SELECCIÓN E INSTALACIÓN DE PUERTAS Y VENTANAS

Solids-control equipment is designed to control the buildup of undesirable solids in a mud system. Rheological and fi ltration properties can become diffi cult to control when the concentration of drilled solids (low-gravity sol- ids) becomes excessive. Penetration rates and bit life decrease and hole problems increase with a high concentra- tion of drill solids.

In an ideal situation, all drill solids are removed from a drilling fl uid. Under typical drilling conditions, low-gravity solids should be maintained below 6 vol%. How serious is the added mass of the drilled solids? The volume of rock fragments generated by the bit per hour of drilling is given by

(1 ) 2 ROP, 4 ls d V S I

where V_ls is the solids volume of rock fragments entering the mud, f is the average formation porosity, d is the bit diameter, and ROP is the rate of penetration of the bit. The fi rst few thousand feet of hole drilled in the US Gulf Coast area usually has a diameter of approximately 15 in. and is drilled in excess of 100 ft/hr. Thus, for an average formation porosity of 0.25, V_ls would be given approximately by

( )( ) S  u ls V 2 3 1 0.25 15/12 (100) 122.7 ft /hr. 4

From Table 3.1, the average specifi c gravity of drilled solids is approximately 2.6, so the density is 2.6 × 62.4 lbm/ft3 _{(density of water) = 162.2 lbm/ft}3_{. At a rock solids inlet volume rate of 122.7 ft}3_{/hr, the drilled} solids mass rate is:

162.2 lbm/ft3 _{× 122.7 ft}3_{/hr × 1 ton/2000 lbm = 9.95 tons/hr.}

Thus, the volume of drilled solids that must be removed from the mud can be quite large.

Formation-friendly low density solids and solids-free water-based fl uid systems can be formulated with a brine base (for density) and polymers for viscosity and fl uid-loss control. The introduction of a clay-free SBF system has further enhanced the well-respected performance of SBFs by providing a fl at rheological profi le that responds quickly to treatment in cold and high-temperature environments. However, achieving the maximum benefi t from running these fl uid systems depends to a great extent on the effi ciency of the rig’s solids-control equipment. Fig. 3.24 shows a typical solids-removal system. Four devices are illustrated:

· Screen shaker · Desander · Desilter · Centrifuge

Desanders and desilters are similar devices called hydrocyclones. Fig. 3.25 shows the various solids-particle sizes and the range of sizes that each device can remove from the drilling fl uid. Note that “mesh” refers to the screen shaker, also called a shale shaker. Not shown is a settling pit where solids are allowed to settle out of the drilling fl uid.

Drilling Fluids 125

The two primary sources of solids are chemical additives and formation cuttings. Formation cuttings are con- taminants that degrade the performance of the drilling fl uid. If the cuttings are not removed, they will be ground into smaller and smaller particles that become more diffi cult to remove from the drilling fl uid. Keep in mind—the smaller the drill-solid particle to be removed, the more diffi cult it is to remove.

Most formation solids can be removed by mechanical means at the surface. Small particles are more diffi cult to remove and have a greater effect on drilling-fl uid properties than large particles. Solids-processing effi ciency is determined by the last piece of equipment processing 100% of the fl ow.

Solids control is accomplished either mechanically with a screen or with the application of time and gravity. Mechanically with a screen means a shale shaker. Time and gravity means either a setline pit or a hydrocyclone. If time is not available, then increasing gravity through centrifugal separation devices is effective. Dilution is another form of solids control, but is generally considered to be a much less effi cient and much more expensive option, which we have already addressed earlier in this chapter.

3.8.1 Settling Pit. One inexpensive solids-control method is to allow a drilling fl uid time to settle. That means circulating through a settling pit. Particles above colloidal size will eventually settle out if in a quies- cent condition. However, the smaller the particle, the longer it will take to settle. In some cases, for silt-sized particles, it may take days. Particle-settling velocities are given in Fig. 3.25.

3.8.2 Shale Shakers. The most common screen device is a shale shaker (Fig. 3.26), which contains one or more vibrating screens through which mud passes as it circulates out of the hole. Shale shakers are classifi ed as circular/ elliptical or linear-motion shale shakers.

Mesh Size. Mesh screen size is the number of openings per linear inch as measured from the center of the wire. For example, a 70 × 30 oblong screen has 70 openings across a 1-in. segment, with 30 openings along the 1-in. line perpendicular to the fi rst segment. A 20 × 20 mesh screen has 20 openings per inch of screen on mutually perpendicular sides. It has relatively large openings. A 220 × 220 mesh screen has 220 openings per inch on both sides. It has relatively small openings. In addition, the mesh material in this case is much smaller in di- ameter than in the 20 × 20 mesh screen, potentially making tearing more likely.

Circular/Elliptical-Motion Shaker. This shaker, also known as a “rumba” shaker, uses elliptical rollers to generate a circular rocking motion to provide better solids removal through the screens.

Linear-Motion Shaker. The development of the linear-motion shaker has made the use of a desander/ desilter almost unnecessary. If 220 mesh screens can be run on the shaker, the fluid can go directly from the shaker to the mud cleaner (that has 250 mesh screens for a finer cut of particles).

Fig. 3.24—Solids-control system (Bourgoyne et al. 1991).

Discard Screen shaker Dilution water Desander overflow Desilter overflow Chemical, bentonite + water Pump suction Centrifuge overflow Feed Feed Feed Discard Discard Discard

A linear-motion shaker uses a straight back-and-forth rocking motion to keep the fl uid circulating through the screens. Four-screen linear-motion shakers confi gured in a series currently provide extremely effi cient removal of drill solids. Very fi ne mesh screens can be run on these shakers, constrained only by the need to preserve the barite in a weighted system.

Actual separation sizes are determined by factors such as particle shape, fl uid viscosity, feed rates, and particle cohesiveness. Some drilling fl uids can form a high surface-tension fi lm on the wires of the screen and reduce the effective opening size of the screen.

Screens are available in 2D and 3D designs. 2D screens can be classifi ed as panel screens, with two or three layers bound at each side by a one-piece, double-folded hook strip, or they can be classifi ed as perforated-plate screens, with two or three layers bonded to a perforated metal plate that provides support and is easy to repair.

3D screens are perforated plate screens with a corrugated surface that runs parallel to the fl ow of fl uid, provid- ing more screen area than the 2D screen confi guration. On a properly designed screen, the fl uid will pass through the openings about midway down the screen surface.

3.8.3 Hydrocyclones. Hydrocyclones are a means to circulate a drilling fl uid around a cylinder at a high rate of speed. In effect, the gravitational fi eld on the drilling fl uid is artifi cially increased, greatly speeding up the settling time of the particles. A typical hydrocyclone is illustrated in Fig. 3.27. Hydrocyclones have been used by the drilling industry for decades, and it is amusing that they were only recently discovered for use in home vacuum cleaners.

Hydrocyclones come in various sizes and shapes. They are usually specifi ed by the size particles they are de- signed to remove. There are desanders, desilters, mud cleaners, and centrifuges. A desander typically has a few

Fig. 3.25—Particle size for solids-control devices (Annis 1974). Reprinted courtesy of ExxonMobil.

Drilled solids Barite Barite Drilled solids Tobacco smoke Centrifuge overflow Dispersed bentonite Silt Desilter underflow Fine sand Coarse sand Beach sand Gravel 20 mesh 60 mesh 200 mesh discard 100 mesh Milled flour

Settling velocity in water at 68°F, ft/min

Drilling Fluids 127

large diameter cones (greater than 6 in. diameter), whereas a desilter has a larger number of small diameter cones (less than 6 in. in diameter). Desanders are designed to remove sand-sized particles and desilters are designed to remove silt-sized particles. Mud cleaners are a combination of a fi ne-screened (roughly 320 mesh) shale shaker under a desilter. It is used for weighted muds because barite tends to be removed with silt-sized particles. By using a mud cleaner, barite can be recovered and reused.

Example 3.10 A mud cup is placed under one cone of a hydrocyclone being used to process an unweighted 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 lbm/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-specifi c-gravity solids by

U U U   U U   ls w ls ls w w ls ls w w ls w ls w m m V V f f V V V V ,

where r- is the density of the slurry, m_lsis the mass of the low density solids, and m_wis the mass of the water. Using the values given in Table 3.1, the average density of low-specifi c-gravity solids is 2.6 × 8.33 = 21.7 lbm/gal, and the density of water is 8.33 lbm/gal. Substituting these values in the above equation yields

17.4=21.7fls+8.33 1( −fls).

Solving for the volume fraction of solids gives

fls=

− − = 17.4 8.33

21.7 8.33 0.6784.

Because the slurry is being ejected at a rate of 1 qt per 30 seconds, the mass rate of solids is

0.6784 qt 30 sec 1 gal 4 qt 21.7 lbm gal 3,600 sec hr 441.6 lbm/hr, × × × =

Liquids and fine solids

Coarse solids Whole mud in

Hydrocyclone

Drilling Fluids 129

and the volume rate of water ejected is

(1.0 0.6784 qt) 30 sec 1 gal 4 qt 3,600 sec 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.

3.8.4 Centrifuge. When installed downstream of properly confi gured shakers, a decanter centrifuge effi ciently removes most of the fi ne particles that traditional solids-removal equipment cannot capture. Typically, the de- canter centrifuge features slender cylindrical- or conical-bowl sections with a relatively large aspect (length/diam- eter) ratio, as shown in Fig. 3.28. A screw conveyor is fi tted inside the bowl for continuous removal of separated solids. Typical bowl speeds are 1,800 to 4,000 rev/min and the developed G-force is between 644 and 3,100 Gs.

The drilling fl uid is fed into the rapidly spinning cylindrical section. Through centrifugal force, the solids form a layer around the bowl wall. The thickness of this layer is determined by a series of discharge weirs at the end of the cylindrical section through which the clean liquid is decanted. The solids, being heavier, collect at the bowl wall. From there they are continuously removed by the screw conveyor. The solids are transported along the conical section (the beach) where they are dried, and then discarded out the discharge ports.

The separation result, solids recovery, solids dryness, and liquid clarity can be optimized during operation. Parameters infl uencing the result are easily regulated and include feed rate, rev/min (G-force), pond depth, and differential conveyor speed.

3.8.5 Summary. A high-effi ciency system for drill-solids removal means drastically reduced mud cost and sig- nifi cantly increased penetration rates, as well as a signifi cantly decreased risk of differential sticking and other associated hole problems. The dilution costs for maintaining a drilling fl uid in perfect condition are directly re- lated to the drill solids remaining in the system after processing by the solids-control equipment. A small increase in solids-removal effi ciency can result in large savings.