Water as a drilling fluid does not qualify as a mud. If there are no hole (formation) problems that prevent its being the most economical drilling fluid; if neither the geologist, palentologist, nor production supervisor have valid objections; and if it is available, water is seldom if ever surpassed. When the formation requires, or a supervisor demands , filtrate control and / or viscosity and /or gels in the drilling fluid, a “mud” is built. Or if the fluid density required is too high for salt water alone , mud properties are required to suspend barites.

The first and most basic difference between an unweighted mud and a weighted mud is that money is spent on the unweighted mud to keep slurry density down , and money is spent on weighted mud to keep slurry density up. Another simple, and usually correct, criterion is that if a drilling fluid does not contain barites it is unweighted and if it does contain barites it is weighted. In the search for simplicity , it is sometimes missed that as the change is made from an unweighted to a weighted mud system the philosophy, or logic, of solids treatment also must be changed as the problems almost completely change. This is shown in Table 1.

Table 1. The Difference In Mud Solids Problems Between Weighted and Unweighted Water Base Muds
Items Weighted Mud Unweighted Mud
Costliest Portion Weight material Liquid, solubles & colloidals
Size of:
Ideal Solids Mostly ultrafine & fine Mostly colloidal
Solids from Bit Colloidal to coarse Colloidal to coarse
Specific Gravity of :
Ideal Solids High Low
Drilled Solids Low Low
Source of :
Needed Solids 1. Commercial ultrafine and fine barites
2.Commercial colloids
1. Commercial colloid
2. Colloidal cuttings(variable)
Detrimental Solids 1. Drilled fines and larger
2. Drilled colloidals
3. Fine and larger barites
4. Colloids mechanically degraded from larger particles.
1. Drilled ultra fines and larger
Detrimental Effects of :
Colloidal Cuttings PV increase (major) Density (minor)
Colloidal Barites PV increase (major) Not applicable
Size Degradation of:
All Solids PV increase (major) removal system
Ultra Fine and Fine Cuttings 1. PV increase (minor)
2. Source of degraded colloids
Same as for fines and larger cuttings , see below;
Medium and larger Cuttings 1. PV increase (minor)
2. Abrasion (variable)
3. Filter cake character
4. Source of degraded colloids
1. Density (major)
2. Abrasion (major)
3. Filter cake character (major)
4. PV increase
Ultra fine and fine barites 1. PV increase (minor)
2. Source of degraded colloids
Not applicable
Medium and larger barites 1. Slight PV increase
2. Abrasion
3. Filter cake character
4. Source of degraded colloids
Not applicable
Figure 1. Separation of WBM and cuttings is mainly based on particle size and relies on shale shakers, hydrocyclones, and occasionally a decanting centrifuge. Most cuttings are sand/gravel-sized and are easily recovered on the shale shaker. However, silt- and clay-sized cuttings are difficult to separate from the barite and bentonite of WBM; hydrocyclones and centrifuges may be required.

The Factors Governing Total Solids Control in Weighted Muds

Although great improvement has been made in solids removal techniques in twenty years, in weighted muds there are overriding factors that cause total solids to vary, sometimes more than purposeful effort by the operator. If the reasons are not understood , the blame (for credit) may fall on the type mud , supervision, removal equipment, etc. , when it actually may be caused by another one or none of those. Table 2 lists the major and minor governing factors.

Table 2. The Factors That Control Total Solids In Weighted Muds (Numbered in Order of Decreasing Influence )
Item Contributes to:
Low Total Solids High Total Sol ids
1a Type formation Medium hard 1. Unconsolidated
2. Very hard
1b Bit cutter type Long teeth 1. All
2. Single diamond teeth
2. Mud density Minimum Above Min imum
3. Bit jet horsepower Adequate lnadequate
4. Annular lift Adequate Inadequate
5. Rig shale shaker screen Constant efficient operation Bypassed
6. Full flow hydrocyclone removal of mediums and larger solids Effectiveness varies with formation & bit Not applicable
7. Rig shale shaker screen mesh Fine Coarse
8. Fine screen used to return to system part of the liquid, colloids , ultrafines , and fines from hydrocyclone underflow (6. above) Secondary separation can not reduce solids Variable increase in total solids, but more than centrifuge salvage (10 blow); no viscosity reduction
9. Removing direct from system a fraction of colloids & liquid with centrifuge Variable total solids effect , but good viscosity reduction Primary separation cannot increase total solids
10. Centrifuge used to return to system the liquid and colloid from the hydrocyclone underf low (6. above). Secondary separation can not reduce solids Variable increase in total solids , but less than screen salvage (8. above ); no viscosity reduction.
11. Chemical treatment to prevent shale cutting dispersion. Variable , but may help screen removal Normally does not , but decreases viscosity-to-total solids ratio.
12. Chemical treatment to disperse shalestone to colloidal size Variable , but can help centrifuge removal of shale . Normally does not , but increases viscosity-to-total solids ratio.
Whether a finer screen will help noticeably in this primary separation depends upon the comparative size relation between the cuttings reaching the surface and the screen mesh and whether or not the finer mesh can be maintained in proper operation. If a ” fine ” screen cannot operate properly at full flow , a coarser screen will maintain lower total solids than a finer screen that is bypassed.

Contrary to popular belief, total solids do not always and consistently tend to run “high” in weighted muds . In drilling a medium hard formation with toothed bits, the necessity to mix new mud for the volume of the high percentage of large cuttings removed maintains a minimum solids content. When the fact is faced that the most economical and trouble-free method of drilling an extremely hard, or an extremely soft, formation with weighted mud will at best involve some higher total solids content than drilling a medium-hard formation, attention can be concentrated on the optimum solids removal with equipment available , control of critical mud properties, and on the truly controllable drilling and completion parameters.

Coarse screen and API fine shaker screen