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Kinematic Viscosity - CS

Figure 9-23. Drilling rate versus kinematic viscosity at 31/2 gal/min. (From Eckels,24 Copyright 1958 by SPE-AIME.)

Because their viscosities are higher than aqueous muds, conventional oil muds tend to give comparatively slow penetration rates. Simpson25 describes a special low-viscosity invert emulsion mud that gave drilling rates as fast as or faster than did aqueous muds in offset wells. Low viscosities are obtained by using not more than about 15% water in the dispersed phase, a low viscosity oil, and a minimum amount of additives such as bentones, oil-dispersible lignites, tall oil soaps, and asphaltenes,25a'25b

The concentration of particulate solids is another mud property that al'fccts drilling rate. High concentrations of solids reduce drilling rates because they increase mud density and viscosity. Use of weighting agents with specific gravities higher than that of barite, such as itabirite and ilmenite (see Chapter 11), has enabled faster drilling rates to be obtained25c d e f g because the volume of solids required for a given mud density is less, and hence the viscosity is lower. Also, these materials are harder than barite, so there is less attrition in the course of drilling, and thus less increase in viscosity.

Much higher penetration rates are experienced as the percentage of solids approaches zero than can be explained by the negligible decrease in viscosity. The reason is reduced CHDP, as discussed later. The actual solids concentration that it is possible to maintain depends on well conditions and the type of mud being used. When drilling in low-permeability sandstones and carbonates that remain true to gauge, it is possible to drill with clear water. Emphasis is placed on the word clear, because field experience has shown that remarkably small amounts of solids can greatly reduce penetration rates.26-27 For this reason, small amounts of a flocculation aid, such as 10% hydrolyzed polyacrylamide co-polymer, are added at the flow line. If proper settling facilities are provided, clear water is obtained at the suction. The settling facilities usually consist of large earthen pits, because the floes are voluminous and their specific gravity is only slightly greater than water. Consequently, their settling rates are low, and they are sensitive to stray currents. Settling efficiency is greatly increased if the pits are baffled to distribute the flow evenly over the whole surface of the pit, and if a weir is placed at the discharge end of the pit to skim clear water from the surface. Because of hindered settling, settling efficiency falls off very rapidly if solids content is allowed to rise much above 1% by volume.

Another type of ultra low-solid fluid (commonly known as milk emulsion) used to drill hard formations, consists of water or brine in which 5% diesel oil is emulsified with an oil-wetting surfactant. The emulsified oil is believed to provide a small measure of filtration control, and to protect the drill cuttings from disintegration by the oil-wetting action. Figure 9-24 shows the faster drilling times and decreased bit wear that have been achieved with this type of fluid in West Texas.28

In most holes, filtration control is necessary, and therefore the drilling fluid must have a colloidal base, which makes maintenance of a low solids content more difficult. Some decrease in rate of penetration is inevitable, but quite high


Effect of Drilling Fluid on Hard Rock Drilling Permian Basin 5000-10000 ft.


Effect of Drilling Fluid on Hard Rock Drilling Permian Basin 5000-10000 ft.

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