Drilled solids are not the same as the weighting material used for increasing the mud weight. The weighting materials like barite, calcium carbonate, or hematite are High Gravity Solids (HGS) whereas drilled solids are Low Gravity Solids (LGS). More LGS in the mud system tend to reduce the Average Specific Gravity (ASG) of the mud and have a detrimental effect on mud properties. Solids in the mud system are measured in lbm/bbl or as volume percentage.

Although drilled solids may initially have a larger particle size, their size rapidly reduces due to chemical dispersion and regrinding because of the mechanical action of the bit and drill string. The rate of disintegration varies with the chemical composition of formation, type of mud, exposure time, circulation rate, deviation, and mechanical abrasion from the drill string.

It is recommended to use a centrifuge to remove LGS from the mud. It is important to remove as many of the drilled solids as possible on the first circulation. If they are not removed early, eventually the drilled solids will disintegrate to a point where they cannot be removed from the mud system by solid control equipment.

A good practice is to maintain the drilled solids in the 6% - 7% range by volume. A higher percentage of LGS in mud systems scramble critical rheological properties of drilling fluid. Some of the negative effects of higher LGS percentage in mud systems are as below:

• Small-size LGS particles tend to increase the viscosity of mud due to higher friction between particles. This increases the Equivalent Circulation Density (ECD). Excessive ECD could fracture the formation and lead to lost circulation

• A higher percentage of LGS in the mud system tends to deposit a thick and sticky filter cake, which increases the potential of getting deferentially stuck across permeable formations.

• It adversely affects drilling performance by causing a reduction in the rate of penetration.

• Increases surge and swab pressures. Higher surge pressure risks causing formation fracture, and partial or total losses. Increased swabbing effect can result in kick situations.

• It also increases the mud bill. If drilled solids are not removed from the system, the only way to maintain the target percentage of LGS in the mud system is dilution. It may seem like a cheaper and easier option, but dilution requires further adding appropriate amounts of other chemicals to maintain the required rheological properties of the mud system, which increases the cost of drilling fluid many folds.