E a r t h A n d E n v i r o n m e n t a l S c i e n c e s , B a h r i a U n i v e r s i t y , I s l a m a b a d

DIFFERENT DRILLING METHODS USED IN HYDROGEOLOGY

CONTENTS

PAGE

1.0 Introduction 1

2.0 Project Objective 2

3.0 Description of Drilling Methods 3

3.1 Methods without Drilling Fluids 3

3.2 Methods that use Drilling Fluids 6

Conclusion

References

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DRILLING METHODS IN HYDROGEOLOGY

1. INTRODUCTION

Various well drilling methods have been developed because geologic conditions range from hard rock to completely unconsolidated material such as alluvial sand and gravel. Particular drilling methods are employed more frequently in certain areas because they are more effective in penetrating the local aquifers and thus offer cost advantages. Drilling procedures may depend on factors such as depth and diameter of well, lithology, sanitation requirement and use of the well (i.e. well dedication). The drilling method is site-specific and depends on the type of logging and testing to be performed. No single method is best for all conditions and applications. Well drilling methods are numerous and only the basic principles and applicability of selected and conventional methods are presented.

2. OBJECTIVE:

The objective of this assignment is to understand the concept of different drilling methods used in hydrogeology, their purpose, advantages and disadvantages and different uses in water exploration.

3. DESCRIPTION OF DRILLING METHODS:

A brief Description of Drilling Methods is below. Drilling methods can be grouped into two general categories:

a. Methods which do not use circulation (drilling) fluids:

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Displacement boring Solid-stem auger Hollow-stem auger Sonic drilling

b. Methods which use circulation (drilling) fluids to carry drill cuttings to the surface

Rotary (direct) Drilling Reverse Circulation Rotary

Drilling (RC) Cable-tool Percussion Air Percussion Down-The-

Hole Hammer

Groundwater well uses are numerous. For the sake of simplicity, the three following categories are proposed:

Monitoring well. Dewatering (production) well. Piezometer or observation well.

The primary consideration of a monitoring well is to obtain a groundwater sample representative of existing conditions and valid for chemical analyses. The sample must not be contaminated by drilling fluid or by drilling or sampling procedures.

3. DRILLING METHODS

3.1 METHODS WITHOUT DRILLING FLUIDS:

a. Displacement boring

Method where a piston or plug-type sampler is forced into the soil to the desired depth, displacing all the material on its path. Upon reaching the desired depth, the sampler is retracted and "grabs' a sample on its way back to the surface.

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b. Solid-Stem Auger

Method consists of drilling a continuous helix into the ground. The torque is provided by a top drive auger-drilling machine, which permits both downward push and retraction. Individual flights are normally 5 feet long. Different drill bits can be attached to the bottom of the auger to meet the formation requirement, which cut a hole ~10 % greater in diameter than the diameter of the auger.

Borehole diameter ranges from 6"-24", and can reach depths up to 400 feet depending on the size of auger used. The method is useful for hard grounds, cobble-rich (this depends on the size of the auger) soil or soft rock. The method is often ineffective in loose ground or below the water table since cuttings are not recovered, although it may be applicable under some circumstances. For those reasons, the method is mostly use as a "starter" method to advance the borehole down to the water table, from which drilling can resume with a more effective, adaptable method.

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c. Hollow-Stem Auger

This is a form of continuous-flight auger where the helices are wound around and welded to a tubular center stem or axle. Drilling proceeds essentially as in solid-stem drilling. When the sections are connected however, the hollow-stem auger will present a smooth, uniform bore throughout its length thus providing an open, cased hole in which samplers can be used or well installation can be performed. Other drilling methods can also proceed within the hollow stem, which can be used as temporary casing to prevent caving.

The use of hollow-stem auger, although common in geotechnical drilling, is limited in groundwater applications. Augers with diameters ranging from 6"-13" can be found (often drillers discuss HSA diameters as the inside diameter of the auger since the outside diameter varies with use; the outside is abraded with use.), with depths reaching up to 120 feet depending on the size used. The method is limited to unconsolidated to semi-consolidated formations. The method applies particularly well to clayey formation and is reasonably "good" to collect representative soil samples.

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d. Sonic Drilling

This relatively new drilling method allows the collection of core samples with speed and precision, and without the need of drilling fluids or air. One of the main advantages of the sonic drill is its ability to continuously core unconsolidated and some consolidated formations with a minimal amount of disturbance and compaction. The samples can be analyzed to provide a detailed stratigraphic profile of unconsolidated formations, including: dry or wet sands and gravels, cobbles and boulders, clays, silts and hard tills. Applications include environmental borings, monitoring well installation, aggregate exploration, rock exploration, methane probes, conductor casing installation, extraction wells, or other applications requiring a borehole diameter of less than 12-inches and depth of less than 500 feet.

High frequency mechanical oscillations, developed in a special drill head, are transmitted as resonant vibrations, along with a rotary action, through the tooling to the bit. The vibratory action fluidizes the soil particles, destroying the shear strength and pushing the particles away from the drill bit and along the sides of the drill string. Similar to a casing hammer or Odex system, the sonic rig drives an outer drill casing and an inner string consisting of drill rods and core barrel.

While coring, the core barrel is advanced before the outer drill casing, without fluid or air. The outer drill casing is then advanced to the same depth; this is best accomplished with water, which aids in pushing soil particles away from the drill bit, and also helps to cool the core barrel. Although dry casing advancement is possible, "drilling dry" can generate heat that affects the sample integrity. The use of water also provides an effective means of combating heaving sands without drilling mud or bentonite. With the outer casing left in place to hold the borehole open, which

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decreases the possibility of cross-contamination by cave-in of up-hole material, the core barrel is extracted and the samples are vibrated out of the barrel into plastic sleeves, stainless steel sample trays, wooden core boxes, or other containers.

The outer drill casing also holds the borehole open while installing monitoring wells, piezometers, vents, observation wells, instrumentation, or other down-hole equipment. Outer drill casing sizes include nominal diameters of 6 and 8 inches, allowing sufficient space to install the common monitoring well sizes of 2 and 4-inches. While constructing wells, the vibratory effect reduces "bridging" of the filter pack and seal, and also reduces the potential problem of "sand locking" and inadvertently removing the well as the outer drill casing is extracted. This positive placement of well construction materials allows for controlled well installations.

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3.2 METHODS THAT USE DRILLING FLUIDS: a. Rotary (direct) Drilling

This method makes use of a constantly rotating bit to penetrate any type of formation to depths that can exceed 1,000 feet. As drilling proceeds, cuttings are removed by a continuous circulation of fluid (either air or water based) that flows down inside the pipe string and up-hole along the annular space between the borehole walls and the pipe string. The penetration rate is often faster and the bit life longer when using air as compared with water based drilling fluids. A drag bit is normally used to penetrate unconsolidated to semi-consolidated sediments; while a cone-type or roller bit is used to drill consolidated rock. The bit can be rotated either by a top-drive or a table-drive system. The rotation speed is adjusted according to the hardness of the formation material.

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The drilling fluids serve several functions, which are principally to: lift and transport drill cuttings to the surface; stabilize borehole walls and prevent caving by the action of pressure and; cool and clean the bit. In some instances however, air tends to cause loosen unconsolidated formations. The method is limited to borehole diameter of less than 24 inches, due to the fluid's viscosity and up-flow velocity that make it difficult to clean out the cuttings.

b. Reverse Circulation Rotary Drilling (RC)

Reverse circulation rotary drilling uses the same principles as direct rotary drilling, except that the flow pattern of the drilling fluid is reversed. In this method, the drilling fluid (air or water) is pushed down in the annular space between the borehole walls and the pipe string, and is expelled upward within the pipe string. The cuttings are pumped to a collection facility at the surface by the use of an inverse coupling that carries cuttings through a discharge pipe. The method has seen few applications in groundwater monitoring work.

Reverse circulation drilling is mostly used in consolidated formation but can also be used in soft consolidated rock. I can also be used in hard rock if using both air and water as drilling fluids. The drilling mud is best described as muddy water and additives may or may not be added to the water. Engineering the correct mud chemistry and viscosity can be critical for some projects. Reverse circulation is a method often used to drill water wells since borehole diameter in excess 24 inches can be drilled to depths greater than for the direct rotary method. Reverse circulation rigs can be either table or top head driven.

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c. Cable-tool Percussion:

Cable-tool drilling is a method in which a bit, hammer or other heavy tool is alternately raised and dropped to strike through the formation by breaking the soil or rock. Cuttings are recovered by adding water to the borehole (slurry), which is intermittently bailed out of the borehole. This method is widely used in water wells but of limited application in monitoring work mainly because the method is slow.

In unconsolidated formations, the pipe (or casing) is advanced behind the bit. The casing diameter is slightly larger than the diameter of the bit and is equipped with a drive shoe at the end of the casing. No casing is necessary in consolidated formations and drilling proceeds by open-hole methods. The minimum borehole diameter is 6 inches because of the need to use large, heavy bits. The maximum diameter is ~ 24 inches, because of the lifting limitation of the hoist structure. Depths can reach several 1,000's feet, but the method is increasingly slow as the blowing impact looses force due to the increased friction between the casing and walls of the borehole.

d. Air Percussion Down-The-Hole Hammer

A method close to the direct rotary method, called down-the-hole, is used for hard rock formation and employs a pneumatic drill at the end of the drill pipe that strikes the rock while the drill pipe is slowly rotated. The percussion effect is similar to the blows delivered by the cable-tool bit. The air used to drive the hammer as the hole is advanced removes cuttings continuously. Unlike the conventional cable-tool bit that is constantly striking previously broken rock fragments, the bit on the air hammer always strikes a clean surface, thus rending the hammer very efficient. The method permits soil sampling as cuttings are delivered to the surface through a discharge swivel.

The method is well suited for boring in hard formations, where there is low risk of caving. Hammer bits 6 inches in diameter are most commonly used, although sizes range up to 17 inches. The down-the-hole hammer requires internal lubrication, which is provided by hydrocarbon lubricant added by means of in-line oilers. This need for lubrication eliminates consideration for monitoring work where hydrocarbons will be sampled.

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ADVANTAGES AND DISADVANTAGES OF DIFFERENT DRILLING METHODS:

DISPLACEMENT BORING:

ADVANTAGES:

Does require heavy equipment (by hand or lightweight equipment). Clean method for shallow well installation.

DISADVANTAGES:

Method limited to shallow depths. Method limited to soft soils and boulder, cobble-free zones. Not efficient if necessary to install several wells. Practical limitation up to ~ 2 " diameter sampler

SOLID-STEM AUGER:

ADVANTAGES:

Rapid and low-cost drilling in clayey formations. Clean method, does not require circulation fluids. No casing necessary where the formation is stable. Allows collection of representative sample in semi-consolidated formations.

DISADVANTAGES:

Practical limitations to 24" diameter Ineffiecient in loose, sandy material (depends on the depth). Inefficient below the water table

HOLLOW-STEM AUGER

ADVANTAGES:

Allows collection of uncontaminated sample in unnsolidated formation. Can be used as temporary casing to prevent caving. Relatively rapid, especially in clayey formations.

DISADVANTAGES:

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Ineffective through boulders. Limited drilling in loose, granular soils, particularly below the water table

where sample recovery can be compromised. Difficult to retrieve a sample in loose, granular soil because cuttings don't

always want to come to the surface. Samples must be collected with a split spoon or a continuous corer, either of which can provide excellent samples if done correctly.

Limited to rather shallow depths.

SONIC DRILLING:

ADVANTAGES:

Drilling can proceed with or without the use of drilling fluids Method can be utilized in unconsolidated and some consolidated formations Minimal disturbance to soil samples. Conventional air rotary or down-hole hammer methods can be employed

through the outer drive casing. The rig can also be operated as a fluid rotary machine.

DISADVANTAGES:

A relatively new method that is not available everywhere. Relatively expensive compared to other drilling methods. Dry casing advancement generates heat that can affect the sample integrity. Maximum nominal diameter of less than 12 inches. Practical depth limitation of less than 500 feet.

ROTARY(DIRECT)DRILLING:

ADVANTAGES:

High penetration rate. Drilling operation requires a minimum amount of casing. Rapid mobilization and demobilization.

DISADVANTAGES:

Use of a drilling fluid, both in terms of sample contamination and water management (in the case of water-based fluids and air injected by gasoline compressors).

Circulation of drilling fluid may be lost in loose/coarse formations, hence making difficult to transport drill cuttings.

Difficult to collect accurate samples, i.e. a sample from a discrete zone since

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the cuttings accumulate at surface around the rim of the borehole.

REVERSE CIRCULATION ROTARY DRILLING (RC)

ADVANTAGES:

Applicable to a wide variety of formations; Possible to drill large-diameter holes, both quickly and economically. Minimal disturbance to the formation due to the pressure being applied inside

and outside the pipe string. Easier recovery of cuttings since the up-hole velocity is controlled by the size

of the drill pipe and less subject to lost-circulation. No casing required during drilling and advantageous when high risks of

caving in. If there is a risk of caving, mucd should be used as a stabilizer. In the case of air drilling, it presents the same risk than regular air rotary, since the flow is down the annular space.

DISADVANTAGES:

High water requirements (not for air drilling). Collection of a representative sample is difficult due to potential material

mixing. Rig size can render access difficult. Need for drilling mud management (not for air drilling)

CABLE-TOOL PERCUSSION:

ADVANTAGES:

In situations where the aquifer is thin and yield is low, the method permits identification of zones that might be overlooked by other drilling methods.

Recovery of representative soil samples at every depth, although samples are disturbed due to the impact of the blow which can affect material several feet below the bottom of the hole.

Allows well construction with low chance of contamination. Borehole can be bailed at any time to determine approximate yield of the

formation at a given depth. Easy access to rough terrain.

DISADVANTAGES:

Slow penetration rate. Due to the constant mixing of water, is is not possible to obtain groundwater

smaples during drilling.

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Expensive casing for larger diameters. Difficult to pull back casing in some geologic conditions.

AIR PERCUSSION DOWN-THE-HOLE HAMMER

ADVANTAGES:

Rapid removal of cuttings. No use of drilling mud. High penetration rate, especially in resistant rock formation (e.g. basalt). Easy soil and groundwater sampling during drilling. Possible to measure yield estimate at selected depth in the formation.

DISADVANTAGES:

Restricted to semi-consolidated to consolidated formations.

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CONCLUSION

One of the objectives while using these methods is to know when you first hit the water and the water is likely to come from which zone? Moreover the difference in present water level and static water level should be determined these provide a lot of information and should be taken under consideration when continuing these methods .Mostly in Pakistan cable tool drilling is used on wide scale as compared to other because this is cost effective and reliable in any area. Only one engine is employed, and no water or mud is needed, yet it will perform all the tasks required to drill, place casing, and develop a well.

However every method has its pros and cons therefore while employing any of these methods due consideration should be given as water is a very valuable commodity for

life.

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REFERENCES

Understanding groundwater and wells (instruction hand book)

http://www.unicef.org/wash/files/04.pdf

http://www.liag-hannover.de/fileadmin/user_upload/dokumente/Grundwassersysteme/

BURVAL/buch/107-122.pdf

http://www.welldrillingschool.com/courses/pdf/DrillingMethods.pdf

http://www.robertsongeoconsultants.com/index.php?page=page&id=92

http://people.hofstra.edu/j_b_bennington/121notes/pdfs/Hydro_Lec._Geophysics.pdf

http://pubs.usgs.gov/of/2004/1366/pdf/ofr2004-1366.pdf

http://www.deq.state.or.us/lq/pubs/docs/tanks/GroundwaterMonitoringWellDrilling.pdf