It has been discussed that the solids problem and the problem of controlling solids in weighted muds is very different from unweighted muds . This has long posed a dilemma to operators who have reached a depth where they did not need a weighted mud , the formation pore pressure required more hydrostatic head than the desilted (and unweighted) mud in the hole , and the operator did not wish the drilling disadvantages of stopping the desilters . This problem has been classic in the range from a low of 9.0 to 9.5 PPG ( 1.08 to 1.14 Specific gravity), at the low weight, up to 10.5 to 11 PPG. This problem is reflected in Figure 6-29. It has been approached three basic ways in the past :
- The most common method has been to stop desilting first, and do basically nothing while mud weight increases from drilled fines and mediums. As higher weight is needed, barites are added , and either a centrifuge is used or discard and makeup is resorted to for viscosity control. This is most adaptable to slower than the fastest drilling rates.
- In faster drilled holes, with short, rapidly increasing transition zones , a similar except much quicker method has been used often . In tensive desilting is carried on until the transition zone is evident. Then, simultaneously, desilting is stopped , decanting centrifuging for colloidal removal is started, and rapid barite weighting is begun. Fast drilling is continued. Several operators have habitually reported increasing weight 3 PPG (0.36 Specific gravity) per circulation. They also report this can be done only with a thoroughly desilted mud.
- A third method is very popular with those who have tried it, but the majority are reluctant to test it for fear of barite cost. Intensive desilting is continued , but barites are added as more mud is needed. This is usually continued to a mud weight of 11 PPG (1.32 Specific gravity ) or more , and often to 13 PPG (1.55 Specific gravity). In this range, desilting usually has been stopped, or the underflow restricted more, or resorted to periodically to thin the filter cake, reduce abrasion , alleviate hole problems, etc. The decanting centrifuge is used as needed for viscosity reduction, or the discard and makeup method is used.
It would seem that the special secondary separation screens may be of benefit for the “Twilight Zone” of neither weighted nor unweighted muds. As shown in Table 6-10, the return of the liquid phase from the screen to the unweighted system will cause some weight increase. If a more rapid increase is needed, barites can be added as necessary. As the transition to a weighted mud is completed additional measures can be taken if needed. This should be tried against Method 3 above to determine comparative drilling advantages.