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the cost of a large one-step dilution is less than frequent small dilutions. The cost of dilution increases rapidly with mud density. An example arrangement of the solids control equipment for an unweighted clay/water mud is shown in Fig. 2.23. 5 The various components arc arranged in decreasing order of clay size removal to prevent clogging. Dilution water is introduced up- stream of the hydrocyclones to increase their separation efficiency. Each device is arranged to prevent newly processed mud from cycling back to the input of the device. Chemical treatment normally is made downstream of all separation equipment. • DRILLING FLUIDS TABLE 2.9-RATED CUT POINT OF HYDROCYCLONES Hydrocyclone Size Rated Cut Point (microns) 97% 90% 70% I 50% I 30% I ~~J --~ero (in.) 6 4 2 40 20 10 IN OVERFLOW • 3°/o • 10% • 30% • 50% • 70% • 95% • 100% CUT POINT IN UNDERFLOW SEPARATION DIAGRAM (Not to Scale) Fig. 2.21-Solids distribution and removal using hydrocyclones. Example 2.13 A mud cup is placed under one cone of a hydrocyclone being used to process an on- weighted mud. Thirty seconds were required to collect 1 qt of ejected slurry. The density of the slurry was determined using a mud balance to be 17.4 Ibm/gal. Compute the mass of solids and volume of water being ejected by the cone per hour. Solution. The density of the slurry ejected from the desilter can be expressed in terms of the volume fraction of low-specific-gravity solids by ms+mw - PsVs+PwVw p= =psfs +Pwfw· Vs + Vw Vs+ Vw Using the values given in Table 2. 7, the average density of low-specific-gravity solids is 21.7 Ibm/ gal 59 TABLE 2.1 0-COMMON DEFLOCCULANTS USED TO LOWER YIELD POINT AND GEL STRENGTH Approximate Deflocculant Phosphates Sodium acid pyrophosphate Sodium hexa· metaphosphate Sodium tetraphosphate Tetra sodium pyrophosphate Tannins Quebracho Alkaline tannate Hemlock tannin Des co Lignins Processed l1gnite Alkaline lignite Chrome lignite Lignosulfonates Calcium lignosulfonate Chrome lignosulfonate LL. 40 ~0 :::i~ Ow 20 Cflu a: w 0.. pH of Maximum Deflocculant Optimal Effective in a 10-wt% Mud Temperature Solution pH (oF) 9.0 175 4.8 6.8 7.5 10.0 11.5 250 3.8 300 400 4.8 9.5 10.0 10.0 350 7.2 7.5 0 20 40 60 80 100 120 140 PARTICLE SIZE DIAMETER, Microns Fig. 2.22-Solids distribution and removal using hydrocyclones. and the density of water is 8.33 Ibm/gal. Substituting these values in the above equation yields 17.4 = 21.7/5 + 8.33(1-/ 5 ). Solving for the volume fraction of solids gives 17.4-8.33 fs = = 0.6784. 21.7-8.33 Since the slurry is being ejected at a rate of 1 qt/30 s, the mass rate of solids is 0.6784 (1 qt) X ~ X 21.7 Ibm 30 seconds 4 qt gal 60 O•lullon Screen Shaker Water Fig. 2.23- Schematic arrangement of solids-control equip· ment for unweighted mud systems.s 3,600 seconds Ibm X =441.6-, hr hr and the volume rate of water ejected is 1 qt (1.0-0.6784) x gal 30 seconds 4 qt x 3,600 s/hr = 9.65 gal/hr. Note that to prevent the gradual loss of water from the mud, 9.65 gal of water must be added each hour to make up for the water ejected by this single cone. 2.3.5 Chemical Additives. Unweighted clay/water muds are controlled primarily by removing inert solids, diluting, and adding bentonite when required to keep the active solids at the proper concentration. However, after using all available methods of solids control, one or more of the mud properties may be at an undesirable value and require selective ad- justment. Chemical additives commonly are used for (1) pH control, (2) viscosity control, and (3) filtrate control. Caustic (NaOH) almost always is used to alter the mud pH. A high mud pH is desirable to suppress (1) corrosion rate, (2) hydrogen embrittlement, and (3) the solubility of Ca2 + and Mg2 +. In addition, the high pH is a favorable environment for many of the organic viscosity control additives. The pH of most muds is maintained between 9.5 and 10.5. An even higher pH may be used if H 2S is anticipated. Flocculation refers to a thickening of the mud due to edge-to-edge and edge-to-face associations of clay platelets. These arrangements of platelets are shown in Fig. 2.24. Flocculation is caused by unbalanced electrical charges on the edge and surface of the clay platelets. When the mud is allowed to remain static or is sheared at a very low rate, the positive and negative electrical charges of different clay platelets begin to link up to form a "house of cards" structure. The hydrated clay platelets normally have an excess of electrons and, thus, a net negative charge. AGGREGATION (FACE TO FACE) DISPERSION APPLIED DRILLING ENGINEERING FLOCCULATION (EDGE TO FACE) (EDGE TO EDGE) DE FLOCCULATION Fig. 2.24-Association of clay particles. 3 Since like charges repel, this tends to keep the clay platelets dispersed. The local positive and negative charges on the edge of the clay platelets do not have a chance to link up. Anything that tends to overcome the repelling forces between clay platelets will in- crease the tendency of a mud to flocculate. The common causes of flocculation are (I) a high active solids concentration, (2) a high electrolyte con- centration, and (3) a high temperature. The concentration of flocculated particles in the mud is detected primarily by an abnormally high yield point and gel strength. At high shear rates, the "house of cards" structure is destroyed. The plastic viscosity, which describes fluid behavior at high shear rates, usually is not affected greatly by floc- culation. The normal range of plastic viscosity for an on- weighted mud is from 5 to 12 cp measured at l20°F. The yield point is descriptive of the low shear rates present in the annulus and greatly affects the cuttings carrying capacity and annular frictional pressure drop. A yield point in the range of 3 to 30 Ibm/ 100 sq ft often is considered acceptable for unweighted clay/water muds in large-diameter holes. This yield point range will enhance the ability of a mud to carry the cuttings to the surface without •. increasing the frictional pressure drop in the annulus enough to cause for- mation fracture. The gel strength is descriptive of the mud behavior when the pump is stopped. The gel strength of the mud prevents settling of the solids during tripping operations. However, an excessive gel requires a large pump pressure to be applied to start the fluid moving and could cause formation fracture. A progressive gel increases with time and is less desirable than ajragile gel (Fig. 2.25). 22.214.171.124 Dejlocculants. Deflocculants (thinners) are materials that will reduce the tendency of a mud to flocculate. The deflocculants are thought to render ineffective the positive charges located on the edge of the clay platelets and, thus, destroy the ability of the platelets to link together. A large number of deflocculants are available. The common types are listed in Table 2.10. None of the defloc- culants are totally effective against all causes of I DRILLING FLUIDS :I: t- <.!) z w a:: 20 15 ~ 10 ...J w <.!) 5 10 20 30 40 50 60 TIME, Minutes Fig. 2.25-Fragile and progressive gel strength. 3 70 flocculation. Since many of the deflocculants are acidic and only slightly soluble in the acid form, they must be used with caustic (NaOH) to increase the pH. Any of the deflocculants can be used to lower yield point and gel strength when flocculation is caused by excessive solids.