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AFE: drilling fluids
Mud cost categories
The prices are based on build cost for a certain mud weight and a daily maintenance expense. These costs vary from different mud types and are dependent on the chemicals and weighting material required and on the base fluid phase, such as water or oil. Miscellaneous cost factors include specialty products such as hydrogen sulfide scavengers, lost-circulation materials, and hole-stability chemicals.
The build cost for a mud system (Fig. 1) is the price for the individual components and mixing requirements. Oil-based muds have a higher build cost than most water-based muds, because of
- The expensive oil phase.
- The mixing and emulsion-stability chemicals.
- The additional barite required to achieve comparable densities with water-based muds.
Fig. 2 shows a comparison of build costs for an oil-based mud (invert type) and a lignosulfonate mud. The total build cost includes purchasing the initial mud system and the expenses involved with increasing mud weight in the well as it is drilled.
The maintenance costs for deep, high-pressure wells are usually larger than the build costs. The maintenance fee includes the chemicals required daily to maintain the desired mud properties. These chemicals include:
- Fluid-loss agents
- Caustic soda
Fig. 2 shows an estimate of empirically derived maintenance costs for invert emulsion, oil muds, and lignosulfonate water muds. The illustration demonstrates that heavy muds can have high daily fees. A system with 1,000 bbl of 16.0-lbm/gal lignosulfonate mud would cost approximately U.S. $2,700 for daily maintenance. In addition, note that the maintenance costs for invert-emulsion muds is significantly less than that for lignosulfonate muds, even though the reverse is true for build costs.
Several additional factors affect mud costs. Small mud companies can often provide less-expensive mud systems than larger companies, although a sacrifice is made occasionally in terms of technical support and mud-problem testing capabilities. In addition, many mud companies offer mud without technical support at a price reduction over mud with engineering support.
Packer fluids are placed between the tubing and production casing above the packer. The fluid is usually a treated brine, but can be an oil mud or treated water-based mud-type fluid. In some cases, a packer fluid will not be used. Although a low-density brine is commonly used, occasionally a higher-density water or mud is used for pressure control.
Special fluids are occasionally used for well-completion purposes. They are usually designed to minimize formation damage. The fluids may be filtered brine, nitrogen, or oil. Costs for these fluids must be considered on a case-by-case basis.