Content      ..      1       2         ..

NATIONAL DRILLING ASSOCIATION DRILLER’S MANUAL (2005) - page 1

Page 14 of 360

bedrock does not naturally buffer (neutralize) this rainfall; for example, in Canada, New England,

northern Minnesota. Florida, southern Norway, and Germany. In time, this phenomenon, called acid rain,

may increase significantly the acidic of near-surface groundwater.

All igneous and metamorphic rocks exposed at or near the Earth's surface are in an unstable chemical

and physical condition, and over geologic time these rocks breakdown into finer and finer components.

Destruction of rocks and the redistribution and deposition of the rock particles play a significant role in

producing three of the four major types of aquifer systems - alluvial, sedimentary, glacial, and

igneous/metamorphic. These particles are entrained and redistributed by the three agents of erosion -

wind, running water, and glacier ice. Running water is the most effective of these agents because it

operates continuously over most land areas of the Earth. Furthermore, it acts both as the primly agent in

the creation of alluvial aquifers and is a major force in building or altering other types of aquifers. Before

any major removal of a rock mass can take place, however, it must be broken down into particles that

can be carried by these agents of erosion. These processes are called rockweathering.

1.3. Weathering

Weathering is the in-situ physical disintegration and chemical decomposition of rocks in response to the

environmental conditions found at or near the Earth's surface. In general, the higher the temperature at

which a rock formed, the more unstable it will be at the Earth's surface. Granite, which crystallizes from

magma at approximately 1,110°F, is much more resistant to weathering than basalt, which forms at

2,190°F. Thus, the weathering rate depends on the temperature at which the rock formed, as well as

climatic conditions, the availability of water, its chemistry, and rate of movement, and the chemistry of

minerals making up the rocks. Because water has such a chemical affinity for almost all substances, and

because of its unusual physical properties, rock weathering proceeds rapidly in the presence of water.

Weathering is not only the first step in the production of alluvial and sedimentary aquifer systems, it is

also the process by which soils are produced.

Most rocks decompose or disintegrate by a combination of both physical and chemical processes.

Physical weathering is disintegration of rocks in place without associated major chemical change. During

physical weathering, Tacks are broken down into numerous small particles having a much larger

combined surface area than the original rock. In cold regions, physical weathering is the dominant

process; in the rest of the world, chemical weathering is far more important. Once the physical size of the

particles is reduced, decomposition is then accelerated by chemical attack on the larger surface area.

Chemical weathering of rocks is most effective in warm, humid regions where it is assisted by

organisms. Few common rock-forming minerals can resist chemical decomposition; quartz and

Page 21 of 360

sandstone, and marble with numerous small-to-large cavities that may be arranged along old depositional

surfaces in the original rock. In almost every case, a mushy clay zone of varying thickness exists

between the glacial deposits and the more competent underlying weathered bedrock.

Massive basalt flows (extrusive rocks) originate in the interior of some continents as a result of rifting

events or so-called hot spots in the upper mantle. Basalt may have high porosity and hydraulic

conductivity, depending on the way individual flows cooled or the length of time between flows. The

openings in a sequence of basalt flows occur in several ways: as cracks formed during cooling where the

hardened crust of a basalt flow has collapsed, as vesicles (gas holes) near the top of each flow, in empty

lava tubes, and in alluvial sediments laid down between flows. Vesicles near the top of individual lava

flows are a mayor cause of high porosity and hydraulic conductivity. Wells in basalt have a much greater

yield potential than do those in intrusive rocks.

In summary, the success rate for wells in igneous and metamorphic terrains is low. Even when elaborate

exploratory methods are employed, the yields from wells may be disappointingly small, and dry holes

are common.

CHAPTER 2.

OCCURRENCE AND MOVEMENT OF GROUNDWATER

The majority of the fresh water in the conterminous United States is found in the groundwater. Estimates

indicate that groundwater represents approximately 85% of our total fresh water. Approximately half of

this groundwater is near surface with an average detention time of 200 years. The other half is much

deeper with a detention time of 10,000 years. The second largest source of fresh water is the water found

in the Great Lakes.

When precipitation falls to the earth in the form of snow, rain, or hail, some portion of it is intercepted

by trees, plants, and buildings. If the storm is brief or of a low-intensity, this water will be evaporated

back into the atmosphere.

Page 22 of 360

During heavier precipitation, water (liquid or frozen) does reach the ground surface and can follow

several different paths as shown in Figure 2.1. The water that flows overland is called surface runoff. It

usually combines with other surface water to form a steam or river. Some of the water may infiltrate the

soil and seep downward until it reaches the groundwater table. Some of this water may eventually seep

into a stream or river. The relative amounts of surface runoff to infiltration depends on the amount of

water already present in the soil. During severe storms (or in the north during the early spring when the

ground is frozen) little or no water may infiltrate the ground. These are the times when flooding is likely.

Typically water is introduced to the soil by infiltration and stream flow, however once the water enters

the soil, it exists in several different modes as shown in Figure 2.2. This commonly used classification

indicates that water is found in two major zones in the soil--a zone of vadose water and a zone of

phreatic water.

Figure 2.2. Classification of subsurface water. (After Davis and DeWiest, 1966)

In the vadose zone, three separate types of water exist: soil water, intermediate vadose water, and

capillary water. The soil water is the primary water used by plants. The depth of the soil water zone

varies from 3 to 30 feet. The intermediate vadose water zone merely represents the region between the

soil water zone and the capillary water zone. The capillary water zone (sometimes called the capillary

fringe) is the zone where water is literally pulled upward by capillary action (which depends on the

surface tension between water, soil particles, and air). The height of capillary rise depends on the size of

the smaller grains of the soil. Capillarity is not effective in coarse grained soils whereas water can be

pulled up 30 feet or more in some clays.

The groundwater table lies at the bottom of the capillary zone and separates the vadose water zone from

the phreatic water zone.

Page 24 of 360

Groundwater may also occur under confined conditions. Groundwater that is confined is isolated from

the atmosphere at the point of discharge by aquicludes. The confined aquifer generally has higher

pressures than atmospheric acting on it causing the water in a well drilled into the formation to rise

above the levy of the formation. Artesian is equivalent to confined. It can refer to either the

water-bearing material, as in confined aquifer, or to the water confined in the material, as in artesian

ground water. The water in a confined material also may be referred to as occurring under confined

conditions or artesian conditions. Confined water is held in the water-bearing material by an overlying

material of low permeability called the confining layer. Confined water will rise in a well to a level

above the top of the water-being material, defining the potentiometric surface at that point. If the

potentiometric surface is above the land surface, the well will be a flowing well.

An aquifer performs two important functions—storage and transmission of water. The water in the pore

spaces is constantly moving at rates ranging from feet per day to feet per year. The shape, size, volume,

and connectivity of the pores and openings affect the ability of the formation to store and transmit water.

Two properties of an aquifer that affect its ability to store water are porosity and specific yield. The

porosity is defined as the ratio of the volume of the pore space to the volume of the geologic material. It

is expressed quantitatively as the ratio of void space to the total volume of porous material. It is stated as

either a decimal fraction or as a percentage. and is dimensionless. For example, if 1 ft³ of sand contains

0.25 ft³ of open space or pores, its porosity would be 25%. Primary porosity refers to the original

interstices created when a material, such as rock or soil, was formed. Typically, primary porosity is the

pore space between grains, pebbles or crystals. Secondary porosity refers to interstices created after a

material was formed. Examples are fractures (joints and faults) openings along bedding planes, solution

cavities, cleavage, and schistosity. Secondary porosity is the dominant form in consolidated materials

such as well cemented and strongly indurated sedimentary rocks and in most and metamorphic or

crystalline rocks.

Table 2.1 Typical Porosities of Geologic Materials

Table 2.1 provides typical values for common geologic materials. Although the porosity represents the

volume of water an aquifer can hold, it does not represent how much water the aquifer will yield.

An example is needed to explain this concept. When water is drained from a saturated sponge by gravity,

the sponge releases only a smell portion of the total volume of water stored in its pores or openings. This

is true of an aquifer as well. The quantity of water that a unit volume of unconfined aquifer yields up to

gravity is called its specific yield.

Page 25 of 360

The amount of water that a unit volume of aquifer retains after gravity drainage is called its specific

retention. The porosity of an aquifer is the sum of the specific yield and the specific retention. Specific

yields are not determined for confined aquifers because the water is not completely removed from the

pores during pumping.

2.1. Permeability

The transmission function of an aquifer is related to its ability to conduct water. The property of an

aquifer directly related to this ability is called its coefficient of permeability or hydraulic conductivity.

The permeability of an aquifer is governed by the size, shape, and connectivity of the pores, as well as

the properties of the fluid moving through the pores. If the pores are small and not well connected, the

ability of a fluid to flow through the aquifer will be low and the aquifer will have a low permeability.

Typical values of the coefficient of permeability are shown in Figure 2.3.

Page 26 of 360

These values are used to predict how much fluid will flow in a given time through a cross section of

aquifer under a specified hydraulic gradient. The gradient is related to how much pressure is available to

cause the water to flow in the aquifer. The higher the gradient, the more water that will flow through the

aquifer. According to D'Arcy (1856), if the gradient is doubled, then the amount of water that will flow

will be doubled The D'Arcy equation used to mathematically describe D’Arcy’s law is shown in

equation 2.1.

q = kiA

(2.1)

where q = discharge (volume/time), k = coefficient of permeability (length/time), i = gradient

(length/length), and A = area through which fluid is flowing (length²).

2.2. Transmissivity

The coefficient of permeability times the aquifer thickness equals the transmissivity. This value describes

the rate of flow through a vertical section of a aquifer under a unit gradient. The transmissivity is

obtained by pumping tests, by laboratory tests, or by estimating the permeability from grain size

measurements. Of these methods, the pumping test proves to be the most accurate.

2.3. Groundwater Flow Velocities

The magnitude of groundwater flow velocities is of little interest to persons primarily concerned with

water yields from a well. However, with the advent of groundwater contamination, the ability to assess

Page 28 of 360

Some judgment is involved in how long to pump a welt or otherwise remove water, before sampling, but

a widely used standard is to remove an amount of water equal to at least degree times the volume of

water standing in the well. Various conditions frequency make it impossible or impractical to purge a

well to the desired extent. The extreme case is when a very low-yielding material is being investigated.

Then it may be difficult to obtain even enough water at a single sampling to perform the desired

analyses. ID any case when a sample is obtained, the duration and rate of water removal prior to

sampling should be recorded along with all other conditions of the well and the water, such as whether or

not the well is actively used and last usage, depth to water before and after purging and sampling, depth

of pump intake or the depth to which other sampling device was lowered, clarity of the water before and

after purging and sampling, water temperature, and so on. Water samples should be analyzed as soon

after sampling as possible.

2.4.2. Piezometers

A piezometer is a specialized type of observation well designed to determine pore pressure in soil, rock,

or other porous material. The piezometer differs from the general observation well in that it is open only

to a particular point in the material so that the water level in the piezometer indicates the hydraulic

pressure at that point. A general observation well on the other hand is usually open to some thickness of

the porous material and is indicative of the average potential in the material over that interval. Pore

pressure is determined by subtracting the elevation head (the distance of the point of measurement above

some arbitrary datum) from the hydraulic pressure (the height of the potentiometric surface above the

same arbitrary datum). The potentiometric surface can be measured in an adjacent observation well or in

a second peizometer at the water surface.

A type of piezometer installation is shown in Figure 2.4. A piezometer may consist of a pipe or casing

that is drilled or driven to the desired depth of measurement .With the screened section only at the

bottom, water can enter only at the depth of interest and will rise in the pipe in accordance with the

hydraulic pressure at that depth. More sophisticated types of piezometers consist of a porous tip sealed

into a soil layer and connected to the surface by fluid-filled tubes. Several of these devices may be placed

at various depths in one boring. For more accurate leadings, a mechanical or electrical pressure

Page 30 of 360

3.1.1. Static Water Level

This is the level at which water stands in a well or unconfined aquifer when no water is being removed

from the aquifer either by pumping or free flow. It is generally expressed as the distance from the ground

surface (or from a measuring point near the ground surface) to the water level in the well. For example,

when the static water level in a well is 15 ft. it means that water stands 15 ft below the measuring point

when there is no pumping. For an artesian well which flows at the ground surface, the static water level

is expressed as a height above the ground surface. When artesian flow is stopped or contained at the

ground surface, the pressure developed is referred to as the shut-in head. If the well has a shut-in head of

3 psi at the surface, it means that the confining pressure will cause the water to rise 7 ft in a pipe

extending above the ground surface.

3.1.2. Pumping Water Level

This is the level at which water stands in a well when pumping is in progress. In the case of an artesian

well, it is the above ground level at which water is flowing from the well. The pumping water level is

also called the dynamic water level as measured in the well.

3.1.3. Drawdown

Page 32 of 360

In confined formations, the saturated thickness of the aquifer is generally not reduced during pumping.

Hydrostatic pressure, however, is reduced in the aquifer, and the pressure drop is greatest at the well

bore. The pressure drop is directly analogous to the dewatering effect in unconfined aquifers.

During pumping, water flows toward the well from every direction. As the water moves closer to the

well, it moves through imaginary cylindrical sections that are successively smaller in area. Thus, as the

water approaches the well, its velocity increases. In Figure 3.2, A1 represents the area of a cylindrical

surface 100 ft from the center of the well and A2 represents the area of a similar surface 50 ft from the

well. Because A₁ is twice A₂ and

the same quantity of water flows toward the pumped well through both cylinders, the velocity V₂ must be

twice V₁.

Darcy's law indicates that the velocity of flow through porous media varies directly with the hydraulic

gradient. As the hydraulic gradient increases, velocity increases as flow converges toward a well. As a

result, the lowered water surface develops a continually steeper slope toward the well. The form of this

surface resembles a cone and is caned the cone of depression. When pumped, all wells are surrounded by

a cone of depression. Each cone differs in size and shape depending upon the pumping rate, pumping

duration, aquifer characteristics, slope of the water table, and recharge within the cone of depression of

the well.

Figure 3.3 shows two cones of depression around pumped wells that illustrate how transmissivity of an

aquifer affects the shape of the cone. In a formation with low transmissivity, the cone is deep with steep

sides and has a smog radius. In a formation with high transmissivity, the cone is shallow with flat sides

and has a large radius. The explanation for these different cone shapes is clear, for greater hydraulic head

Page 33 of 360

(feet of head) is required to move water through a less permeable formation than through a more

permeable formation.

Figure 3.4 shows the levels at which water would be found in observation wells drilled at various

distances from a pumped well. Only one side is shown; the other side is similar. This curve is called the

drawdown curve and represents the lower limits of the cone of depression. In an unconfined aquifer, it

represents the level to which the formation remains saturate In a confined aquifer, it represents the

hydrostatic pressure in the aquifer. Drawdown at any given point is the difference between the water

level indicated by the curve and the static water level.

Head loss is a term used to describe the difference in head (pressure) that is required to cause flow from

one point to another in an aquifer; it is a measure of the force required to overcome resistance to flow.

Page 35 of 360

Page 36 of 360

The transmissivity and storage coefficients are especially important because they define the hydraulic

characteristics of a water-bearing formation. The coefficient of transmissivity indicates how much water

will move though the formation, and the coefficient of storage indicates how much can be removed by

pumping or draining. If these two coefficients can be determined for a particular aquifer, predictions of

great significance can usually be made. Some of these are:

1. Drawdown in the aquifer at various distances from a pumped weld.

2. Drawdown in a well at any time after pumping starts.

3. How multiple wells in a smell area will affect one another.

4. Efficiency of the intake portion of the well.

5. Drawdown in the aquifer at venous pumping rates.

3.3. Cone of Depression

Page 37 of 360

When water is pumped from a well, the initial discharge is derived from casing storage and aquifer

storage immediately surrounding the well (Figure 3.6). As pumping continues, more water must be

derived from aquifer storage at greater distances from the well bore. This means that the cone of

depression must expand. The radius of influence of the well increases as the cone expands. Drawdown at

any point also increases as the cone deepens to provide the additional head required to move the water

from greater distances. The cone expands and deepens more slowly with time, however, because an

increasing volume of stored water is available with each additional foot of horizontal expansion.

Figure 3.7 illustrates how the cone of depression expands during equal intervals of time. Assume that

after 10 hours of pumping the radius of the cone is 400 ft and its depth is 6 ft at the well bore. At the end

of 20 hours, the cone's radius has expanded to 570 ft and its depth has increased to 6.3 ft. In the second

10 hours, the cone has only extended outward an additional 170 ft and deepened by an additional 0.3 ft.

An additional radial expansion of only 130 ft and an increase in depth of only 0.2 ft occurs in the next 10

hours. Calculations of the volume of each of the cones would show that cone 2 has twice the volume of

cone 1, and cone 3 has three times the volume of cone 1. This occurs because, at a constant pumping

rate, the same volume of water is discharged from the well during each 10-hour interval Thus, the

increase in volume of the cone of depression is constant over time if the well is being pumped at a

constant rate and the aquifer is homogeneous.

It is evident from this example that after some hours deepening or expansion of the cone during short

intervals of pumping is barely discernible. This often leads observers to conclude that the cone has

stabilized and will not expand or deepen as pumping continues. The cone of depression will continue to

enlarge, however, until one or more of the following conditions is met:

1. It intercepts enough of the flow in the aquifer to equal the pumping rate.

2. It intercepts a body of surface water from which enough additional water will enter the aquifer to

equal the pumping rate when combined with all the flow toward the well.

3. Enough vertical recharge from precipitation occurs within the radius of influence to equal the pumping

rate.

Page 40 of 360

Page 43 of 360

11. Wells can be drilled in formations where lost circulation is a problem.

12. Wells can be bailed at any time to determine the approximate yield at that depth.

Some disadvantages of the cable tool method include the following:

1. Penetration rates are relatively slow.

2. Casing costs are usually higher because heavier wall or larger diameter casing may be required.

3. It may be difficult to pull back long strings of casing in some geologic conditions, unless special

equipment is available.

4.2 DIRECT ROTARY DRILLING

Page 45 of 360

The tricone bit, used as an all-purpose bit in every type of formation, has conically shaped rollers on

spindles and bearings set at an angle to the axis of the bit. Another design has four rollers; two are set at

an angle and two are nonnal to the vertical axis of the bit. The cutting surfaces of all roller bits are

flushed by jets of drilling fluid directed from the inside (center) of the bit. The jets can be sized so as to

maximize the cutting action of the bit. The jets are also effective in breaking up or washing away soft

formation materials.

The bit is attached to the lower end of the drill pipe, which resembles a long tubular shaft. The drill

string usually consists of four parts: the bit, one or more drill collars or stabilizers, one or more lengths

of drill pipe, and, in table-drive machines, the kelly. Selection of the bottornhole assembly will depend

on the physical conditions of the geologic materials. These include dip of the formation, presence of

faults or fractures, and drillability of the formation.

Each drill collar is a heavy-walled length of drill pipe; one or more drill collars are used to add weight to

the lower put of the drill-stem assembly. The concentration of weight just above the bit helps to keep the

hole straight, and provides sufficient weight for the bit to maintain the proper penetration rate. Drill

collars fitted with stabilizer bars or rollers are even more effective in drilling straight boreholes.

Stabilizers are an important component of the bottom-hole tools. To be effective in maintaining straight

holes in soft formations, the stabilizer must have large wall contact. Increased contact can be achieved by

using stabilizers with longer and wider blades, or by using longer stabilizers. The flow of drilling fluid

upward around the stabilizer must not be restricted too much, however, because cuttings may pack

around die stabilizer. This leads to sticking and a possible loss of circulation if back pressure builds up.

Weakening of the formation structure can also result from the pressme increase. Accumulation of

Page 49 of 360

with large passageways are often used to pump the drilling fluid because they can handle cuttings

without excessive wear on the pump. In operation, however, most of the cuttings do not actually enter

the pump but bypass it by means of an eductor system. An uphole velocity of at least 150 ft/min is

recommended. The fluid returns to the borehole by gravity flow. It moves down the annular space

between the drill pipe and borehole wall to the bottom of the hole, picks up the cuttings, and reenters the

drill pipe through ports in the drill bit.

In the reverse circuladon romy method, die drilling fluid can best be described as muddy water rather

than drilling fluid; drilling fluid additives are seldom mixed with the water to make a viscous fluid

Suspended clay and silt that recirculate with the fluid are mostly fine materials picked up from the

formations as drilling proceeds. Occasionally, low concentrations of a polymeric drilling fluid additive

are used to reduce friction, swelling of water-sensitive clays, and water loss.

To prevent caving of the hole, the fluid level must be kept at ground level at all times, even when drilling

is suspended temporarily, to prevent a loss of hydrostatic pressure in the borehole. The hydrostatic

pressure of the water column plus the velocity head (inertia of the water moving downward) outside the

drill pipe support the borehole wall. Erosion of the wall is usually not a problem because velocity in the

annular space is low.

4.3.1. During Drilling

Water infiltrates the permeable formations surrounding the borehole. Some of the fine particles

Page 51 of 360

2. Reverse rotary rigs and components are usually larger and thus more expensive.

3. Large mud pits are required.

4. Some drill sites are inaccessible because of the rig size.

5. For efficient operation more personnel are generally required than for other drilling methods.

4.4. AIR DRILLING SYSTEMS

Two different drilling methods use air as the primary drilling fluid—direct rotary air and down hole air

hammer. In conventional reverse circulation methods, air is used as an assist but not as e primary drilling

fluid. In the air rotary method, air alone lifts the cuttings from the borehole. A large compressor provides

air that is piped to the swivel hose connected to the top of the kelly or drill pipe. The air, forced down the

drill pipe, escapes through smut pons at the bottom of the drill bit, thereby lifting the cumags and cooling

the bit. The cuttings are blown out the top of the hole add collect at the surface around the borehole.

Injecting a small volume of water or surfactant and water (foam) into the air system controls dust and

lowers the temperature of the air so that the swivel is cooled. Air drilling can be done only in

semiconsolidated or consolidated materials. Therefore, to achieve the capability to operate in completely

unconsolidated as well as consolidated formations, air rotary drilling machines are often equipped with a

mud pump in addition to a high capacity air compressor. Conventional water based drilling fluids are

then used when drilling through the overlying, caving formations above the bedrock (or more

consolidated formations), whereas air is used once bedeck has been reached. Thus drillers are utilizing

various options of drilling technology to adjust to the different physical characteristics of the formation.

In many instances, cawing may have to be installed through the overburden to avoid caving or excessive

erosion of the borehole wall after changing to air circulation.

Cuttings are removed by grinding the material finely enough so that the uphole velocity of the air is

sufficient to lift them to the surface. The lifting capacity of the air can be enhanced by adding a small

amount of surfactant and water solution to the air. Larger cuttings can then be removed, thereby

increasing the drilling rate. Foam also reduces loss of air to the formation. Suggestions for use of various

drilling fluid additives are presented in Chapter 5.

Roller-type rock bits, similar to those designed for drilling with water-based fluids, can be used when

Content      ..      1       2         ..

//////////////////////////////////////////