of the molecule accounts for the strong tendency to form hydrogen bonds and the devolpment of structure in liquid water. Dissociation of salts, acids and bases occurs in water. Hydrate sheaths reduce the attraction between the oppositely charged ions. Reactions between water and clay-surfaces and the effect of electrolytes dissolved in the water on the clay-water interaction are the primary subjects for any study of drilling mud performance.

Fortunately, water is usually readily available at relatively low cost. At some locations, however, the quality is such that the composition of the mud must be altered, or the water must be treated to counteract some dissolved substance. In such instances, consideration should be given to the cost involved and to possible alternatives. Removal of excess mud from the location may be a major item of expense in some areas. If so, the mud program should be designed to minimize the increase in volume without sacrificing performance.

Only rarely is water metered to record the quantity used in the mud. In the few instances when the water has been measured, daily consumption has been amazingly large, in some cases amounting to a replacement of as much as 60% of the volume of mud in the system. When a weighted mud is being used, the volume of water added can be estimated from the quantity of barite that is required to maintain the density. A conservative guess as to the quantity of water used annually in drilling is over twenty million tons.

Materials to Increase Density

An important function of drilling mud is the control of formation fluid pressure to prevent blowouts. The density of the mud must be raised at times to stabilize incompetent formations. Any substance that is denser than water arid that does not adversely affect other properties of the mud can be added to raise the density to some extent. Cost is important, but there are other practical restrictions on the material to be selected. The solubility of salts limits their range of usefulness, and there are other problems associated with the use of such systems. Various finely-ground solid materials, as listed in Table 11-1, have been used to successfully raise drilling mud density.

Obviously, the specific gravity of the weighting agent is of primary importance, especially in very heavy muds.11 The fractional volume occupied by the added solid is a major limiting factor in its use. Figure 11—1 shows the effect of the specific gravity of the weighting material on the solids concentration of weighted muds. For example, the solid content of mud weighted to 19.0 lb/gal (2.28 g/cm3) with material having a specific gravity of 4.2 is 39.5% by volume, as compared with 30% by volume for a material of 5.2 specific gravity.

Several factors in addition to chemical inertness and specific gravity affect the use of a substance as a weighting material. First, the substance should be available in large quantities. It should be easily ground to the preferred particle-size distribution, and relatively nonabrasive. It should also be moderate in cost, and not injurious or objectionable to the drilling crew or

Table 11-1

Materials Used to Increase the Density of Drilling Muds

Material

Principal Component

Specific Gravity

Hardness, Moh's Scale

Galena Hematite Magnetite Iron Oxide

BaS04 FeCOj SrS04

0 0

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