Shown below is a sketch of a typical flow loop for rotary drilling rigs. The drill solids are generated by drilling new hole and hole enlargement. Because drill solids accumulate in the annulus, the drill solids are at their highest concentration there. A portion of the solids are removed by the solids removal equipment and a portion are retained. This is called separation of solids'.
Because of the difficulties in removing solids, few old solids, i.e., those which have been recirculated and reduced in size, can not be mechanically separated; and only about 60% of the new drill solids can be separated. A fraction of the drill solids are removed by the discarding of mud. This is called 'discarding solids'.
Chemicals, barite, and water are added to reduce the concentration of drill solids; but, this does not remove the drill solids. This is called 'dilution of drill solids'. The solids which are retained in the mud are recirculated. Drill solids concentration is the ratio of the volume of the drill solids to the volume of the mud including the drill solids, and is reported as percent drill solids.
-no vj Volume Drill Solids Solids = volume Mud
Discarded mud & drill solids
One major problem exists in the optimization of mud costs. It is the selection of the concentration of recirculated drill solids to be allowed in the mud. As drill solids are allowed to increase, drill rate decreases and mud consumption decreases. A decrease in drill rate means more bit to drill a section of hole, more rotation time, more trip time, and more support time. Thus, cost per foot for the rig goes up and mud costs goes down. A balance is required.
For a weighted mud the recirculated solids concentration can be estimated by comparing the barite and water added to the mud while drilling a section of hole with the computed amounts required. For an un-weighted mud, the comparison must be based on only the water added.
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