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NATIONAL DRILLING ASSOCIATION DRILLER’S MANUAL (2005) - page 2

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4.5. IN-VERSE DRILLING

A recent innovation for the top-head drive, direct rotary machine involves the addition of an air assist by

using a special 6-in inside diameter, side discharge swivel assembly and 5-7/8 in drill pipe with built-in

air channels. This equipment permits compressed air to be injected through an injection stem into air

channels mounted outside the drill pipe and then into the drilling fluid as it moves up inside the drill pipe

(Figure 4.6). Thus, the drilling fluid and cuttings are assisted to the surface by an airlift inside the 6-in

diameter conductor (drill) pipe. This method is known as the In-Verse system and converts a direct

rotary, towhead drive machine into a reverse circulation rig. Use of the In-Verse system can increase the

capacity of a direct rotary rig to drill large-diameter wells. Depending on the rig, boreholes from 20 to 30

in can be drilled routinely. If the pulling capabilities of the rig are sufficient, enough torque is available,

and larger bits can be accommodated under the centralizer, boreholes of 30 to 60 in are possible in

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Dual-wall pipe can be driven into place in loosely consolidated materials by a steam, gasoline, or diesel

operated pile hammer as the formation is being cut by a drive bit. Air or water is forced down the

annulus to lift the cuttings to the surface through the inner pipe. If bedrock is reached, drilling may be

continued by direct rotary methods using the dual-wall pipe as temporary casing. The pile driving

method is generally not used in the water well industry because the hammering compacts unconsolidated

formation materials. In addition, the method may not penetrate deeply enough for most water well

applications.

More frequently, dual-wall pipe is set by standard reverse circulation methods using a top

head-drive unit. The top-head drive should deliver about 4,500 to 5,000 ft-lb of torque to be effective.

Down-the-hole air hammers and tricone bits can be used to cut the formation. As in the pile-driving

method, air or water lifts the cuttings. Surface casing is not needed when the dual-wall system is used.

The outer pipe of the dual-wall system must be able to operate within the normal tensile, column, and

collapse pressures associated with rotary drilling. The inner pipe is under little physical stress, but the

abrasion caused by earth materials moving up the pipe from the bit causes wear. In practice this abrasion

will generally cause the inner pipe to wear out more rapidly than the outer pipe. The inner pipe can be

replaced if necessary.

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If dual-wall casing is being set by a top-head drive, several different types of bits can be used, but the bit

size is normally one nominal size larger than the drill pipe. Thus, the space between the outer pipe and

the borehole wall is small and the pipe partially (or totally) supports the wall like a conventional

stabilizer. The bit is counted into a permanent sub that has ports for passage of the drilling fluid. If a

tricone bit is used, the drilling fluid passes upward through the inner part of the bit. A bit-wear sleeve is

attached as close as possible to the cutting face and serves as a wear ring. The drilling fluid passes from

the annular space between the two pipes, through a predrilled bit sub, and is discharged toward the

cutting surface along the periphery of the bit sleeve; after entraining the cuttings, the fluid passes upward

through the inner pipe.

When a tricone bit is used, the formation sample passing upward through the inner casing originates

from a small vertical section of the formation. In the use of a down-the-hole hammer, however, the bit

extends 4 to 5 ft out from the bottom of the dual-wall pipe. Air is forced down inside the hammer, out

the ports, and then passes up around the outside of the hammer shaft and into a special type of crossover

channel (interchange) sub and then into the inner casing (Figure 4.8). Thus, the formation sample or

water sample passing up the pipe can originate over a longer vertical section (3 to 4 ft) of the formation.

It must be remembered, however, that this distance is still small when compared with intervals sampled

by other types of rotary air drilling.

At the surface, drilling fluid enters the annular space between the inner and outer pipes by a special side

inlet swivel. Drilling fluids can consist of dry air, air and water, air and water with surfactants, or water

with clay or polymers. When air is used, velocities in the dual-wall system average 4,500 to 6,000 ft/min.

After passing down the annular space and up inside the inner pipe, air passes with the formation sample

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Three drilling procedures can be followed when using the casing driver (1) the drill bit and casing

advance as a unit, (2) the casing is driven first (in unconsolidated materials only) and then the plug in the

casing is drilled out, and (3) the drill bit advances beyond the casing a few feet, is withdrawn into the

casing, and then the casing is driven.

As drilling commences using the first procedure, the cone-type bit protrudes out the bottom of the

casing, but rarely more than 12 in. Cuttings are blown up the short open hole into the casing, and pass

out the top through a horizontal tube during drilling, the casing is simultaneously driven into the ground;

that is, the casing advances at the some rate as the drill bit. The dealer adjusts the pulldown and distance

the bit is outside the casing according to the rate of advance and speed of cuttings removal. Occasionally

the bit may be pulled up within the casing for a few moments to allow the air pressure to blow out the

cuttings. Cuttings removal is facilitated by periodically adding small volumes of water if the borehole

has not encountered water. This method is particularly suitable for drilling in stratified deposits that hove

large differences in particle size, for example, sand and silt to boulders.

In the second procedure, the casing is driven into the ground approximately 0.5 to 1.5 ft and the plug in

the casing is then drilled out. The casing is usually driven only short distances so that each formation can

be identified and sampled. During the casing-driving procedure, the drill bit is withdrawn inside the

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State or federal laws may dictate the minimum length of grout required for various casing diameters for

certain types of wells. The drilling contractor should become familiar with specific regulations for the

type of wells drilled.

4.11.1. Proportioning Cement Grout

Laboratory tests indicate that 5.2 gal of water are needed to hydrolyze one 94-1b sack of portland

cement. This mixture produces a slurry weight of 15.6 lb/gal. An advantage of using the proper

water-cement ratio is more effective bridging of cement particles in the pores of permeable formations,

which prevents excessive penetration of the grout into these formations. Although thinner mixtures with

more than 6 gal per sack are used for grouting foundation materials, this ratio is less suitable for well

work. Shrinkage increases with greater water content, because water is squeezed out of the thinner

mixtures by pressure against fine sand or other permeable formation materials. Cement will settle out of

the slurry if the ratio is greater than 10 gal per sack of cement. Water used for grout should be free of oil

and other organic material.

Bentonite clay can be added to the cement to hold cement particles in suspension, reduce shrinkage, and

improve fluidity of the mixture. Approximately 3 to 5 lb of bentonite should be mixed with 6.5 gal of

water per sack of cement. If the amount of bentonite exceeds 6 percent, excessive shrinkage of the

cement will occur. It is best to mix the bentonite and water few, then add cement to the clay-water

suspension.

4.11.2. Mixing the Grout

It is important that grout be mixed thoroughly and be free of lumps. Some drillers use small portable

grouting morphines that combine both the mixing and pumping operations. Many of these machines are

equipped with a positive displacement pump because this type of pump can work efficiently against

much greater head pressures with lithe loss in emplacement volume. Most drillers avoid using the mud

pump on their rotary rigs because of the abrasive qualities of the cement and the difficulty in removing

all traces of the cement from the pump after completing the cementing operation.

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Grout can be placed through a string of small diameter pipe (tremie or grout pipe) placed outside the

casing. The casing is lowered into the hole with centering guides attached. Care must be taken to align

the centering guides along the entire length of casing to be grouted so that the tremie pipe can pass by

them. The lower end of the casing should be closed with a drillable plug or driven into clay so the grout

cannot enter. To overcome the buoyant effect of the slurry, the casing may be filled with water or be held

down by the weight of the drill rig.

Grout can be placed by gravity through a tremie pipe, but pumping is preferred because the required

volume of grout can be introduced rapidly and with little chance of leaving voids in the grout. Pump

pressure must equal the hydrostatic pressure of the Bout the fluid friction in the grout pipe and annular

space.

For shallow holes where the grout is placed by a positive displacement pump, the cementing operation

may be completed in a single step; that is, the position of the tremie pipe is not changed as the annulus is

filled. If a centrifugal pump is used or if the hole is deep, the tremie must be raised periodically so the

hydraulic head created by the cement does not exceed the working pressure of the pump. Usually the

tremie is withdrawn one or more joints at a time, but the bottom

of the tremie should always remain beneath the surface of the cement. The rate of tremie withdrawal will

depend on the pumping rate and the volume of the annulus. The depth to the top of the grout can be

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detected by using a weighted line or a weight indicator. The volume (and therefore the height) of the

grout can also be estimated by knowing the volume of material in the hopper before grouting begins.

The grout pipe must be large enough so that all the grout can be placed before hardening begins. A 3/4-

or 1-in grout pipe may be used, although 2 in pipe is used for deeper holes. The borehole should be 4 to 8

in larger than the casing to accommodate the grout pipe. Initially, the pipe should extend to the bottom of

the annular space and should remain submerged in the slurry while the grout is being placed (Figure

4.11). Should the tremie become plugged, the output pressure can be increased, the tremie can be raised

to reduce the pressure at the bottom of the line, or it can be vibrated or struck to dislodge the stuck

material. If operations are interrupted for any reason, the pipe should be raised above the grout level and

not be lowered into the slurry again until all air and water in the pipe have been displaced by grout.

4.11.5. Tremie Pipe Inside Casing (Inner String Method)

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When the use of a grout pipe outside the casing is impractical, grouting may be done by using a grout

pipe installed temporarily within the casing (Figure 4.12). In the oil-well industry, this is referred to as

the inner-string method of cementing. A cementing plug (float shoe) is attached to the bottom of the

casing, which permits the grout to pass into the annular space but prevents it from leaking back into the

casing while grouting or after removing the grout pipe.

In the grouting process, the casing is filled with water and suspended just above the bottom of the

borehole. Grout is pumped through the grout pipe and—float shoe and forced upward around the casing.

When cement appears at the surface, displacing all other fluid in the annular space, the grout pipe is

disconnected from the float shoe. Cement is washed out of the pipe by pumping water through it before

removing it from the well. Because calcium residues may have a deleterious effect on the

viscosity-building characteristics of some drilling fluid additives, the casing should be completely

flushed with clean water after completing the cementing operation.

4.11.6. Casing Method of Grouting

The casing method of grouting, in which the slurry is forced down the casing and into the annular space,

has been adopted from the oil-well industry. In one method, two spacer plugs are used. One plug,

introduced first, separates the cement slurry above from the drilling fluid in the caning; the other

separates the slurry from water pumped in above it to wash the slurry from the casing (Figure 4.13).

After punning water or drilling fluid through the casing to circulate fluid in the annular space and clear

any obstructions from the hole, the first plug is inserted and the casing capped. A measured volume of

grout is then pumped in, the casing is opened, a second plug is inserted, and the casing recapped. A

measured volume of water is then added and pushed to the bottom of the casing, forcing most of the

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polymer prevents the clays from hydrating immediately, and once the particles are evenly distributed in

the water the viscosity remains low enough so the slurry can be pumped for about 20 minutes. The

granular Bentonite should be mixed gently into the water with a paddle, not a mixer or pump; these latter

devices will break up the particles and cause the viscosity of the slurry to increase prematurely.

Bentonite grouts should be mixed in batches so they can be pumped before the slurry becomes too

viscous. Ideally, the diameter of the suction hose should be as large as possible. In most cases, the slurry

reservoir is above the pump intake so that hydrostatic pressure created by the reservoir makes the pump

operate more efficiency. The pump and all piping should be flushed with clean water after each batch of

grout is pumped into place. The volumes of bentonite, polymer, and water for various annulus sizes, per

100 ft of depth, are given in Table 4.3.

Bentonite grout has several advantages over cement grout. It has a faster setting time, no heat of

hydration, a lower hydrostatic pressure (specific gravity is 9.2 for the grout given in Table 4.3), and the

cost is one-third that of cement. Also, Bentonite will adhere to both walls of the annulus, whereas cement

will adhere finely only to the soil.

There are several 1imitadons on the use of Bentonite grout. Bentonite grouts cannot be used when the

borehole is underreamed, because the "set" taken by the grout is not sufficient to withstand the vertical

hydrostatic pressures. Thus, the grout may eventually flow into the underreamed section.

Another limitation is that Bentonite gout should not extend so close to the ground surface that it can dry

out and shrink because of low soil moisture. Cement is always used at or near the top of the borehole.

The presence of salt water will cause Bentonite grout to flocculate and thereby lose viscosity. Organic

acids can also destroy the impervious character of the grout seat

4.12. PLUMBNESS AND ALIGNMENT

A well should be both straight and plumb, although in practice any borehole of substantial depth may not

be perfectly straight perfectly plumb. A straight well is one in which each casing section is joined to

adjacent sections in a manner that maintains perfect alignment. A borehole that is plumb is one whose

center does not deviate from an imaginary vertical line running from the ground surface to the center of

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plumbness is normally allowed in practice. By custom, a deviation from plumbness of two-thirds the

well's inside diameter per 100 ft is allowed and thought to be reasonable, considering the inherent

difficulties of drilling in earth materials. The U.S. Environmental Protection Agency (1975) has

suggested that wells should be constructed so that the borehole deviation from plumbness is 1 degree or

less per 50 ft when using drift indicators.

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operation may be determined by the contractor’s ability to control the physical characteristics of the

drilling fluid.

5.1. TYPES OF DRILLING FLUIDS

Drilling fluids used in the water well industry include water-based and air-based systems (Table 5.1).

Water-based drilling fluids consist of (1) a liquid phase, (2) a suspended-particle (colloidal1) phase, and

(3) cuttings entrained during drilling. The colloidal phase may range from less than 1 percent to as much

as 50 percent by volume. Air-based drilling fluids may consist of only a dry air phase, but more often

they contain some water to which a surfactant (soap) is added to produce a foam. Occasionally a small

amount of clay or polymer may be added to stiffen the foam. Thus, the primary drilling fluids, water and

dry air, may be used alone, but a great variety of additives are available to modify their physical and

chemical properties so they will perform more satisfactorily.

¹ Suspended particles that are approximately 0.0005 to 0.5 microns in size, do not settle out of the liquid

rapidly, and are not readily filtered.

(There are 25,400 microns per inch.)

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3. Cool and lubricate the drill bit. Fluids circulating through the drill string cool and lubricate the bit,

thereby avoiding unnecessary bit wear and reducing maintenance.

4. Control fluid loss. All water-based drilling fluid systems must control drilling fluid loss in highly

permeable formations by creating a nearly impermeable clay filter cake or polymeric film on the

borehole wall. Insufficient filter cake or polymeric film deposition may allow excessive fluid loss or

even complete loss of circulation.

5. Drop cuttings into a sealing pit. As the drilling fluid is circulated through the sealing pit, cuttings

should drop out so they are not recirculated. The gel strength of the drilling fluid is the primary factor

controlling the rate of settlement. Gel strength is a measure of the fluid's ability to suspend cuttings when

the fluid is at rest. The flow rate in the settling pit is also important and is controlled by the shape and

size of the settling pit.

6. Facilitate acquisition of information about the formation being penetrated. Drilling fluid systems

should facilitate the recovery of representative cuttings and permit accurate geophysical logging of the

well.

7. Suspend cuttings in the borehole when the drilling fluid is not being circulated. During the time the

drilling fluid is not in motion, cuttings tend to settle in the borehole. If the rate of settlement is excessive,

cuttings may settle around the drill bit or stabilizer and jam the rotation of the drill string when drilling is

resumed.

During drilling, the principal objective is to maintain the drilling fluid in a suitable condition in spite of

changing downhole or surface conditions and the continuous addition of suspended drill cuttings. In

most cases, continuous monitoring of the drilling fluid is necessary to achieve the best results.

5.3. PROPERTIES OF WATER-BASED DRILLING FLUIDS

The drilling fluid properties listed in Table 5.2 should be understood thoroughly by the drilling

contractor. Regardless of which drilling fluid system is used, its effectiveness will depend upon the

contractor’s ability to anticipate the chemical and physical changes taking place during drilling and to

make modifications as required. At a minimum, all rotary drilling crews should be able to measure

drilling fluid density and viscosity, and understand the relationship of these properties to hole stability,

cuttings removal and fluid-loss control. The physical and chemical behavior of bentonite and polymers

differs significantly. These differences are examined separately as each drilling fluid property is

discussed below.

5.3.1. Density

Control of drilling fluid density is a fundamental factor in successful well drilling. Density is defined as

the weight per unit volume of fluid. Thus, the terms density and weight can be used interchangeably. In

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Under ordinary conditions, the maximum density that can occur in a clay system as a result of the

entrainment of solids during drilling is about 11 lb/gal. Further increases in density while maintaining a

proper solids/fluid ratio requires the introduction of higher density material so that less solids by volume

are needed for a specific density. Barite, wide a specific gravity of 4.2 to 4.35, is a standard weighting

material and is much heavier than clay additives and most formation materials, which have specific

gravities of 2.6 to 2.7. Barite particles are sized so as to remain suspended in the drilling fluid, but are

not small enough to affect the flow characteristics of the fluid.

To control the flow of water into the borehole, the contractor should increase the density of the drilling

fluid before reaching the confined formation. The additional drilling fluid density required to equalize

the confined pressure is determined by:

Drilling fluid density = weight of water · height of water above-ground level depth to top of confined

aquifer

The calculated drilling fluid density will only balance the confined pressure, however, and thus a safety

factor of 0.3 lb/gal is usually recommended. The total added density should be enough to control any

potential collapse of the formation during circulation of the drilling fluid and withdrawal of the drill

pipe.

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Viscosity of a fluid can be measured by a viscometer or a Marsh funnel. A certain volume of drilling

fluid is allowed to drain from a special funnel into a cup; the flow time is recorded and calibrated against

the time required for an equal volume of water to drain from the funnel (about 26 seconds at 70°F;).

These values, called apparent viscosities, are approximate and are good only in a relative sense.

Viscosities should be no higher than necessary to efficiently lift cuttings to the surface and control fluid

losses. Although drilling conditions can vary greatly, a Marsh funnel viscosity of 35 to 40 seconds will

usually be satisfactory in fine sand formations. If coarse sediment (gravel) is encountered, viscosities

must be substantially increased so that coarser particles do not have to be finely ground to be lifted by

the drilling fluid.

5.3.3.1. Viscosity of Drilling Fluids Made with Clay Additives

The viscous nature of drilling fluids made with clay additives originates from the small size of clay

particles (less than 4 microns) and their relatively large surface areas. Most clay particles have a platelike

structure; groups of these platelets are common. Clay particles generally swell when exposed to water

because the electrically unbalanced water molecules are strongly attracted to the plate surfaces and

thereby force the plates apart. This results in the clay particles occupying a larger space, which leads to a

more viscous fluid. Different types of clay have a wide range of hydration potential. Clays that hydrate

effectively are preferred because they produce a low-solids drilling fluid with high viscosity. Clays such

as montmorillonite, kaolinite, and illite are the primary clays used for fresh-water drilling fluids,

although montmorillonite is the only clay of these three that is available commercially. The

viscosity-building characteristics of sodium montmorillonite are the greatest of any clays, because the

sheets of atoms making up the flat clay particles are much thinner and come apart more easily in water

than those of other clays. Clays used for drilling fluids are rated by their yield, which is defined as the

number of 42-gallon barrels of drilling fluid with an apparent viscosity of 15 centipoise produced by

2,000 lb of clay. Water at 68°F has a viscosity of 1.005 centipoise. The term “bentonite" is used as a

commercial name for clays blat are predominancy sodium montmorillonite. Wyoming bentonite is most

common drilling fluid additive used in the water well industry.

5.3.3.2. Viscosity of Drilling Fluids Made with Polymeric Additives

In recent years, the use of natural and synthetic polymeric colloids in drilling fluids has increased. A

polymer is a long-chained chemical compound consisting of many small molecular units (monomers)

combined together. Polymers can be either natural or synthetic, usually have a high molecular weight,

and form chains of monomers several thousand units long. When the chains become tangled, they tend to

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Natural and synthetic polymeric drilling fluids have virtually no gel strength. This lack of gel strength

assures that cuttings removal is exceptionally good at the surface, wear on the mud pump by abrasive

material is minimized, and pumping pressures are minimized during normal circulation and resumption

of circulation. The drilling contractor should, however, clear the borehole of cuttings before circulation

is stopped, to prevent them from accumulating around the drill bit.

5.3.6. Filtration

Another of the principal requirements for a drilling fluid is to prevent fluid loss by forming a filter cake

or low-permeability film on the porous face of the borehole. The sealing property depends on the amount

and nature of the colloidal materials in the drilling fluid. The filter cake produced by clays and the thin

film created by polymeric colloids are physically dissimilar, because the size and shape of the particles

differ and their ability to hydrate is significantly different. Colloidal particles and suspended cuttings

entrained during drilling are important components of the total solids that create a filter cake or film.

Thus, the filtration properties of all drilling fluids are, in part, supplied by materials derived from the

borehole.

When drilling begins, hydrostatic pressure in the borehole causes the drilling fluid to flow into porous

formations. For drilling fluids made with clay additives, the fluid and some clay particles initially enter

the formation unhindered; but as the suspended solids and cuttings continue to close off the pores, clay

particles filter out and form a cake on the borehole wall. As the remaining pores around the borehole

become clogged with particles, progressively smaller volumes of water can pass into the formation

(Figure 5.2). In time, a filter cake effectively limits water flow through the borehole wall except in

highly permeable zones where lost circulation is apt to occur.

During drilling, the thickness of the clay filter cake will vary according to the rate of erosion caused by

the rotation of the tools and by the uphole velocity of the drilling fluid. In addition, thicker filter cakes

will form in formations that have higher hydraulic conductivity. When circulation stops, the filter cake

will continue to build up on the wall of the borehole.

The nature of this filter cake or film and the way it farms are quite different when drilling fluids are

prepared with polymers. Guar gum is a polysaccharide that provides natural fluid-loss control. This

property is derived from both the soluble guar gum particles (sols) dispersed in the drilling fluid and the

insoluble cell-wall residue (insols) of the gum.

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Many drillers, however, use single-reservoir pits that serve both of these functions. The velocity of the

drilling fluid as it moves through the mud pit during the storage internal must be as low as possible. This

can be achieved by changing the direction of flow as the drilling fluid moves through the pits, as well as

by deepening part of the pit or by using baffles and overflows. Deeper, rather than wider, trenches are

more satisfactory in reducing drilling fluid velocity. When a single pit is used, some drillers will slope

the bottom of the pit downward toward the pump suction point to slow the velocity and create a place for

the cuttings to settle. The suction hose must be mounted above the bottom of the pit. The bottom of an

excavated mud pit should be sealed with a plastic film or a compacted layer of clay.

5.4. AIR DRILLING

Many water wells are now drilled with air because of the relative simplicity and effectiveness of air

systems and the increasing number of rotary rigs equipped with air compressors. The earliest attempts to

use air as a circulating medium during the 1950's showed that significant increases in penetration rates

and bit life could be obtained. Air drilling is now recognized as a primary method to reduce drilling time

and therefore the cost of a well. The mechanics of air drilling are more difficult to understand than

typical water-based systems because the drilling fluid is compressible and often contains water, an

incompressible fluid, and other special additives.

To drill with air, the drilling rig must have access to an adequately sized compressor and a water pump

that can inject up to 10 gal of water and chemicals into the air stream. On some rigs, a special pump is

used to inject surfactants into water before the water is injected into the drill rods.

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The enormous variety of chemical and physical conditions that can exist in boreholes, and the large

number of commercial products available to remedy specific problems, preclude a simple prescription

for successful use of drilling fluids. Certain problems, however, occur regularly in typical geologic

formations when using ordinary additives. Swelling clays, for example, are a major problem when

drilling with rotary systems. The Corcoran Clay in the San Joaquin Valley of California, and the Laramie

Formation in the Denver Basin, are notable examples. As the clays hydrate and expand, the borehole is

partially or completely plugged, and occasionally the drill string may become stuck. The section below

contains recommendations for solving clay swelling and other common drilling fluid problems.

PROBLEM: Inadequate cuttings removal from borehole.

RECOMMENDED ACTION:

1. Clays and polymeric solids in water.

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4. Screen construction methods should permit the use of a wide variety of materials that are

compatible with differing groundwater environments to minimize corrosion and incrustation.

5. If constructed of metal, screens should be of single-metal construction to minimize

galvanic corrosion.

6. Screens must be sufficiently strong to withstand stresses normally encountered during and

after installation.

6.1. Continuous-Slot Screen

The continuous-slot screen is widely used throughout the world and is the dominant screen

type used in the water well industry. It is made by winding cold-rolled wire, approximately

triangular in cross section, around a circular array of longitudinal rods. The wire is attached

to the rods by welding, producing rigid one-piece units having high strength characteristics at

minimum weights. Welded screens are commonly fabricated from Type 304 and Type 316

stainless steel, monel, galvanic or ungalvanized low carbon steel, and thermoplastic

materials, mainly PVC and ABS or alloys of these materials.

Slot openings for continuous-slot screens are manufactured by spacing successive turns of

the outer wire to produce the desired slot size. These screens are typically fabricated in slot

sizes ranging from 0.006 to 0.250 in. Width of the openings can be held to close tolerances in

the all welded manufacturing method; allowable variations from the designated (ordered) slot

size generally range from 0.001 to 0.002 in, depending on the screen material and screen

size. Most high-quality screen manufacturers are concerned with slot variation because sand

pumping problems may occur if too many slots are significantly oversized slot control

quality is usually checked by comparing the designated size versus the average finished size.

All slots should be clean and free of burrs and cuttings.

Slot openings have been designated by numbers which correspond to the width of the

openings in thousandths of an inch A No. 10 slot, for example, is an opening of 0.010 in. Slot

size may also be expressed in metric units; for example, 0.010 in equals 0.25 rain. For

sm~l-diameter screens covered with wire mesh, the number of openings in the mesh per inch

me designated by gauze numbers. The size relationship of slot number and gauze number is

shown in Figure 6.1.

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For continuous-slot screens, individual slot spacing can be varied during fabrication. In fact,

a single section of screen may be made with many different slot sizes if geologic conditions

require these variations. In this way, maximum use of the hydraulic conductivity of each

stratum is possible.

Each slot opening between adjacent wires is V-shaped, resulting from the special shape of

the wire used to form the screen surface. The V-shaped openings, designed to be

nonclogging, are narrowest at the outer face and widen inwardly; they allow only two point

contact by any sand grains with a diameter larger than the slot size. Thus, oversized particles

are retained outside the screen and cannot close off the openings. Any sand grain that will

pass through the narrow outer part of the V-shaped opening enters the screen without

wedging in the slot. In screens with cut slots without V-shaped design, entering particles

often turn or twist sideways and, once lodged in the slots, the available intake area of the

screen is considerably reduced, causing either lower yield or greater drawdown (Figure 6.2).

Continuous-slot screens provide more intake area per unit area of screen surface than any

other type. For any given slot size, this type of screen has maximum open area Table 6.1

gives representative open areas of venous continuous-slot screens with differing outer wire

sizes (face widths) and slot sizes. Note that as the wire face width increases for a given slot

size, the open area decreases. However, larger wires increase the collapse strength of the

screen for any given diameter.

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