08/10_rock excavation handbook / water well drilling

8. Water Well DrillingROCK EXCAVATION HANDBOOK

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The importance of fresh water has inspiredpeople throughout history to develop variousmethods of tapping these valuable groundwater resources. Many of these methods arestill used today because different types ofaquifers and environments (physical, financialor political) require different drilling tech-niques.

8.2. METHODS

CABLE TOOL DRILLING

Cable tool drilling is the oldest percussivedrilling method and was developed by theChinese about 4,000 years ago. The techniqueis still used today, but the original bambootools have been replaced by steel tools, andhuman and animal-generated power has beenreplaced by gasoline and diesel engines.

Cabletool rigs (also called spudder rigs) drillby repeatedly dropping and lifting a string ofheavy tools into a hole. The reciprocatingmotion required for lifting and dropping is imparted by the vertical movement of a spuddingbeam (FIGURE 8.2.-1.). The loosened material or rock cuttings are mixed with water andremoved from the hole periodically by a bailer or sand pump.

The bailer is a section of pipe with a check valve at the bottom. The cuttings open the valvewhen the bailer is lowered to the bottom. The valve closes automatically when the bailer islifted. The sand pump is a bailer equipped with suction pipe and plunger. The plunger cre-ates a vacuum that opens the check valve at the bottom and sucks sand and cuttings intothe bailer.

The drill string for cable-tool drilling consists of 5 components: drill bit, drill stem, drillingjars, swivel socket and cable (FIGURE 8.2.-2.). The drill bit (chisel) crushes the rock andmixes the cuttings. The drill stem is a heavy section of pipe that adds weight to the stringand guides the bit in the hole. Drilling jars consist of two steel bars that are linked together.The free sliding jars are used to loosen a stuck drill bit on the upward stroke. A swivel socket connects drilling tools to the cable and transmits the rotation of the cableto the drill string so that the drill bit crushes fresh rock in every downstroke. The cablecarries and rotates the drilling tools by twisting the swivel socket on every upstroke.

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BAILER

FIGURE 8.2.-1. Cabletool rig.

8.1. GENERAL

Water is necessary for sustaining life on earth. All life depends on it, not only humanbeings. About 97% of the earth’s fresh water supply is located underground (glaciers and icecaps excluded).

Most ground water that we use comes from rain and snow. Some water that falls on theearth’s surface penetrates the soil and becomes subsurface water bearing formations(FIGURE 8.1.-1). Useable ground water also includes water trapped in sediments and waterthat vaporizes from molten rock as it cools below earth’s surface.

Subsurface water with formations that are capable of yielding sufficient water for wells arecalled aquifers, which can be found in gaps and pores between particles and grains in allu-vial or unconsolidated formation or in cracks, and joints in consolidated formation. Althoughground water exists almost everywhere, it is not always accessible to tap it. Layers contain-ing water may be impermeable, thus preventing water from flowing into the well.

SAND

SLATE (Impermeable)

SANDSTONE (Aquifer)

GRANITE

OCEAN

SubmarineSpring

RAIN

Evaporationwhile falling

Clouds and Water Vapor

SUNSource of Heat

DAM

EVAPORATION

Gro

und

wat

er fl

ow

FAULT

Freshwater/SeawaterInterface

Ground waterflow

Swamp

Percolation

Run off

FIGURE 8.1.-1. Hydrologic cycle.


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ROTARY DRILLING

Rotary drilling is the method in which arotating bit crushes or cuts rock under heavydown pressure. This method was developedto increase drilling speed and depth in mostformations. The drill string, which has anattached bit at the bottom, is rotated byrotary table or hydraulic top head. Theweight of the drill string provides therequired feed force for the bit. In topheaddrive rigs, the weight can also be applied bya hydraulic feed system at the top of thehole. As the drill bit penetrates the hole,drilling fluid is forced down to the bit whereit flushes the cuttings out of the hole.Depending on the drilling fluid and channelused to force it down to the bit, drilling canbe divided into the following main types: airdrilling, reverse air drilling, air hammerdrilling (DTH), direct mud circulation drillingand reverse mud circulation drilling.

Air Drilling

Air drilling is a rotary drilling method thatuses air, air/water or an air/water/foamingagent mixture as the hole cleaning fluid. Airdrilling has the advantage of reducingdrilling costs by increased penetration rates,longer bit life and reduced well development(cleaning) time. Air for hole cleaning is pro-duced by a compressor that is either on thedrill rig or a portable unit. Air is directeddown the drill pipe where it cleans the faceof the bit and flushes the cuttings out ofthe hole through the annulus between thedrill pipe and hole wall. The pressurerequirement for air drilling depends on thewater head in the hole, but is typically 100 -150 psi (7 - 10.5 bar). The air volume need-ed for hole cleaning depends on the holeand drill pipe diameter calculated by the

FIGURE 8.2.-3. Auger drilling

Although cabletool drilling is a slow method and can´t competewith modern rigs, it has maintained its popularity in certainapplications and geographical locations. Cabletool machines aresimple, mechanical machines that can be repaired in almost anyworkshop. Initial investment costs are low and most spare partscan be locally produced. Cabletool rigs also have the advantagewhen drilling in alluvial formation that contains boulders becausethe casing is set during the drilling process. The casing stabilizeshole walls and prevents boulders from falling onto the bit.

BORING

Well boring is the non-percussive drilling of shallow wells in soilor unconsolidated formation with mechanical hole cleaning. Twomain types of boring are commonly used in water well drilling:bucket drilling and auger drilling.

In bucket drilling, the excavated material is collected in a cylin-drical bucket that has auger-type cutting blades at the bottom.The bucket is connected to a kelly bar (square cross-section tub-ing), which is rotated by a truck-mounted ring gear. Once thebucket is full, it is raised by a winch to the surface and dumpedbeside the rig. This procedure continues until the desired depthhas been reached. The well diameters drilled with a bucket drillvary from 18” - 48” (457 mm - 1219 mm) and typical depths areless than 150´ (45.7 m).

In auger drilling, the drill string consists of one or several sec-tions of tubing equipped with welded spiral flanges, calledflights. When auger pipes are rotated, a special bit or cutterhead, which is attached to the leading auger pipe, cuts the hole.The flights then carry the soil or cuttings to the surface (FIGURE

8.2.-3.). The cutter head is usually 2” (51 mm) larger in diameterthan the flights. Augers can be divided into two types: solidstem and hollow stem. The latter is mainly used in explorationdrilling, but is becoming more popular in well drilling due to itsability to support the hole walls while screen, filter packing andpermanent casing are set through the center hole. The commonauger outside diameter in water well drilling application is 6” -14” (152 mm - 356 mm) and the depth range is 40´ - 120´ ( 12.2m - 36.6 m). Drilling rigs for auger drilling are usually topheaddriverotary drills with adequate torque capacity. Large diameter augers require drilling rigs that are similar tothose used in bucket drilling.

FIGURE 8.2.-2.Cable tool rig drillstring.

Cable

Swivelsocket

Drilling jars(In closed Position)

Wrenchsquare

Drill stem

Watercourse

Drill bit


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Air lifting capacity can also be improved by adding a foaming agent to the air/water mist.The air/water/foam method carries larger cuttings, therefore improving the penetration ratein larger holes where the bailing velocity is insufficient to clean the hole properly. Bailingvelocity in the hole annulus can be as low as 150 fpm (0.76 m/s) if foam is used.

Air drilling in water well applications is not especially common because it is limited to semi-consolidated or consolidated materials and it can not compete with DTH hammer drilling inmedium hard or hard formations.

A typical air drilling rig comesequipped with a mast, holdback/ feedsystem, rotation unit, air compressor,prime mover (which may be a truckengine or a separate deck engine),hydraulic system or mechanical drivesystem and truck (FIGURE 8.2.-5.).The holdback/feed system is neededto move the drill string up and down,and to hold it during the drillingprocess. In deep holes, the weight ofthe drill string can be too heavy forthe drill bit and must be reduced bythe holdback system. The holdback/feed system in top-drive machines isoperated by a hydraulic cylinder andchain or cable. Rotary-table rigs use awinch to carry the drill pipe loadwhich means that feed force otherthan the drill string weight can notbe applied.

A rotation unit can be comprised of either a hydraulic top driveor table drive, which is oftena mechanical system. The top drive rotates the drill string from the top. The table drive islocated at deck level on the drill rig (FIGURE 8.2.-6.). Table drive requires a special drillsteel joint (also called a kelly) to rotate the drill pipe. A kelly is the first section of the drillsteel after the swivel and must be removed each time a drill pipe is added. The kelly’s crosssection is usually square, but can also be hexagonal or round with lengthwise grooves.

Usually, the prime mover in an air drilling rig is a deck-mounted diesel engine that powersall drilling functions through a hydraulic or mechanical drive system. Mud drilling rigs and small air drilling rigs that may require separate, portable air compressors, are often run off atruck engine.

FIGURE 8.2.-5. Hydraulic top-drive air drilling rig.

following formula:

183.3 • CFMBV = —————————

D2 - d2

where BV = bailing velocity in fpmCFM = compressor output volume in cfmD = hole diameter in inchesd = drill pipe diameter in inches

In normal drilling conditions, bailing velocity for well drilling application should be 3000 -7000 fpm (16 - 35 m/s). Bailing velocity can also be determined from FIGURE 8.2.-4..

The formula above shows that the available compressor limits the maximum hole size.Bailing velocity can also be adjusted by the drill pipe, but a larger pipe diameter weighsdown the drill string, thus limiting the maximum drilling depth. Hole cleaning is enhancedby adding small amounts of water to the flushing air. Water also suppresses dust from thehole and makes the entire drilling process more environmentally friendly.

FIGURE 8.2.-4. Calculating bailing velocity when hole, pipe size and compressor volume are known.


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DOWN-THE-HOLE (DTH) HAMMER DRILLING

DTH drilling is actually a percussive method, but is usually classified as a rotary drillingmethod because rotary air drilling rigs are used.

A pneumatic hammer and bit, attached to the lower end of the drill string, hit the rock atthe bottom of the hole at a high frequency. Air with high pressure, which is directed downthe drill pipe, operates the DTH hammer and cleans the face of the bit so that it always hitsa clean surface. The drill string, hammer and bit are rotated slowly by the top head. The rig’sfeed system keeps the bit firmly on the rock. Since DTH drilling is a percussive method, itrequires neither high torque or bit load.

Today, the optimum operating pressure for DTH hammers is 350 psi (24.6 bar), which setsthe requirement for air compressors. Necessary air volume depends not only on hole size, butalso on the size of the DTH hammer. The bigger the hammer is the more air it requires tohold 350 psi (24.6 bar) operating pressure (FIGURE 8.2.-8.). The DTH hammer’s penetrationspeed is proportional to the operating pressure.

FIGURE 8.2.-7. Rotary drill string.

The drill string in rotary drilling consists of several components, each with its own importantfunction: top sub, drill pipe, cross-over sub, drill collar, stabilizer, bit sub and bit (FIGURE

8.2.-7.). The top sub is an adapter between the top head or swivel and the drill pipe (threads mayvary). The drill pipe conveys the rotation torque and drilling fluid to the bit. Drill collars addweight to the bottom of the drill string and keep it and the hole straight. Drill collars alsoplay a very important role at the top of the hole when drilling with table-drive machinesbecause they don’t have any feed force. A cross-over sub is an adapter that is needed whenthe thread of the drill collar and the stabilizer is not the same. The stabilizer, usuallyattached next to the bit and has a diameter of 1/4” smaller than the bit diameter, is used toguide the drill bit so the hole stays straight. The bit sub is another adapter between the sta-bilizer and the drill bit. In DTH drilling, the hammer is located next to the bit at the bot-tom; stabilizers and drill collars are not normally used. Reverse circulation air drilling is yetanother type of air drilling and is further explained in chapter 9.2.2.

AIRCOMPRESSOR

ROTARYTABLEDRIVE

KELLYBAR

COMPRESSOR

FIGURE 8.2.-6. Hydraulic top-drive and mechanical table-drive rigs.


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When selecting the correct mud pump, two factors must be considered: mud volume for cre-ating the necessary bailing velocity, and pressure for compensating pressure loss in the drillstring and surface equipment. The following formula is used to calculate the required mudpump volume capacity:

24.5 • GPMBV = ————————

D2 - d2

where BV = bailing velocity in fpmGPM = mud pump displacement in gpmD = hole diameter in inchesd = drill pipe diameter in inches

The recommended bailing velocity is 80 -150 fpm (0.41 - 0.76 m/s). A lower velocityresults in a slower drilling speed. This factmust often be accepted in water welldrilling. A mud pump necessary to create 80fpm or more bailing velocity in a medium orlarge diameter well is too big and heavy tobe mounted on a truck.

The mud pump pressure requirement dependson the hole size, drill pipe size, hole depth,mud pump displacement and type of surfaceequipment (in other words, everythingbetween the first drill pipe and mud pump).Detailed pressure loss calculations are fairlycomplicated and usually the losses are onlyestimated using tables and graphs shown inwell drilling manuals.

Drilling mud has the following main functions in the well drilling process:

- Drill bit cooling and lubrication- Cuttings removal from the hole- Hole wall support and caving prevention- Sealing hole wall to reduce fluid loss- Blow-out prevention (oil and gas exploration)

Correct mud properties are essential for a successful well drilling process, therefore, each wellmust be individually designed. Main mud drilling properties include density (specific weight),

SETTLING PITSUCTION PIT

MUDPUMP

FIGURE 8.2.-9. Direct-circulation mud drilling.

The most common sizes of DTH hammers for water well drilling are 6” and 8”, but sizes suchas 4” and 5” in the lower end are not unusual. Also 10”, 12”, 15” and even 18” hammer areused in water well drilling, however the operator must compromise the 350 psi (24.6 bar)operating pressure. Self-contained well drilling rigs do not have a compressor that is bigenough to hold the optimum pressure in large hammers and the connection of an additionalportable compressor into the air system is not feasible in most cases due to high operatingcosts.

In well drilling applications, the DTH hammer has the fastest penetration in hard formation.Even if the hammer operates at less than the optimum pressure level, penetration is fasterthan in air or mud rotary drilling.

DIRECT CIRCULATION MUD DRILLING

In the direct rotary mud method, the drilling process is similar to air drilling except theflushing fluid is water or a water/bentonite clay mixture (also called drilling mud). The aircompressor is replaced by a mud pump that pumps mud through a rotating drill pipe and abit to the bottom of the hole. The fluid picks up the cuttings and then flows upward throughthe space between the drill pipe and the hole wall to the ground surface. At the surface, themud flows to the settling pit where the cuttings fall down to the bottom. From there, theclean surface mud flows through a ditch to another pit where the suction of the mud pumpis located (FIGURE 8.2.-9.).

FIGURE 8.2.-8. Down-the-hole hammer air consumption (Sandvik XL and SD series).


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The mud pump is usually a centrifugaltype which can handle the cuttingswithout excessive wear. Reverse-circu-lation rigs are always rotary table typebecause they have more rotationtorque and can handle larger drill pipediameters.

Another form of reverse-circulationdrilling is air-lift drilling, which is suit-able for top-head drive machines(FIGURE 8.2.-12.). This system consists of a side dis-charge swivel that is mounted under-neath the top drive, and special drillpipes equipped with built-in air chan-nels. Instead of the centrifugal mudpump, compressed air is used to circu-late the drilling fluid. Air, injectedthrough pipe’s air channels, enters thepipe and creates a flow that brings thefluid and cuttings to the surface andthrough the discharge swivel to set-tling pit.

DRILL BIT

DRILL PIPE WITH BUILT-IN AIR DUCTS

SIDE DISCHARGE SWIVEL

TOP DRIVE

FIGURE 8.2.-12. Air-lift reverse circulation drilling.

0

1

2

3

4

5

9 14 19 24

5Ó 10Ó 15Ó 20Ó

Alluvial

Soft

Medium

Hard

Very hard

ROCK HARDNESS

HOLE DIAMETER

AUGER DRILLING

DTH DRILLING

DIRECT CIRCULATION MUD DRILLING

BUCKET DRILLING

REVERSE CIRCULATION MUD DRILLING

FIGURE 8.2.-11. Selection guide for rotary well drilling.

viscosity and filtration. Mud density affects the hydrostatic pressure, pumping pressure, lift-ing capacity of cuttings, and the stability of the hole walls. Excess density may cause damageto the aquifer by forcing solid material into the formation. Likewise, insufficient density maycause the walls to cave in.

Mud viscosity affects the pumping pressure, lifting capacity of cuttings and the settling timeof cuttings in the mud pit. Mud viscosity should be low enough for rapid cuttings removal in settling bit, but high enough to ensure proper hole cleaning.

Filtration properties consist of the mud´s ability to form a thin low-permeability filter cakeon the hole walls. Walling prevents drilling fluids containing solid content from entering thewater bearing bed and plugging the aquifer. A thick filter cake can restrict the passage oftools.

The direct circulation mud drilling method can be used in almost any kind of formation ordrilling depth. There are however places where bentonite mud has been prohibited for envi-ronmental reasons. Additionally, if the formation has many cavities and cracks, mud drillingcan not be used due to the risk of circulation loss.

REVERSE-CIRCULATION MUD DRILLING

The reverse-circulation method was developedto overcome hole size limitations in directmud drilling. As the name indicates, the differ-ence between direct and reverse mud circula-tion is the direction of the fluid flow insidethe drill pipe (FIGURE 8.2.-10.). The fluid andcuttings are sucked through the drill pipe tothe surface by a mud pump and then dis-charged to the settling pit. Pit fluid flowsback to the bore hole and down to the bitbetween the drill pipe and the hole wall dueto gravitational pull. Drilling fluid usually con-sists of water or very light mud.

The reverse circulation system is used mainlyin large diameter wells in unconsolidated rock(FIGURE 8.2.-11.). Whereas hole size in directcirculation is limited by the mud pump capaci-ty, in reverse circulation even a medium-sizedmud pump can create enough bailing velocityto bring the cuttings up because the innerdiameter of the drill pipe is relatively small.

FIGURE 8.2.-10. Reverce-circulation mud drilling.

SETTLING PIT SUCTION PIT

MUDPUMP

KELLYBAR

ROTARYTABLEDRIVE


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WELL SCREEN, CASING AND GRAVEL PACKING PLACEMENT

A well screen is a section of slotted steel casing that allows water from the aquifer to enterthe well. Well screens are used in unconsolidated formation to prevent sand from enteringthe well. If the aquifer formation is very fine, gravel packing is placed around the screen tomake the material around the screen more permeable.

Well casing has several functions in the well. It houses the pumping equipment, works as a conduit for upward flowing water and seals contaminated surface water from the well.

The well screen, casing and gravel packing are set down in the following way based on theformation (FIGURE 8.3.-1.):

A) Alluvial or unconsolidated formation

The casing is set down after each diameter in telescopic drilling. The screen and gravel pack-ing are set on the level of aquifer.

B) Consolidated formation - large diameters

If used, the surface casing is set after the hole has been completed. Screens and gravelpacking are not used.

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SURFACE CASING(CONDUCTOR PIPE)

CEMENTGROUT

CASING

WELLSCREEN

WATER BEARINGAQUIFER

GRAVELPACKING

ALLUVIAL OR UNCONSOLIDATEDFORMATION

CONSOLIDATED FORMATION

UNCONSOLIDATEDOVERBURDEN

SURFACE CASING

FIGURE 8.3.-1. Well designs in alluvial and consolidated formations.

8.3. WELL DRILLING PROCESS

DRILLING PREPARATION

Preparing for a drilling project starts as soon as the location of the well has been decided. The location is chosen based on geological, hydrological ormerely functional reasons. Preparations include:

- Opening access road to the site- Transportation of drilling tools and accessories

(drill pipe, bits, hammers, casing, screens, bentonite,foaming agent, etc.) to the site

- Transportation of water to the site, if otherwise unavailable

- Mud pit and ditch digging (mud drilling)- Rig set-up (leveling, raising the mast, building

the starting drill string, etc.)

DRILLING

The actual drilling procedure varies a lot depending on the formation and purpose of thewell. The following three main types are identified:

A) Alluvial or unconsolidated formation

Drilling is performed in several stages starting with a large diameter bit. After each stage,the casing is run down and drilling continues with a smaller bit through the casing. Thedepth of each stage depends on the drilling rig’s holdback capacity (how much each size ofcasing it can handle).

B) Consolidated formation - large diameters

Drilling starts with a pilot hole, which is drilled to the final depth. The hole is then openedwith a reamer bit to the final diameter. It may be necessary to ream several times with different reamer sizes to reach the final diameter. A top hole is drilled prior to the pilot holeif surface casing is used.

C) Consolidated formation - small to medium diameters

Consolidated formations are often covered with a shallow layer of soil. In order to getthrough this layer, the well is started driving casing simultaneously with the drilling. (Tubexoverburden drilling or casing hammer). Once solid rock is reached, the drilling continues withthe desired diameter to the final depth.


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C) Consolidated formation - small to medium diameters

If necessary, the casing is set simultaneously with the drilling until solid rock is reachedafter which the casing is no longer needed. Screens and gravel packing are not used in thistype of well.

WELL DEVELOPMENT

Well development repairs the damages done to the aquifer during the drilling operation sothat the hydraulic properties can be restored. Additionally, it alters the basic physical proper-ties of the formation so that more water can flow into the well. If well development is notproperly performed, the aquifer’s water yield may not be adequate. The life span of a waterpump may also be limited in a poorly developed well due to a high clay and sand content inthe water.

There are several methods of developing wells, include surging with a plunger, air lifting andjetting. In surging, the plunger forces water from the well into the surrounding formation onthe downstroke. The upstroke pulls water back into the well drawing all particles smallenough to pass through the screen along with it. Air lifting is similar to air lift reverse cir-culation except that it uses clean water. Jetting is a method in which high pressure water ispumped through small nozzles into the formation.

TEST PUMPING

Test pumping consists of determining the performance characteristics of the well and thehydraulic parameters of the aquifer. In a well performance test, yield and drawdown aremeasured so the capacity of the well can be calculated. An aquifer test is performed to esti-mate the affect of the new well to the aquifer. Test pumping also provides valuable informa-tion for correct pump selection.

PUMP INSTALLATION

The last operation in the well drilling process is pump installation. Pumps are installed inwater wells to lift water to the surface and pump it to the point of use. Pump installation isoften a side business for drilling contractors, but there are also companies that specialize init. Pump selection is made by matching well performance characteristics, customer require-ments and pump specification.

08/10_rock excavation handbook / water well drilling

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