Description of Drilling Methods

• Displacement boring
• Driven wells
• Solid-stem auger
• Hollow-stem auger
• Sonic drilling

• Rotary Drilling
  • Rotary (direct) Drilling
  • Reverse Circulation Rotary Drilling (RC)
  • Dual-wall Reverse Circulation Drilling
  
  
• Percussion Drilling
  • Cable-tool percussion
  • Air percussion down-the-hole hammer
  • Air percussion casing hammer
  • ODEX percussion down-the-hole hammer
  
  

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

• Depth of drilling: all drilling methods have certain limitations;
• Sample recovery: type of samples desired, i.e. soil, groundwater, disturbed or undisturbed, frequency of sampling, yield estimation;
• Target lithology: well installation completed in unconsolidated of consolidated formation.

Health and Safety
• Level of contamination
• High yield of formation may produce high pressures;
• Underground fire hazards in gaseous areas
Access and Noise
• Terrain roughness;
• Space and height limitations;
• Municipal noise ordinance
Disposal of drilling fluids and cuttings
• Contaminated cuttings and groundwater may have to be handled as hazardous wastes and be transported to landfill or special waste disposal facilities.
Lithology and Aquifer characteristics
• Soil type (sand, clay, boulders)
• Depth to water table
• Depth to bedrock
Cost

Methods without drilling fluids:

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.

PROS:
• Does not require heavy equipment (by hand or lightweight equipment);
• Clean method for shallow well installation;
CONS:
• 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.
Similar to the above method is "Direct Push Technology" or DPT. A common trade name is GeoProbe. DPT does not require heavy equipment, most units are pickup mounted or ATV mounted for easy accessibility.

Driven wells

Driven wells can be installed only in relatively soft formations. Well points can be driven either by hand or by a hammer to depths reaching 50 feet. Points can also be driven out the bottom of larger diameter casing when the aquifer has been reached. Points may be set to greater depths if the screen is protected by casing during driving; the casing is then pulled back to expose the screen.

PROS:
• Cost effective;
• Easy access in most conditions;
CONS:
• Limited to shallow depths (< 50 feet);
• Limited to unconsolidated, soft formations relatively free of cobbles or boulders;
• May require pre-drilling a hole of slightly greater diameter that the well point.

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.

PROS:
• 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;
CONS:
• Practical limitation to 24" diameter;
• Inefficient in loose, sandy material (depends on the depth);
• Inefficient below the water table (depends on the depth).

Hollow-stem auger (HSA)

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.

PROS:
• Allows collection of uncontaminated sample in unconsolidated formation;
• Can be used as temporary casing to prevent caving;
• Relatively rapid, especially in clayey formations;
CONS:
• 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

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 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.

PROS:
• 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;
• Good recovery of quasi-continuous 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.
CONS:
• 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.

Methods that use drilling fluids:

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.

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.

PROS:
• High penetration rate;
• Drilling operation requires a minimum amount of casing;
• Rapid mobilization and demobilization;
CONS:
• 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 the cuttings accumulate at surface around the rim of the borehole.

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.

PROS:
• 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, mud 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.
CONS:
• 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).


Dual-wall Reverse Circulation Drilling

Dual-wall reverse circulation is a sub-type of rotary drilling similar to RC, in which two concentric drill pipes are assembled as a unit to create a controlled annulus. This method is widely used in groundwater monitoring work. The drilling fluid (air- or water-based) is pumped through an outer swivel down through the annulus of the bit where it is deflected upward into the center pipe. The cuttings are carried upward through the inner pipe and surface swivel, and can be collected as a sample or pile up on the ground. The method allows collection of geologic samples of known depth within the formation, which is delivered through a cyclone at the surface.

The method is applicable to both rotary drilling and percussion (down-the-hole) drilling methods. Because the fluids at all time circulate within a controlled space, the problem of lost-circulation is minimized and cuttings can be recovered at all times. The method is applicable in virtually any type of geologic formation and does not require the use of a surface casing. In general, borehole diameters do not exceed 10 inches and depths can reach in excess of 1,400 feet.

PROS:
• Good sample recovery due to controlled up-hole fluid velocity;
• Fast penetration in coarse alluvial or broken, fissured rock;
• Possible to obtain continuous representative samples of the formation and groundwater;
• Easy estimate of aquifer yield at many depths in the formation;
• Reduction of lost-circulation problems;
CONS:
• Practical borehole diameter limited to 10 inches;
• Maximum depth of ~ 1,400 feet, although greater depths can be achieved in hard rock;
• Possible to dry out or to not detect a thin of low-yield aquifer;
• Possible sample contamination due to the oil used in the air-compressor unless quality air filters are used (this is true for all air methods, unless the contractor uses filters).


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.

PROS:
• 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.
CONS:
• Slow penetration rate;
• Due to the constant mixing of water, it is not possible to obtain groundwater samples during drilling;
• Expensive casing for larger diameters;
• Difficult to pull back casing in some geologic conditions.


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.

PROS:
• 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.
CONS:
• Restricted to semi-consolidated to consolidated formations.


Air percussion casing hammer

This method is also called drill through casing driver. It combines the borehole stability of the cable-tool rig and the speed of an air rotary rig. The use of a casing driver permits the casing to be advanced during drilling, but both drilling and driving can be adjusted independently depending on the nature of the formation. Drivers can be used to drive upward to remove the casing or expose the screen for well installation. The method is particularly suitable for drilling in stratified deposits that have large differences in particle sizes.

Cuttings are blown upward inside the casing and discharged through a pipe at the surface. When air-drilling techniques are used, it is easy to see how much water is being blown out with the cuttings at a given depth. From this observation, it is possible to assess when the borehole is deep enough to produce the desired yield.

The air-operated casing hammer requires lubrication (hydrocarbon) by means of in-line oilers. Unlike the down-the-hole hammer, the casing hammer does not input oil into the formation directly as oiling proceeds at surface. The method is thus applicable for groundwater monitoring work. It is possible, however, that oil can be added indirectly to the formation if proper cleaning of the casing is not performed.

PROS:
• Wells can be drilled in unconsolidated materials that could be difficult to drill with cable-tool or direct rotary method;
• No water-based fluid (drilling mud) is required in unconsolidated materials;
• Representative formation and groundwater samples can be collected;
• Borehole is fully stabilized during drilling operations through the use of casing;
• Rapid penetration rates even in difficult drilling conditions;
• Lost circulation problem is rarely a concern, except in very loose materials (e.g. mine waste rock);
• Operates well in cold weather;
CONS:
• Method does not permit yield measurements during drilling;
• When groundwater static levels are low, the high air pressure in the hole can prevent water from entering the borehole; a "rest" period is necessary to assess the true static level;
• Relatively expensive method (increased cost of driving casing in);
• Very noisy (driving of casing);
• Borehole diameter limited to 12 inches.


ODEX percussion down-the-hole hammer (Odex, Stratex, and Tubex are trade names)

ODEX is an adaptation of the air-operated down-the-hole hammer. It uses a swing-out eccentric bit to ream the bottom of the casing. The percussion bit is a two-piece bit consisting of a concentric pilot bit behind which is an eccentric second bit that swings out to enlarge the hole diameter. Immediately above the eccentric bit is a drive sub that engages a special internal shouldered drive shoe on the bottom of the ODEX casing. The ODEX is thus pulled down by the drill stem as the hole is advanced. Cuttings blow up through the drive sub and casing annulus to a swivel conducting them to a sample collector or onto the ground.

Drilling limitations are essentially the same as for the down-the-hole hammer method.

PROS:
• 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;
• Advantageous in unconsolidated formations with a high risk of caving (this is the probably the most important feature).
CONS:
• Practically restricted to unconsolidated formations.
• Relatively a more expensive method