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Drilling fluid solids control for mud treating


Solids control is the most important function in mud treating. Control of mud properties centers around the treatment necessary to counteract the continual influx of drilled solids into the active mud system. Almost all of the costs of treating a mud can be directly attributed to solids control.


Undesirable solids increase drilling costs in a number of ways in addition to increased mud costs. If not properly controlled, they lead directly to such problems as lost circulation. Differential pressure sticking, and reduced penetration rate.


There are basically three mechanisms for reducing the solids content of a drilling fluid. These are dilution, screening, and settling or centrifuging. Dilution is accomplished by adding liquid to reduce the concentration of solids in a given volume. Solids can also be reduced by passing the mud over a screen and discarding those particles that will not pass through the screen openings. Solids can be separated from a fluid by settling or by imposed centrifugal force. This is accomplished in settling pit, hydrocyclone, and centrifuge.


Our basic aim in solids control is to reduce the formation solids content of a mud to a practical limit that will allow us to maintain the filtration and flow properties in a range to best handle all of the drilling problems. This means that we must pay special attention to control of the particles less than 2 microns in size, since they are dominant in determining the mud properties.


Unfortunately, the small particles are the most difficult to remove. To reduce the concentration of these particles, it is necessary to either dilute and build mud volume or to centrifuge the mud. When centrifuging, these small particles are discarded along with a portion of the liquid phase which contains a large portion of the chemicals in the mud. Both dilution and centrifuging are expensive. Centrifuging is normally considered to be less costly in muds weights above 12 lb./gal.


The problems and cost of removing the small particles makes it imperative that we remove as much of the large particles as possible before they are reduced to the fine size range. This is accomplished primarily by screening weighted muds and by using desanders and desilters on unweighted muds. The cost of controlling solids goes up as the mud density increases.