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Borehole Drilling

Services

A borehole is the generalised term for any narrow shaft drilled in the ground, either vertically

or horizontally. A borehole may be constructed for many different purposes, including the

extraction of water or other liquid (such as petroleum) or gases (such as natural gas), as part

of a geotechnical investigation or environmental site assessment, for mineral exploration, oras a pilot hole for installing piers or underground utilities. Boreholes used as water wells are

described in more depth in that section.

In the engineering and environmental consulting fields, the term is used to collectively

describe all of the various types of holes drilled as part of a geotechnical investigation or

environmental site assessment. This includes holes advanced to collect soil samples, water

samples or rock cores, to advance in situ sampling equipment, or to install monitoring wells

or piezometers. Samples collected from boreholes are often tested in a laboratory to

determine their physical properties, or to assess levels of various chemical constituents or

contaminants.

Typically, a borehole used as a well is completed by installing a vertical pipe (casing) and

well screen to keep the borehole from collapsing. This also helps prevent surface

contaminants from entering the borehole and protects any installed pump from drawing in

sand and sediment. When completed in this manner the borehole is then more commonly

called a well: whether it is a water well, oil well or natural gas extraction well.

Drilling has undergone a substantial evolution from the early days and is now a very

specialised and technical activity requiring substantial skill and professional expertise to

achieve the desired result.

There are various methods for drilling a borehole but the most common methods used todayare rotary drilling, percussion drilling and sonic drilling. With the exception of sonic drilling,

there are different methods for each of the other categories. In addition there are also

different flushing methods for the removal of cuttings from the borehole i.e. direct flush or

reverse flush and different and different flushing media such as air, foam or mud. In the case

of percussion the cuttings are removed by mechanical methods. Possibly the most common

drilling methods used is the rotary method utilising air or mud flush. Using the rotary method

with air flush and an air operated hammer (percussion) mounted in the drill string has the

advantage of being fast, efficient and very controllable. However, different situations may

require different applications, technologies and methodologies to reach a successful project

completion, and that's where the industry experience and expertise of the Drilcorp team can

help. With their skill and knowledge, you can avoid expensive mistakes in your project andrest assured that your borehole will be successfully drilled on time and within budget.

Groundwater Abstraction Boreholes

When drilling a borehole that will ultimately be used for groundwater abstraction purposes,

the process becomes much more than just making a hole in the ground.

A water borehole is a specially engineered hole in the ground, designed to allow water to

flow into this hole and permitting a pump to be installed inside the hole to facilitate the

abstraction of water. Critical to this engineered design are the steps taken to prevent the

surrounding geological formations to collapse and seal off the borehole, which could result inthe entire borehole drilling process being started from scratch, or at a minimum, cleaning out

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the hole to recommission it.

Water abstraction is a team effort; the driller is there to drill and construct a borehole, while

the actual location of the water is a task that is carried out by the scientific team members,

led by a hydrogeologist. The scientists have to study the geological formations and

determine how and where the driller should install the borehole. At Drilcorp we have the bestdrillers working with the most accomplished scientists; which together make a formidable

team.

One of the most important aspects of drilling is the absolute necessity to achieve a vertical

borehole. The skill involved in maintaining a vertical borehole as the drill passes though

sand, gravel, clay and hard rock formations to its final destination is one that is only acquired

through many years of drilling experience.

Drill rods can weigh several tons and if too much pressure is applied while drilling or the drill

spins too quickly, it can wear out the bit or overload the rig. Conversely if insufficient

pressure is applied or the spin speed is too slow, it may not penetrate the rock. Add to thisthe complication that several different geological types of strata may be encountered in a

single borehole installation, requiring differing drill pressures and spin speeds and you can

see why this is a task for experienced operators, and why Drilcorp put so much emphasis on

experience and training in their workforce.

Because the drill rods are quite heavy and can weigh several tons; if the drill pushes too

hard or turns too fast, the drill bit will wear out too fast; if it does not push hard enough, it

won't penetrate. Quite often several rock layers are encountered in a single borehole, each

requiring different drilling pressures and speeds. When water is encountered the drilling

becomes even more complex, with the driller having to use his expertise to make vital

adjustments to the pressures and drill speeds in a very short timescale.

The two main types of drilling are:

Rotary drilling using the mud flush technique.

In rotary drilling, a drill bit, which is made from toughened materials such as tungsten, is

attached to a drill pipe. Once drilling has commenced and the bit is breaking through the

substrata, the drill fluids are circulated through the drilling pipework into the borehole and

back to the surface, at the same time washing the drilling residue or cuttings upwards and

out of the hole. This fluid also serves as a formation stabiliser preventing possible cave-in of

unstable sands or crumbly rock before the well casing or well screen is installed. In additionthis fluid acts as a lubricant for the drill bit. As the drill passes through water bearing rock

formations water will flow into the hole. Drillers, or hydrogeologists on site will carefully

monitor the depth of water "strikes" and keep a note of the formations in which they occur.

Rotary drilling using air percussion and flush technique

This technique uses compressed air to operate a down-hole air hammer on the end of the

drill string that helps to break up the rock formation. The compressed air that is used to

operate the down-hole air hammer also blows the crushed rock fragments out of the hole to

the surface along with any water that flows into the hole during drilling. An experienced driller

will realise that water will flow from the hole at specific intervals can be caused by thecompressed air forcing minor water deposits to the surface. Only when the driller is certain

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that he has reached the right aquifer will he stop the drilling process.

Costs

The cost of a borehole can be significantly influenced by the applied design as well as the

difficulty to construct a borehole in a specific geological formation.

Drilcorp will advise you of the most cost-effective method of drilling a borehole after they

have carried out a primary site survey.

While our solution may not be the cheapest in initial costs, in the long term we will provide

you with a solution that will repay its investment many times over.

Drilcorp will specify systems, applications and construction methods that will ensure the

maximum functionality and longevity of your installation, keeping it clean, functional and

operational for many years to come with the minimum of maintenance.

Drilcorp's reputation is second to none, our services are actively sought and retained by

some of the most prestigious companies in the World and our byword is excellence in

service and delivery.

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Hand auger - general

Auger drilling in action

The hand auger consists of extendable steel rods, rotated by a handle. A number of different

steel augers (drill bits) can be attached at the bottom end of the drill rods. The augers are

rotated into the ground until they are filled, and then lifted out of the borehole to be emptied.

A different auger can be used for each formation (soil) type. Hand augering can be done

both by using a heavy tripod and winch (such as the Vonder rig), or with lighter materials.

Above the water table, the borehole generally stays open without the need for support.

Below the water table a temporary PVC casing may be used to prevent the hole from

collapsing, and can be emptied either with an auger or a bailer. The permanent well casing

is then installed, while the temporary casing is removed.

Another option is to use the well casing used as support for the borehole as the permanent

casing (direct installation), although in that case a gravel pack cannot be used.

History and social context

Suitable conditions

The hand auger is suitable for unconsolidated formations: sand, silt and soft clay. Stiff clays,

hard materials and gravels are difficult or impossible to drill through and to remove (unless

augering is combined with percussion).

Augers can be used up to a depth of about 15-25 meters, depending on the geology.

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Auger drilling in temporary casing

Advantages Disadvantages/limitations

- Easy to use above the groundwater table - It may be very difficult to remove the

temporary casing. If clay layers are

penetrated, it is very difficult to use a

temporary casing, as it sticks to the clay.

- The borehole does not stay open if a

collapsing sand layer is encountered below a

clay layer (through which the temporary

casing could not penetrate)

Auger drilling in temporary casing

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Technical specification

Operation

It takes quite some expertise to drill a completely straight borehole.

MaintenanceManufacturing

All tools can be made in local workshops (with local materials) and are commercially

produced in western countries (but more expensive).

Suppliers

The tools for Hand augering can be made in local workshops.

Chafagane Maiga - Light and effective hand auger equipment.

Eijkelkamp Agrisearch Equipment - Drilling equipment and manuals on hand auger andpercussion gouges

Van Reekum Materials - Drilling equipment, hand auger and percussion drilling -

Different bailers and augers

Cost

Country experiences

The hand auger is widely used in Niger, Chad, Senegal, The Gambia, Uganda, Zimbabwe,

Tanzania, Nigeria, Central America and western countries.

Manuals

Hand Augered Garden Wells, Jonathan Naugle, Lutheran World Relief, third edition

1996. Manual on Hand augering, well construction, manufacturing tools.

Self-help wells, R.G. Koegel, FAO 1985. Internet manual on hydrogeology, well

constructions, all drilling techniques

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ROTARY DRILLING

In the rotary method, the hole is drilled by a rotating bit to which a downward force is applied.

The bit is fastened to, and rotated by, a drill string, composed of high quality drill pipe and

drill collars, with new sections or joints being added as drilling progresses.

The cuttings are lifted from the hole by the drilling fluid which continuously circulated down

the inside of the drill string through water courses or nozzles in the bit, and upward in

annular space between the drill pipe and bore hole.

At the surface, the returning fluid (mud) is diverted through shale shakers, desilters,

desanders and series of tanks or pits which treat the fluid. In the last of these pits the mud is

picked up by the pump suction and repeats the cycle. Figure 1 shows the basic components

of a rotary drilling rig.

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Making a connection, the process of adding a new joint of pipe to the drill string is shown in

Figure 2. Periodically the pipe must be removed from the hole in order to replace the bit.

The operation is illustrated in Figure 3.

The basic drilling components are:

1. Derrick, mast and substructures

2. Drawworks

3. Mud pumps

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4. Prime movers

5. The drill string

6. Bits

7. Drilling line

8. Miscellaneous and auxiliary equipment

Derrick, mast and substructures

The function of a derrick is to provide the vertical clearance necessary to the raising and

lowering of the drill string into and out of the hole during the drilling operations.

Derricks are rated according to their ability to withstand two types of loading:

1. Compressive loads

2. Wind loads

The allowable compressive load of a derrick is computed as the sum of the strengths of thefour legs. Derricks with load capacities from approximately 86,000 to 1,400,000 lb,

depending on steel grade and leg size are available.

Allowable wind loads for API derricks are specified in two ways, with or without pipe setback.

With pipe setback, the wind may be blowing perpendicular to it, which is essentially a pipe

wall. This is the worst possible condition.

Wind loads are calculated by the formula:

P = 0.004V2

where P = wind load, lb/ft2

V = wind velocity, mph

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Calculation of Derrick Loads

The block and tackle arrangement for a rotary rig is shown in Figure 4. Assuming that the

system is frictionless, the following relationship are apparent:

Wn

2nFd

+=

where Fd = total compressive load on the derick

n = number of lines through the travelling block (those supporting

W).

W = hook load

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The derrick load is always greater than the hook load by the the factor of (n+2)/2 due to the

two additional lines (drawworks and anchor) exerting a downward pull.

During hoisting:

vL = nvA

where vL = velocity of line being spooled (or unspooled) at the drawworks

during hoisting.

vA = hook velocity

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Drawworks (hoist)

The drawworks or hoist is the key piece of equipment on a rotary rig. The functions of the

drawworks are:

1. It is the control centre from which the driller operates the rig. It contains the

clutches, chains, sprockets, engine throttles and other controls wich enable the rig

pwer to be diverted to the particular operation at hand.

2. It houses the drum which spools the drilling line during hoisting operations and allows

feed-off during drilling.

Drawworks are commonly designated by a horsepower and depth rating.

e

1x

33,000

vWHP h=

where W = Hook load, lb

vh = hoisting velocity of travelling block, ft/min

33,000 = ft.lb/min per horsepower

e = Hook to drawworks efficiency

Hook to drawworks efficiencies are commonly between 80 to 90%, depending on the

number of lines in use.

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Mud Pumps

the function of the mud pumps is to circulate the drilling fluid at the desired pressure and

volume. The pump normally used is the reciprocating piston, double acting, duplex type.

The term double acting denotes that each side of the piston does work, while duplex

refers to the number of pistons (two).

The superiority of the piston type pump for drilling service is due to the following features:

1. Ability to handle fluids containing high percentages of solids, many of which are

abrasive.

2. Valve clearance will allow passage of large solid particles (typically lost circulation

materials) without damage.

3. Ease and simplicity of operation and maintenance. Liners, pistons and valves may

be replaced in the field by the rig crew.

4. Wide range of volume and pressure available by using different liner and piston

sizes.

Prime Movers

The bulk of rig power is consumed by two operations:

1. Circulation of the drilling fluid

2. Hoisting

Fortunately these requirements do not occur at the same time and the same engine perform

both jobs.

The prime movers used are the steam engines, electric motors and internal combustion

engines. The most commonly used is the internal combustion engines such as the

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automotive type (multicylinder, light flywheel) diesel and gas engines capable of rapid

acceleration.

The Drill String

The rotary drill string includes the components as in Figure 5.

Fig. 5: Schematic diagram of drill string components and

bit. Bit load is furnished by heavy walled large-diameter

drill collars.

Rotary bits

The bit is the part that drill the hole. Basically there are three types of drill bits, these are the

drag type, rolling cutter type and the diamond bit. The most common is the rolling cutter

type. The diamond bit is commonly used in hard formations.

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Drilling line

The rotary drilling line afford a means of handling the load suspended from the hook during

all drilling operations.

The maximum load occurs when running casing, although fishing operations frequently

require line pulls in excess of the drill string weight.

Travelling Block, Hook and Swivel

The travelling block is the travelling pulley assembly that connects the drilling line to the

hook and swivel. The swivel must suspend the drill string and allow rotation at the same

time.

Blowout Preventer (BOP)

The main function of a blowout preventer is to furnish a means of closing off the annular

space between the drill pipe and casing.

It is not always possible to predict the exact manitude of pressures which will be

encountered in the drilling of a well. Consequently it is not uncommon to encounter pressure

greater than those imposed by the drilling fluid, with the result that formation fluids flow into

the bore hole and eventually to the surface.

This effect is called a blowout, and is one of the most feared and expensive accidents which

can occur in well drilling.

Most blowout preventers are either hydraulically or pneumaticaly operated, with manualoperation available as a safetry precaution.

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