The Use of Centrifuges with Unweighted Drilling Fluids
With properly designed and engineered unweighted muds, which have low total solids content, in which particle size is not a primary cause for concern, the objective of centrifuging is the removal of drilled solids. Massive dilution is required to compensate for the incorporation of these solids if their concentration is to be controlled at the low levels that are normally desired. Centrifuging, and separating the underflow, can remove significant quantities of otherwise inseparable solids. This significantly reduces dilution requirements and drilling waste volume.
To control the density of a freshwater-base fluid at 8.8 lb/gal, 27.6 barrels of dilution are required by the incorporation of each barrel of drilled solids. With seawater, and a 9.0 lb/gal drilling fluid, each barrel of incorporated solids requires 37.5 barrels of dilution. Regardless of the type of drilling fluid in use, the preparation and eventual disposal of large quantities of drilling fluid can be very costly. Economics almost always favors the removal of undesirable solids, rather than the reduction of their concentration by dilution.
With unweighted fluids, solids loads are low, and torque rarely presents a problem. Hence, it is best to operate the centrifuge with high g force and a deep pool to maximize solids separation. Since residence time is a factor in solids separation, the maximum feed rate, which reduces residence time, may not produce the best results. The maximum efficient rate rarely exceeds 250 gpm of feed fluid plus dilution. Dilute the feed as necessary to control the funnel viscosity of the effluent at 35–37 sec/qt. This can be expected to control the low-shear-rate viscosity at levels low enough to permit efficient separation. Target processing capacity should be about 25% of the circulation rate. This may require more than one centrifuge. If multiple centrifuges are used, they must be operated in parallel, not in series.
The Use of Centrifuges with Weighted Drilling Fluids
With the unavoidably higher solids content of weighted muds, the increase of gel strengths and viscosities, as well as the degradation of filter cake quality associated with the diminution of particle size with time, can become a serious problem. The problem is caused by the increasing concentration of colloidal and near-colloidal particles that are too fine to be separated from the base fluid. Centrifuges are used to selectively remove these fine solids.
In this application, the underflow is returned to the mud and the overflow; the liquid and finest solids are separated and are either stored for later use as packer fluid, to be reconditioned through dilution and used in another drilling application, or discarded.
Processing 10 –15% of the circulating volume is usually sufficient. The capacity of machines designed for this application is much less than that of the high-volume centrifuges designed for use with unweighted fluids. Dilution of the feed mud is almost always necessary with these fluids. As with unweighted muds, the dilution should reduce the effluent viscosity
to 35–37 sec/qt.
Due to the high solids content of weighted fluids, torque is often a problem when processing it. Torque can be reduced by increasing pond depth to permit some mud to spill over and become mixed with the separated solids, through reduction in rpm or running a higher conveyor differential, or by processing at a reduced feed rate continuously, rather than at a higher rate intermittently.
At higher mud weights, solids loading in the centrifuge can be reduced by processing the drilling fluid through hydrocyclones and feeding the centrifuge with the overflow (while returning the underflow to the mud system). The temporary removal of the coarser solids, which would remain in the mud in any case, lightens the load on the centrifuge, permitting more efficient isolation and separation of the colloidal and near-colloidal solids.
