Water Supply Learning
Objective: This document is intended to work as an organization of the technical knowledge that is
required to design a well but is not intended to be a design outline.
1. Ground Water and Geology
1.1. Aquifers
1.1.1.Confined
1.1.2.Unconfined
1.1.3.Leaky
1.2. Aquifer Properties
1.2.1.Porosity
1.2.1.1.
Specific yield
1.2.1.2.
Specific retention
1.2.2.Hydraulic Conductivity (permeability)
1.3. Exploration
1.3.1.Aerial geology
1.3.1.1.
Geological map: Distribution of various rock formations over an area permits
and understanding of the subsurface structure. Rocks of different permeability can
influence the surface drainage patterns in such a way that the patterns are easily
seen from the air. Seepages also show up along faults and other major fractures.
1.3.1.2.
Remote sensing: using electromagnetic radiation of the earth to obtain
information about the earth’s surface and near surface structure
1.3.1.3.
Seismic and electrical resistivity surveys
1.3.1.3.1.
Seismic: Gives a physical shock though the earth’s surface and the shock
waves are refracted by different geological layers. The time that it takes to
return to the surface is recorded by “geophones”. This helps especially to
determine the depth to the bedrock and the water table. The shock wave is
higher in saturated materials. This will find the highest permeability in an
aquifer.
1.3.1.3.2.
Resistivity Analysis: uses four electrodes which are placed in the ground
in a straight line. Using a battery an electrical current is injected into the
ground through two “current” electrodes and the travels through the earth.
The amount of resistance of the rock layers is measured by the drop in voltage
at the two “potential” electrodes. Resistivity readings are also affected by
quantity of ground water.
1.3.1.3.2.1. Helps to determine the extent and depth of the buried stream channel,
indicating the most promising sire for the well.
1.3.1.3.3.
Lastly at least one borehole is needed so that the physical
measurements can be related to the geology. Geophysical measurements can
be related to the geology. Well or water “logging” involves lowering or raising
an instrument probe into the well and making measurements of physical
properties surrounding rocks or of the borehole itself.
2. Ground Water Flow
2.1. Ground Water Flow Mechanisms
2.2. Transmissivity
2.3. Ground Water Flow Toward Wells
2.4. Recharge and Boundary Effects
3. Drilling Technology (Depends on drilling conditions, depth of borehole, access to drilling site,
availability of supplies)
3.1. Cable Tool (i.e. Standard or Percussion Method)
3.1.1.Simple lifting and dropping action of a string of specially design drilling tools in to the
borehole based on gravity, weight of drill bit, and rotation from the drill string steadily
break up and cut through the formulations below.
3.1.2.Not the quickest method but is is very economical and dependable and likely to remain a
viable method. Maintenance is minimal which males it also reliable
3.1.3.Used primarily for water wells, the cable tool drills are ideal for drilling though
unconsolidated materials such as sand and gravel, through most boulders, and through
many rock formations that are fissured or cavernous.
3.1.4.Cable Tool Rigs are manufactured in many different sizes and are rated according to tool
weight and depth of hole. The rig must have specifications that correspond with the type
of drilling to be encountered
3.1.5.Components of the cable tool rig: Each component is connected by means of tool joints
and the combined weight of the tool string. The connections are made by means of
tapered, threaded pin joints that fit into the conforming box joint.
3.1.5.1.
Tool string: made up of the rope socket, which attaches the tools to the cable,
and the length is adjusted through adjustments made to the stoke length. The
adjustments are made with wrist pins on the spudding beam to which the pitman
arm is attached. The pitman arm is attached to the spudding gear which in turn
imparts the reciprocal motion to the cable and the tool string. The cable is attached
to the spudding beam and the spudding beam is driven by a pitman arm.
3.1.5.2.
The drilling jars
3.1.5.3.
Drill stem (serves as a guide to the tool string)
3.1.5.4.
Drill bit (does the actual cutting or boring at the bottom of the hole): The cutting
and turning of the drill bit is a result from the elasticity of the lay of the stem cable.
Its elasticity under tension causes the cable to unravel partially or unravel clockwise
motion (if it is wound in a clockwise fashion). When the bit strikes the bottom of the
hole it has turned a few degrees and makes a new cut into the formation. For this
reason left lay cable is used because the natural turning action automatically tightens
the tool joints each time the bit strikes the formation.
3.1.6.The component selection is based on
3.1.6.1.
Well size
3.1.6.2.
Capacity of the drilling rig
3.1.6.3.
Height of the mast
3.1.6.4.
Type of formation
3.1.6.5.
Joint size
3.2. Rotary: The rotary drilling process involves boring a hole by using a rapidly rotating bit to which
a constant, downward force is applied.
3.2.1.Overview: The drill bit is supported and rotated by a hallow stem composed of a high
quality stem, through which a drilling fluid is circulated by means of a suitable pump, the
fluid is forced though the bit openings and in the process, cools and lubricated the cutting
assembly. Because the fluid is under pressure, it naturally travels the path of least
resistance up the hole-stem annulus to the surface. On its way the fluid carries the hole
cuttings in suspension up and out of the well. The cutting slurry is then diverted to a
nearby mud pit where cuttings drop from suspension. The flowing mud is rerouted, via a
flexible hose, down through the hollow drill stem, thus completing the cycle.
3.2.2.Components of Rotary Drill
3.2.2.1.
Drill Bit (the drill bit operates at the lower end of the drill stem, which usually
consists of 3 basic parts: one or more drill collars directly above the bit, one or more
lengths of drill pipe and the Kelly)
3.2.2.1.1.
Star or the fishtail drag type drill bits are used for soft, unconsolidated
formations such as sand and clay. (50 to 150 rpm with bit weights from 1,000
to 1,400 lbs. p/in of bit diameter)
3.2.2.1.2.
Roller or cone type bits are used for drilling through hard, consolidated
formations. (30 to 50 rpm with 2000lbs to 5000 lbs per inch bit diameter)
3.2.2.1.2.1. The drill collars are basically heavy-walled lengths of drill pipe that
concentrate the desired amount of weight at the lower end of the drill
stem. This weight contributes to the bit’s effectiveness and helps keep
the hole vertically straight.
3.2.2.1.2.2. Drill Pipe: usually in 20 foot sections and is high strength seamless
tubing preferably made of carbon manganese steel or a molybdenum
alloy steel. Sizes for water drilling range from 2 3/8 to 4 /1/2 inches and
represent the outside diameter of the tubing. A practical guideline is for
boreholes under 10 in diameter the tool-joint outside diameter should be
approx. 2/3 that of the borehole diameter. The drill pipes are usually the
main cause of failure due to the large amount of stress and threading that
is done.
3.2.2.1.2.3. Kelly (located directly above the drill pipe which passes through the rigmounted rotary table. The Kelly is engaged by the corresponding rotary
table orifice and receives its turning power. The end of the Kelly connects
to a water swivel from which eh entire drill stem hangs.
3.2.2.2.
Drilling fluid (the consistency of the fluid has effects on the drill time and the
actual quality of the well. Will also affect the maintenance of the components such
as the drill bit and mud pump but can also determine the success or failure of the
well system.)
3.2.2.2.1.
Gas based fluid: dry air, air as a mist containing droplets of water or
mud, foam which is bubbles f air surrounded by water and a emulsifying agent,
stiff foam containing film-strengthening material such as organic polymers and
bentonites
3.2.2.2.2.
Water based fluids: contain several dissolved substances, such as
alkalites, salts, and surfactants in addition to the droplets of emulsified oil and
various insoluble solids carried in the suspension.
3.2.2.2.3.
Oil Based fluids: contain oil soluble substances, emulsified water and oil
insoluble materials in suspension. Not used as much in water wells as oil wells.
3.2.2.2.4.
Functions of the drilling fluid
3.2.2.2.4.1. They remove the cuttings that accumulate below the rotating bit
3.2.2.2.4.2. They transport cuttings up the borehole to the surface where the
suspended cuttings can be separated from the fluid
3.2.2.2.4.3. They maintain hole stability
3.2.2.2.4.4. They cool the bit
3.2.2.2.4.5. They prevent fluid entry from the porous rocks penetrated
3.2.2.2.4.6. They reduce drilling fluid losses into permeable and loosely cemented
formations
3.2.2.2.4.7. They lubricate the mud pump, bit bearings, and drill string
3.2.2.2.4.8. They reduce wear and corrosion of the drilling equipment
3.2.2.2.4.9. They assist in collecting and interpreting information from the cuttings,
cores, and borehole geophysical surveys
3.2.2.2.5.
In the field: drilling mud can refer to any fluid ranging from muddy
water to a clay-water mixture t a specially prepared drilling fluid.
3.2.2.2.6.
The filter cake is what forms on the borehole wall. The making of the
cake is one of the main reasons to use a drilling fluid. The mud liner assists in
retaining unstable, soft formations and consequently helps to prevent borehole
sides from collapsing into the well. As the cake builds it protects from the
eroding effects of the drilling fluid. The outward hydrostatic pressure of the
drilling fluid column is what prevents cave-ins from occurring. The hydrostatic
force is the main reason for the cake formation. Properties of the cake are
determined by the drilling fluid pH, viscosity, density and gel strength.
3.2.2.2.7.
Control of Drilling Fluids: Maintain pH between 8.0 and 9.0. Measuring
the viscosity, mud density, gel strength, filtration characteristics, and sand
content allow the drilling fluid to be prepared properly too minimize drill time
and minimize drilling costs.
3.3. Air Rotary
3.3.1. Air rotary drilling works the same as the mud rotary drilling except that air is used instead
of drilling fluid. Compressed air is forced downward through the hollow drill stem and then
exits via the ports in the drill bit.
3.4. Drilling with Foam
3.4.1.Just as with dry air but with foam
3.4.2.Advantages of air drilling with the use of a foaming agent are:
3.4.2.1.
It reduces air volume and pressure requirements
3.4.2.2.
It increases well-cleansing capabilities and borehole stability
3.4.2.3.
It reduces hydrostatic head
3.4.2.4.
It provides hydrostatic head
3.4.2.5.
It provides a method for drilling in zones of extreme loss of circulation
3.4.2.6.
It suppress dust
3.5. Reverse Circulation Drilling
3.5.1.Employs the same principles as found in the conventional rotary methods but reverses the
flow circulation. This means that the fluid is sucked up through the parts in the drill instead
of the fluid being forced out.
3.5.2.This method has proven helpful in deep sand or gravel formation areas. Not used in hard
rock areas. Ideal in large diameter, high capacity wells for municipalities, industry,
irrigation etc. Hole diameter of at least 24 inches
3.6. Dual Tube Method (Reverse circulation rotary)
3.6.1. Based on the use of a double-walled drilling pipe that is capable of carrying air or drilling
foam down to the bottom of the borehole as well as carrying cuttings out of the borehole.
The double-walled drill pipe is constructed of concentric flush-jointed pipe. The drill bit
and drill stem are rotated by a top-head drive unit that rotates the drill pipe at the top of
the drill string.
3.7. Air-Percussion Rotary Drilling (Downhole-hammer drilling)
3.7.1.Method is identical to the conventional air rotary methods except that the main source of
energy for fracturing rock comes from a percussion machine connected directly to the bit.
This single cylinder reciprocating air engine is driven by the same circulating air that is used
to remove cuttings.
3.7.2.Selection if proper drill pipe is critical. Selecting a large size drill pipe can achieve equal
hole-cleaning efficiency while using less air for drilling. The annular space becomes smaller
and subsequently less air pressure is needed to maintain the same feet per minute
velocity.
3.7.3.Most efficient in consolidated rock formation that do not require casing.
3.7.4.Prolonged drilling in wet clay may plug the air holes in the bit and stop the hammer
operation.
3.7.5.Depth is limited by the diameter of the hole and volume of the compressor in use. The
depth at which cuttings can be removed from the hole is governed by the weight of the
rock and the volume of water in the hole.
3.8. Jet Drilling
3.9. Hollow Rod Drilling
4. Water-Well Design
4.1. Well Diameter
4.2. Selecting the Casing Type
4.2.1.Plastic Materials
4.2.2.Steel
4.2.3.Fiberglass
4.2.4.Physical Forces
4.2.4.1.
Tension (Hanging without bottom support)
4.2.4.2.
Column Loading : If the casing is lowered to set at the bottom of the borehole,
the force exerted by that action changes the force acting on the casing
4.2.4.3.
Collapse Pressures: Horizontal in nature and are that which will cause the casing
or screen to fold inward, when gravel is packed too quickly. Once everything is set
the collapse pressures balance each other out and eliminate the risk of collapse.
5. Intake Design
5.1. Overview: The intake portion of the well allows ground water to be pumped from it. As a water
well is pumped it first removes water that is stored in the cased portion of the well. Then
ground water must move from the aquifer into the well, passing through the openings of the
intake structure to replace the water that has been removed, Ground water is driven into the
well by the head difference between the water outside the well, in the aquifer, and inside the
well created by pumping the well. So, if the intake restricts the flow of water from the aquifer
into the well, the amount of water available is reduced proportionally. Higher head differences
are needed to force the same quantity of water through the intake. This is similar to water
through a pipe where the cross sectional area is reduced.
5.2. Intake Materials: Screens of bronze, steel, stainless steel, Monel or plastic. Being at the working
end of the well the intake is subjected to a more intense attack of corrosion from ground water.
5.3. Intake Types ( bridge-slot and louvered-slot design, the wire wrapped screen design and the
crude torch-cut slot method of well screening)
5.3.1.Homemade Openings (saw cut, machine cut or torch-cut slot)
5.3.1.1.
Size depends on
5.3.1.1.1.
Type of cutting tool
5.3.1.1.2.
Width of the cutting portion of the tool
5.3.1.1.3.
Conditions under which the pipe is slotted
5.3.1.2.
Usually irregular in shape and pattern
5.3.2.Punch Intake Openings
5.3.2.1.
Overview: A piece of pipe is used as the base of the intake. A small slit is cut into
the pipe. A form then pushes or punches the pipe outward, forming the opening.
Bridge slots and louvered slots are the most common punched slot intakes used in
the water-well industry.
5.3.2.2.
The bridge consists of a strip of metal that is pushed out from the intake body,
leaving two gaps on either side of the strip. For the best configuration the design of
the pipe is not done in the field but instead by a manufacturer.
5.3.3.Wire Wrapped Screen
5.3.3.1.
Overview: This yields the largest amount of open area and the least resistance
to flow when compared to the other types of screens.
5.3.3.2.
Manufactured by winding a shaped wire around a cylinder of evenly spaced
rods. It has the appearance of a cage with rods forming the vertical component and
the wire forming the horizontal component of the screen.
5.4. Basics of Filter Design
5.4.1.Gravel Packed (gravel or sand packed filter)
5.4.1.1.
An engineered, predesigned artificial filtering pack.
5.4.1.2.
The gravel is placed near the intake portion of the well during the well
construction phase and acts as a filtering media, holding back the aquifer material,
while the intake holds the gravel pack in place
5.4.1.3.
Best when the aquifer consists of a fine, uniform material or when the aquifer is
deep and unconfined
5.4.1.4.
Sieve size: an expression of the coarseness of the sample in relationship to the
other samples.
5.4.1.5.
Uniformity Coefficient (the ratio of the 90 percent retained and the 40 percent
retained of the sample)
5.4.1.6.
Designing Gravel-Packed Wells
5.4.1.6.1.
High-production wells should be gravel-packed when the following
conditions exist:
5.4.1.6.1.1. The aquifer is fine and uniform. The effective size is less than 0.010
inches and the uniformity coefficient is less than 3
5.4.1.6.1.2. The aquifer is comprised of loosely cemented sandstone. Grains of sand
will continually be taken off the rock face during pumping if the well is
not gravel-packed in an effort to stabilize the formation
5.4.1.6.1.3. The aquifer is very heterogeneous. That is, the aquifer contains
numerous thin layers of gravel and sand in a highly laminated fashion.
Gravel packing a well will reduce the concern over exact intake placement
in the well during construction.
5.4.1.6.1.4. The well is being completed using the reverse-circulation method of
drilling. Reverse circulation boreholes are very large, making naturally
developed wells impractical to complete.
5.4.1.6.1.5. The well is being designed for maximum production with little or no
maintenance; Gravel-packed wells will allow water to enter the well more
freely, thereby reducing the effects of corrosion and incrustation.
5.4.1.6.1.6. In a think artesian aquifer a smaller-diameter screen can be used if it is
gravel-packed.
5.4.1.6.1.7. If the aquifer is very dirty with finer sand or silt, gravel packing will help
improve the well’s yield by opening up the intake’s slot openings and
allowing for a better development of the aquifer.
5.4.1.7.
5.4.2.Naturally developed well (tube well)
5.4.2.1.
Uses the natural aquifer material to create a filtering zone around the intake
portion of the well, keeping back the finer portion of the aquifer material, while the
intake holds the filtering zone in place.
5.4.2.2.
Best for shallow, unconfined aquifers that contain a mixture of fine and coarse
material.
5.4.2.3.
Design of Naturally Developed Wells
5.4.2.3.1.
Thick, coarse aquifers having uniformity coefficient values greater than
3 and an effective size greater than 0.010 inches are prime candidates for being
completed using a natural development design.
5.4.2.3.2.
Once the coarse material begins to collect around the intake portion of
the well, the remainder of the material begins to collect around the intake
portion of the well, the remainder of the material will begin to impinge on the
coarser material until, and finally, the aquifer material is stabilized around the
intake.
5.4.3.Intake Length: The relationship of the overall well-design factors and the how the well fits
into the aquifer
5.4.3.1.
Must first understand Darcy’s Law of ground-water flow
5.4.3.1.1.
???????
5.4.3.2.
Unconfined Aquifers
5.4.3.2.1.
Flow into the well is caused by a lowering of a the water in the well,
causing water to flow into the well by the forces of gravity
5.4.3.2.2.
Typically using a screen length equal to one-third to one-half of the total
unconfined aquifer thickness will take advantage of the gravity flow into the
well, while exposing enough aquifer material so that acceptable water
production can occur. Typically, the well is designed with a length of one-third
of the aquifer exposed.
5.4.3.2.3.
In no case should the length excess 50 percent of the saturated aquifer
thickness
5.4.3.2.4.
Pumping equipment can be placed either above or below the intake,
never within the intake portion of the well because this could cause damage to
the intake and perhaps complete failure of the well. If the pumping water level
in the well is of concern, a sump or well extension can be designed into the well
structure that will house the pumping equipment below the intake. This may
cause need for the pump to be cooled.
5.4.3.3.
Confined Aquifers
5.4.3.3.1.
The well is penetrating an aquifer in which the ground water is under
pressure so the length of the intake can ideally be 100% of the confined aquifer
to take full advantage of the pressures that are built up in the aquifer.
5.4.3.3.2.
Placement of the intake should be in the most permeable zone of the
aquifer.
6. Constructing Water Wells
6.1. Installation of the permanent casing
6.1.1.Overview:
6.1.1.1.
The sequence of mixing and placement of cement slurry between the casing and
walls of bore-hole, either by gravity or by pump pressures. The grouting aims at
filling the openings to make it impervious to percolating water. In water well
construction where the sealing of a particular stratum is desired a cement slurry
mixture is pumped into the casing string. The slurry forces out the drilling fluid (mud)
contained in the string and squeezed into the annular space between the well and
the casing (casing clearance) until it reaches the design and desired level mark.
6.1.2.Benefits are
6.1.2.1.
It anchors the casing inside the bore-hole
6.1.2.2.
It protects the outside of the casing from corrosion
6.1.2.3.
It supports the insecure rocks, liable to caving and sloughing
6.1.2.4.
It ensures permanent isolation of zones, to be trapped from the unwanted ones.
6.1.2.5.
It provides better development of the productive aquifers zones, because the
rest of the aquifers are sealed off by filling the annulus between the two casings
6.1.2.6.
It prevents polluted surface water or low quality waters from other aquifers,
through seepage along the outside of the casing contaminating the well
6.1.2.7.
It augments the supporting power of the string
6.1.2.8.
It provides impervious seal along the well casing
6.1.2.9.
By squeezing drilling fluids out of the annular space, through the use of plugging
mixture, it prevents the migration of gas or liquid for one zone to the other
6.1.2.10. It helps to prevent blow-out from high pressure gas zones behind the casings
6.1.2.11. It seals off “lost circulation zones” or other difficult formations to continue
further drilling
6.1.2.12. It aids in providing a base for fracturing or any other future work for the life of
the bore-well
6.1.3.Planning and Implementing a cementing job
6.1.3.1.
First know conditions of well
6.1.3.2.
Arrange for adequate supply of equipment and materials before drilling
6.1.3.3.
Experience of the driller in charge of the machine to modify the modalities and
approach as per requirements at site
6.1.3.4.
Cementing program is to be planned before the start of drilling and then
necessary steps taken for its execution during drilling
6.1.3.5.
Conditioning of mud is to be done to achieve the required characteristics. This is
most important while running the casing and doing the cementing.
6.1.3.6.
The method by which the casing is to be run, its position, the selection and
location of surface connections needs to be planned and looked into for actual
cementing job
6.1.3.7.
Selection of amount and type of cementing material to be in conformity with
well conditions
6.1.3.8.
Adequate number of grout pumps, mixing units, accessories, plugs and proper
techniques and procedures would ensure the satisfactory completion of the
cementing job
6.1.4.Types of cement slurry mixtures
6.1.4.1.
Native clay
6.1.4.2.
Cement grout slurry
6.1.4.3.
Bentonite mud
6.1.4.4.
Hydrated lime
6.1.5.Cement grouting
6.1.5.1.
Methods Gravity grouting method
6.1.5.2.
Pressure grouting
6.1.5.3.
Fluid displacement method
6.1.5.4.
Casing method of grouting
6.1.5.5.
Outside tubing method
6.1.5.6.
Materials
6.1.6.Installation of the well screen
6.1.7.Installation of the gravel pack
6.1.8.Installation of the grouting (seals the well from contamination and helps to stabilize t he
casing and the surrounding formation.)
6.2. Testing for Yield and Quantity
6.3. Water-Well Development
6.4. Water-Well Maintenance
6.5. Consolidated Rock-Well Design
Bibliography