The two primary sources of solids (particles) are chemical additives and formation cuttings. Formation cuttings are contaminants that degrade the performance of the drilling fluid. If the cuttings are not removed they will be ground into smaller and smaller particles that become more difficult to remove from the drilling fluid.

Most formation solids can be removed by mechanical means at the surface. Small particles are more difficult to remove and have a greater effect on drilling fluid properties than large particles. The particle size of drilled solids incorporated into drilling fluid can range from 1 to 250 microns (1 micron equals 1/25, 400 of an inch of 1/1000 of a millimeter.

Oil muds can be diluted with base oil (or clean oil mud) and water muds can be diluted with water (or clean water mud) to keep the concentration and surface area of solids within bounds. Two approaches for dilution are:

1. Dump and dilute continuously while drilling. This is the most expensive approach to solids control in most situations.
2. Dump periodically and dilute while drilling. This is more cost effective than the first approach. Certain practices can be applied to make it less costly.

The total costs of dilution are: the cost of the water hauled to the rig, the cost of converting that water into a mud of correct density, plus the cost of disposal of the mud that was dumped. To make dilution less expensive, these practices should be followed:

1. minimize the total volume of mud to be diluted.
2. dump (displace) the maximum possible dirty mud before adding water and materials, and
3. do as much dilution as possible in a single step- not a series of small dilutions. Therefore, as mud becomes more expensive, dilution becomes a less attractive option and mechanical separation should be pursued.

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