During a drilling operation, drilled solids or cuttings generated at the bit mix and interact with the initially formulated drilling fluid and bring about changes in its density, rheology, and other properties. These changes are affected by such factors as the flow-rate, density, size distribution and the degradation tendency of the drilled solids These factors, in turn, are determined by such interacting variables as lithology, fluid properties, penetration rate, and the wellbore geometry.
The solids-laden fluid returning from the hole is process continually to restore its properties and keep its density arid rheology within the specific margins before returning it to the hole. The mud processing usually involves three operations, namely, drilled solids removal, dilution, and addition of suitable mud additives. Optimum mud processing can be defined as the best combination of solids control equipment, dilution and additives to maintain the desired fluid properties and achieve drilling objectives with minimum cost.
Mud processing involves more than solids control, but control of the drilled solids is the most critical element of mud processing. Inadequate solids removal can lead to increase in mud density. Dilution is then required to maintain the density within the permissible range. In turn, the dilution process requires more mud additives and creates surplus mud that ends up in the reserve pit. This situation, typical of surface hole drilling, does reduce the rental costs for solids control equipment, but this saving is usually insignificant compared to the increased costs for dilution, disposal and mud additives. To reduce the costs of dilution and disposal in the case of inadequate solids control, one could let the mud density increase, but · then the adverse effects, such as the potential risk of losing mud to weaker zones, decrease in penetration rate, and increased trouble cost due to stuck pipe need to be weighed against the savings from not providing adequate solids control.
In this regard, it must be noted that maintaining mud density is just one of the objectives of mud processing. It is equally important to maintain or control the other rheological parameters such as viscosity, yield value, gel strength, fluid loss, and the solids contents for cutting transport and suitable wall cake. In weighted muds, for example, one could achieve the desired high density by loading the mud with drilled solids instead of adding enough weighting material, but such a weighted mud would obviously be unacceptable. Ideally, one would like to control solids in a mud by removing all the “undesirable” drilled solids and adding to the mud only those solids that are absolutely necessary to obtain the desired properties. Recent trends indicate increasing use of polymers instead of solids to obtain the desired rheology for drilling fluids.