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The following article will guide you about how to construct and design water wells.
Selection of Site for a Well:
The factors to be carefully studied before selecting a site for sinking a well are:
(i) Topography.
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(ii) Climate.
(iii) Vegetation.
(iv) Geology of the area.
(v) Porosity, permeability and alteration of rocks.
(vi) Joints and faults in rocks.
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(vii) Folded strata.
(viii) Proximity of any tank, river, spring, lake, unlined channels, reservoirs, etc.
(ix) Existing wells in the vicinity.
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(i) Topography:
The valley regions are more favourable than the slopes and the top of the hillock.
(ii) Climate:
The annual rainfall of region, intensity of sunlight, maximum temperature and humidity are of considerable value. Areas having heavy or moderate rainfall favour more water to percolate in the soil and pervious rocks layers, and get stored on impermeable layers to form aquifers.
Intensity of summer days evaporates and depletes ground water through direct evaporation from shallow depths and evapotranspiration through plants. Areas of scanty rainfall and severe summer do not favour storage of ground water at shallow depth and the wells do not yield much water. Semi-arid zones are more favourable than the arid zones.
(iii) Vegetation:
Vegetation can flourish well where the ground water is available at shallow depths. The trees of forests draw their requirements directly from the ‘zone of saturation’. Such plants are known as ‘phreatophytes’. Some plants can exist under arid conditions by absorbing the soil moisture (intermediate or vadose water) in the zone of aeration and store water in their thick fleshy leaves and stems.
Such plants are known as ‘xerophytes’. By studying the vegetation of the area, the condition of ground water can be assessed. This vegetation indicates large storage of ground water at shallow depths whereas bald hillocks with large number of xerophytes indicate the scarcity of ground water at shallow depths.
(iv) Geology of the Area:
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Areas comprising thick soil or alluvium cover, highly weathered, fractured, jointed or sheared and porous rocks indicate good storage of ground water, whereas bald hillocks of massive igneous and metamorphic rocks or impermeable shales indicate paucity of ground water.
(v) Porosity, Permeability and Alteration of Rocks:
Highly porous, permeable and altered zones of dense rocks encourage storage of ground water. Massive rocks do not permit the water to sink.
(vi) Joints and Faults in Rocks:
The movement of water is through joints, fractures, fissures and cracks which are interconnected. Wells sunk into the highly jointed rocks or along the fault plane yield copious supply of water. Faults in certain areas behave as barriers for the movement of ground water and create artesian conditions where flowing wells and springs are commonly noticed.
(vii) Folded Strata:
When the rocks are folded into anticlines and synclines, the synclines are favourable for storage of ground water in the previous layers and water is stored under pressure under artesian conditions. Wells sunk from the top of synclined hills so as to reach the pervious layers will be successful. On the contrary, a well sunk in the anticlinal valley will be a failure since it will be at a point of ‘ground water divide’ and the water flow will be away from the crest of the anticline towards the synclinal basin.
(viii) Proximity of Tank, River, etc.:
These water bodies serve as sources of recharge and the water is stored in the previous layers. The wells sunk in these areas yield water throughout the area. For example, ‘percolation dams’ are built in Maharashtra and the wells-are sunk in the zones of seepage.
(ix) Study of Existing Wells in the Vicinity:
The subsurface geology, rock formations, depths, fractures, etc., can be observed in the existing wells in the neighbourhood. The depth of water table and the yield can be assessed by observing the water levels. Care should be taken to see that no dykes, veins or faults are situated in between the existing wells and the proposed well.
In addition to the above factors, aerial photographs and hydrogeological maps are helpful in making rapid reconnaissance of the area, where a large-scale well sinking programme is on hand.
Besides, geophysical methods of exploration of ground water, namely, magnetic, seismic, gravity and electrical methods are also employed, of which electrical resistivity method is found to be more helpful in the selection of well sites. Subsurface exploration by test drilling and studying the various rock formations at different depths and their water bearing properties can be done by more sophisticated methods.
Spacing of Wells:
If the wells are situated very closely, the supply of water will be greatly affected, due to interference, when both the wells are pumped simultaneously. As shown in Fig. 3.4 an open well is situated in the land of ‘A’. A borewell is sunk very close to the open well in the land of ‘B’, and water is pumped from the borewell. This creates a large ‘cone of depression’ around the bore well. The water in the open well is depleted rapidly and it can get water only when the pumping is stopped and fast recuperation takes place (unless the open well is further deepened).
Otherwise the open well remains a dry well though the water table is at shallow depth. So it is always advisable to space the wells beyond the ‘radii of cones of depression’ of the adjacent wells. This is roughly estimated to be around 200-300 m in alluvial areas and around 75-150 m in hard rock areas.
Design of Water Well:
A water well has to be designed to get the optimum quantity of water economically from a given geological formation. The water requirements for the particular scheme—rural water supply, agricultural or industrial needs to be carefully determined. The choice of open wells or bore-wells (tube-wells) and the method of well design depends upon topography, geological conditions of the underlying strata, depth of ground water table, rainfall, climate and the quantity of water required.
A water well design involves selection of proper dimensions like the diameter of the well and that of the casing, length and location of the screen including slot size, shape and percentage open area, whether the well has to be naturally developed or a gravel pack is necessary, design of the gravel pack, selection of screen material etc. Screened wells in unconsolidated formations involve consideration of more design details when compared to wells in consolidated rock formations.
Good water well design aims to ensure an optimum combination of performance and long service life at reasonable cost. For instance it is not economical to design a well to yield 2,000 lpm to serve a suburban home requiring 70 lpm. On the other hand, the use of correct sizes of well casing and well screen, choice of materials of good quality, and strength and proper development of the well, will reduce long term power costs due to higher rates of pumping and maintenance costs and increase the useful life of the well.
The size of the well should be properly chosen since it significantly affects the cost of well construction. It must be large enough to accommodate the pump that is expected to be required for the head and discharge (yield) with proper clearance (of at least 5 cm around the maximum diameter of the bowl assembly) for installation and efficient operation.
Also the diameter must be chosen to give the desired percentage of open area in the screen (15 to 18%) so that the entrance velocities near the screen do not exceed 3 to 6 cm/sec so as to reduce the well losses and hence the drawdown, to exclude the finest particles of sand from migrating near the slots and prevent incrustation and corrosion at the strainer slots. In deep wells which have both high static and pumping water levels, the well diameter can be reduced below the level of the lowest anticipated pump setting during dry weather, particularly in artesian aquifers where the artesian head is relatively high.
It can be seen from the Dupuit’s equation [Eq. (5.7)] for steady state flow conditions (constant drawdown), that the yield of the well is-
Where R is the radius of influence and rw is the radius of the well. For R = 300 m, a 60 cm well will yield only 25% more than a 15 cm well, and 12% more than 30 cm well, which shows that drilling a large diameter well will not necessarily mean proportionately large yields. Recommended well diameters for various yields are given in Table 10.1.
The depth of a well and the number of aquifers it has to penetrate is usually determined from the lithological log of the area and confirmed from electrical resistivity and drilling time logs. An experienced driller can decide the depth at which drilling can be stopped after being advised by the hydro-geologist who analyses the samples collected during the drilling. The well is usually drilled up to the bottom of the aquifer so that the full aquifer thickness is available, permitting greater well yield. The poor quality aquifer is backfilled or sealed so that this water will not migrate upward when the well is pumped.
Design of Well Screen:
The design of the well screen consists of the length of the screen, its location, percentage open area, size and shape of this slots and selections of the screen material.
Screen Length:
The optimum length of the well screen is chosen in relation to the aquifer thickness, available drawdown and stratification of the aquifer. In homogeneous artesian aquifer about 70 to 80% of the aquifer thickness is screened. The screen should best be positioned at equal distance between the top and bottom of the aquifer. In the case of nonhomogeneous artesian aquifer, it is best to screen the most permeable strata. In the case of homogeneous water table aquifer, the well screen is positioned in the bottom portion of the aquifer, -since the upper part is necessarily unwatered to form a hydraulic gradient for flow into the well.
Selection of screen length is something of a compromise between two factors—a higher specific capacity can be obtained by using as long a screen as possible, while more available drawdown results by using as short a screen as possible. Theory and experience have shown that screening the bottom one-third of the aquifer provides the optimum design. The principles of design in a nonhomogeneous water table aquifer are the same as in the case of non-homogeneous artesian aquifer.
Slot Size:
The size of the slots depends upon the gradation and size of the formation material, so that there is no migration of fines near the slots and all the fines around the screen are washed out to improve permeability. In the case of naturally developed wells the slot size is taken as 40 to 70% of the size of the formation material. If the slot size selected on this basis becomes smaller than 0.75 mm, then it calls for an artificial gravel pack. Artificial gravel pack is required when the aquifer material is homogeneous with a uniformity coefficient less than 3 and effective grain size less than 0.25 mm.
The pack-aquifer ratio, i.e., the ratio of the 30 or 50% size of the gravel pack material to the 30 or 50% size of the formation material, is kept at 4:1 if the formation is fine and uniform, and 6:1 if the formation is coarse and non-uniform. The gravel-pack material should have a uniformity coefficient less than 2.5. The design procedure of selecting the gravel material is to determine the point D30 of the gravel pack which is equal to 4 to 6 times the D30 of the aquifer material obtained from the mechanical analysis of the aquifer material, and then drawing a smooth curve through this point (corresponding to D30 of the gravel pack) representing a material with a uniformity coefficient of 2.5 or less. This is the gradation of the gravel pack to be used. The slot size of the strainer (well screen) is kept at 10% size (Dw) of the gravel pack material, to avoid segregation of fine particles near the strainer openings.
The width of slots ranges from 1.5 to 4 mm and the length 5 to 12.5 cm. A ratio of 5 of the 50% sizes of the gravel pack and aquifer material has been successfully used in water wells. The thickness of the gravel pack shall be between 10 to 20 cm. The gravel pack material should be clean, rounded, smooth and uniform, consisting mostly of siliceous rather than calcareous material for which the allowable limit is usually up to 5%. Particles of shale, anhydrite and gypsum are also undesirable in the pack material. The maximum grain size of the pack material should be less than 10 mm. Usually the size of the pea gravel varies from 4 to 8 mm.
Screen Diameter:
After the length of the screen (depending upon the aquifer thickness) and the slot size (based on the size and gradation of the aquifer material) have been selected, the screen diameter is determined so that the entrance velocities near the well screen will not exceed 3 to 6 cm/ sec to prevent incrustation and corrosion and to minimise friction losses. The entrance velocity is calculated by dividing the expected yield of the well by the total area of openings in the length of the screen chosen.
Selection of Screen:
The mineral content of the water, presence of bacterial slimes and strength requirements are some of the factors which govern the selection of the screen material. The screen material should be resistant to incrustation and corrosion and should have strength to with-stand the column load and collapse pressure. The principal indicators of corrosive ground water are low pH, presence of dissolved oxygen, CO2 > 50 ppm, CI > 500 ppm.
The principal indicators of incrusting ground water are total hardness > 330 ppm, total alkalinity > 300 ppm, iron content > 2 ppm, and pH > 8. Slime producing bacteria are often removed with chlorine treatment. This is followed by acid treatment to re-dissolve the precipitated iron and manganese. The selection of the screen material also depends on the quality of ground water, diameter and depth of the well and the type of strata encountered. Some of the commonly used types of screens are shown in Fig. 10.1.
The continuous-slot type of well screen is made by winding cold-drawn wire, approximately triangular in cross-section, spirally around a circular array of longitudinal rods. The V-shaped openings facilitate the fine particles to move into the well during development without clogging them. This type has the maximum percentage of open area per unit length of screen, and the slot openings can be varied by adjusting the spacing of the wires wrapped. These screens are being made of metal such as GI, steel, stainless steel and various types of brass.
The Louver-type of screen has openings in the form of shutters. There is a tendency of the openings being blocked by the fine particles during development. This type of screen is, therefore, best used in artificially gravel-packed wells.
The rectangular slot or the slotted pipe screen in produced by cutting slots, vertical or horizontal, with a sharp saw, oxyacetylene torch, or by punching with a chisel and die or, a casing perforator. Some of the limitations of the slotted type well screens are wide spacing from the strength point, resulting in a low percentage of open area, lack of continuity and uniform size of the openings; the slots or perforations made in the steel pipe may be more readily subject to corrosion at the jagged edges and surfaces, and the chances of blockage of such openings are high. This type of screen is least expensive. Slotted PVC pipes are finding increasing use since they are light and easy to handle and are not subjected to corrosion. The use of slotted PVC pipes is generally limited to small diameter wells because of their relatively low strength and difficulty in providing proper fittings.
The pipe-base well screen or metallic filter point is made by using a perforated steel pipe. A wire mesh is wrapped around the perforated pipe and is in turn is covered by a brass perforated sheet. The percentage of open area in this type is usually low and the perforations are blocked by incrustation. This type of screen is relatively inefficient.
In the Cauvery delta the coir-rope screen is sometimes employed as an inexpensive substitute for other types of screens. Coir rope is wrapped tightly around a circular array of steel flat or rods. The life of the coir ranges from 7 to 8 years and can be increased by treating the coir with cashew shell oil. Hand boring sets are used for constructing coir rope screen wells. Coir rope screen is lowered into the casing pipe and the outside casing is pulled’ out. Coir rope screen does not need gravel packing and development, but at the same time gives very good supply. Coir screen is used in shallow wells where the depth generally does not exceed 12 to 15 meters.
The best type of opening is the V-shaped slot that widens towards the inside of the screen, i.e., openings beveled inside. Regarding the choice of the screen material, steel has good strength but it is not corrosion resistant. Brass has fair to good resistance to corrosion but has only half or less the strength of steel.
However, for most situations, the strength of a well- made brass screen is adequate. Stainless steel has excellent strength and is highly resistant to most corrosive conditions. Well screens of corrosion resistant alloys such as Everdur metal, type 304 stainless steel and silicon red brass should be used in all except temporary installations. Metals used in fabricating screens and their resistance to corrosion are given in Table 10.2.
Design of Water Well (Cases):
Case (a)—Confined Aquifer:
From Table 10.1, for an anticipated well yield of 900 lpm a 20 cm well is recommended. The screen may be located in the aquifer which lies between depths 36- 45 m. The thickness of the aquifer is 9 m which has a grain size mostly in the range 0.6-2 mm and is classified as coarse sand as per IS scale.
The screen of 6.75 m length may be centrally located in coarse sand aquifer. From the mechanical analysis data for the aquifer sample, a grading curve is plotted on a semi-log paper, Fig. 10.3. From the grading curve, the effective size D10 = 0.69 mm and the uniformity coefficient Cu = 2.94. Artificial gravel pack is not required since Dw > 0.25 mm and Cu > 2.5 and the well may be naturally developed when the slot size should be kept at D50 or D60 = 1.75 to 2.03 mm, say, 2 mm.
Case (b)—Water Table Aquifer:
Recommended well diameter = 20 cm
Length of screen I = 1/3 × 9 = 3m located at the bottom one-third of the coarse sand aquifer. Assuming 15% open area for the screen, the entrance velocity (Ve) to obtain an yield of 900 lpm is given by-
(900 x 1000)/60 = 0.15 (π × 20 × 3 × 100)Ve
Ve = 4.78 cm/sec
This is slightly on the higher side. To bring this below 3 cm/sec, adopt a screen length of 4.5 m with a maximum of 18% open area, when V is given by-
900 × 1000/60 = 0.18 (π × 20 × 4.5 × 100)Ve
Ve = 2.95 cm/sec, which is permissible
Artificial gravel pack is not required and the slot size may be kept at 2 mm as in case (a).
Gravel-Pack Design:
If an artificial gravel pack is desired since Cu < 3.0 for the coarse sand aquifer, then
D30 (gravel pack material) = 4 to 6 times D30 of aquifer material
= 4 to 6 times 1.27 mm, as read from the grading curve
= 5.08 mm to 7.62 mm
With these points for D30 for the gravel pack material smooth curves are drawn such that Cu for the gravel pack material is 2.5. The shaded area in Fig. 10.3 show the recommended gravel pack material; clean pea gravel of size to 10 mm may be used. The slot size is kept at Dw of the gravel pack material which is 4.2 mm. The thickness of the artificial gravel pack may be 15-20 cm.
Installation of Well Screens:
The method of installing well screens is influenced by the design of the well, the drilling method and problems encountered during drilling.
The common methods adopted in the case of naturally developed wells are given in the following:
1. Pull-Back Method:
In this method, the casing is driven to the full depth of the well. Then the screen is lowered inside the casing and allowed to rest on the bottom. The casing pipe is then pulled up far enough to expose the full length of the screen in the water bearing formation. Using the swedge block, the lead packer provided at the top of the well screen is expanded to make a sand-tight seal between the screen and the inside of the casing, Fig. 12.1. This method is commonly used in cable-tool drilled wells as well as in rotary drilled wells.
2. Open-Hole Method:
In this method, the casing is first sunk to a depth a little below the desired position for the top of the well screen. An open hole is then drilled in the water-bearing sand, the casing being filled with the mud fluid. The well screen is then lowered in position and the lead packer is swedged to the casing, Fig. 12.2. This method is applicable to rotary drilled wells.
3. Bail-Down Method:
In this method, the casing is driven to the intended position of the top of the screen, Fig. 12.3. A bail-down shoe with special connection fittings is fitted to the bottom of the screen. A string of bailing pipe is screwed on to the coupling of the bail-down shoe and the screen is suspended on this string. The screen is then lowered inside the casing till it reaches the bottom of the bore.
Using a bailer or sand pump through the bailing pipe, sand is bailed out from below the screen, when the screen settles down. Before removing the bailing pipe, the special nipple on the bail-down shoe is plugged and after removal, the lead packer is expanded with the swedge block. This method is applicable to rotary drilled well as well as cable-tool drilled well.
4. Wash-Down Method:
The well casing is first set to the desired depth. A high velocity jet of light-weight drilling mud or fluid issuing from a special wash-down bottom fitted to the end of the screen loosens the sand and allows the screen to sink, Fig. 12.3. The sand is brought up around the screen and into the casing with the return flow of the fluid. After the screen reaches the desired depth, water is circulated through the wash pipe to remove the drilling mud. The lead packer is expanded after removing the wash pipe.
5. Driving:
After setting the casing to the desired depths, the well point with a turned coupling or a packer attached is dropped through the casing and is driven by a driving weight. When driving relatively long well points, a long driving bar attached to a string of pipe or a stem is employed to deliver the driving force directly on the solid bottom of the screen, Fig. 12.4.
The methods generally adopted in the case of artificially gravel packed wells are the bail-down, open-hole and double-casing methods. The double-casing method, which is most commonly adopted, uses one string of casing corresponding to the outside diameter of the gravel pack (well casing) and a second string of alternate lengths of plain pipe and slotted pipe (well screen to face different aquifer locations). The outer casing is first sunk to the full depth of the well. The inner string consisting of the casing, the well screen and the bail plug at the bottom is then lowered to the bottom. Gravel is put in the annular space around the screen.
After filling a certain depth with gravel, the outer casing in pulled back a short distance and the well is developed by compressed air. The steps of placing more gravel, raising the outer casing and developing by compressed air are repeated till the level of gravel envelope is sufficiently above the top of the uppermost screen. The outer casing can be pulled out completely or some length of casing can be left in the top portion, Fig. 12.5.
Recovering Well Screens:
It sometimes becomes necessary to remove a well screen from an abandoned well for re-use in another well, replace an encrusted screen after cleaning by chemical treatment or by a new screen. Considerable pulling force has to be applied to the screen to overcome the grip of the water-bearing sand around it. The transmission of this force to the screen for dislodging and recovering without deforming it is provided by the sand-joint method.
In this method sand is placed in the annular space between a pulling pipe and the inside of the well screen to form a sand lock or sand joint by tying 5-10 cm strips of sacking to the lower end of the pulling pipe immediately above a coupling ring welded to the pipe, and arranging evenly the upper end of the sacking strips around the top of well casing as the pulling pipe is lowered into the well near the bottom of the screen, Fig. 12.6.
The pulling pipe is moved up and down and a small stream of water is injected while pouring sand to prevent bridging. About two-thirds of the screen is filled with sand. The pulling pipe is then gradually lifted to compact the sand and develop a firm grip on the inside surface of the screen. Ultimately the screen is pulled out and the sand joint is broken at the surface by washing out the sand with a stream of water. Sizes of pulling pipe and quantity of sand required for sand joints are given in Table 12.2.
Pre-treatment of the screen with acid by filling the whole length of the screen with a mixture of equal proportions of hydrochloric or muriatic acid and water, using a string of black pipe or plastic pipe and allowing to stand for several hours or overnight, serves to loosen the rust and incrustation and thus reduces the forces required to obtain initial movement of the screen.
Fishing Operations:
During drilling it may so happen that the bit may be struck, a tool may be deposited in the bore hole due to breakage. These can happen even to the most capable driller using the best drilling equipment. It is desirable to recover them immediately so that drilling may progress without losing much time. The recovery process is called fishing operation. Fishing operations require a great deal of skill and ingenuity of the driller.
The precautions that should be taken against such accidents are proper care and use of drilling tools, following the detailed instructions given in the manufacture’s catalogues, regular lubrication with a good grade of lubricant free from acid or alkali, to make joints firmly but not with excessive pressure as this can result in broken boxes and pins, proper screwing of the tool joints, the threads being thinly coated with a light machine oil and putting screw caps to avoid sticking of the mud, removing all tools immediately away from the bore hole lest they may be tipped into the hole, and exercising utmost care in caving and boulder formations and crooked holes.
In anticipation of the inevitable fishing job, it is necessary to record the exact dimensions of everything used in the well so that information will be at hand for designing or selecting a suitable fishing tool. If the fishing becomes much complicated and difficult, the cost of the fishing tools and the time consumed may greatly exceed the cost of the tools and even the hole, and it may be found economical to move the drill and start a new hole. There are many types of fishing tools suitable for different fishing jobs and special tools have to be manufactured if they are not already there. Many of the tools are rarely used.
Well Development:
Well development is the process which causes reversals of flow through the screen openings so as to wash out the fines and rearrange the formation particles in a naturally developed well and form a graded filter (reversed filter) with rings of increasing porosity and permeability towards the well in an artificially gravel packed well, so that ultimately the well will yield clear sand-free water.
But causing reversals of flow around the screen, the tendency for several small particles to bridge between large particles is overcome. Fig. 12.7 illustrates how the aquifer material has been rearranged after proper development has been made, along with the grain size distribution curves of the aquifer material before and after development. It can be noted that all particles smaller than 1 mm have been removed by the development with an effective size of 1 mm as against 0.33 mm of the original formation, and a uniformity coefficient of 1.4 against 2.9, both of which imply a greatly increased permeability.
Proper development increases the well efficiency and the well loss coefficient C determined by conducting a step drawdown test is indicative of proper development.
Some of the methods of well development in vogue are:
(i) Mechanical Surging using Solid Type or Value Type Surge Plunger:
In this method the plunger is operated up and down in the casing like a piston in a cylinder, which produces the required alternate reversals of flow.
(ii) Using Compressed Air:
This method will prove to be rapid and effective when properly used under favourable conditions. The process involves in combination of surging and pumping. By means of sudden release of large volumes of air, a strong surge is produced by virtue of the resistance of water head, friction and inertia. Pumping is done with an ordinary air lift.
The equipments required are:
(a) Air compressor capable of developing a maximum pressure of 700—1000 kN/m2 and capacity of providing about 6 litres of free air for each litre of water at the anticipated pumping rate.
(b) Pumping (educator or drop) pipe and air line with suitable means of raising and lowering each independently of the other.
(c) Accessories such as flexible high pressure hose, relief valve, quick opening valve, pressure gauge etc.
The arrangement of the pumping pipe and air line is shown in Fig. 12.8. Best results are obtained when the submergence of the air line is about 60%.
Submergence is calculated by dividing the length of the air line submerged in water by the total length of the air line, i.e., if an air line of length 50 m is submerged in water up to 30 m, then the submergence is (30/50) 100 = 60%. The efficiency of development drops off when the submergence becomes less than 60%. The recommended sizes of pumping pipe and air line for various sizes of well is given in Table 12.3. On wells of 10 cm and smaller size, the well casing itself serves as a delivery pipe, and 18-25 mm airline is used.
Development is started by lowering the pumping pipe to 0.6-1.0 m above the bottom of the screen. Then the air line is held about 0.6-1.0 m above the bottom of the educator pipe and water is pumped out by air. As soon as the pumped water appears sand-free, the valve at the air receiver tank is closed and the air line is dropped so that its lower end is about 0.6-1.0 m below the bottom of the educator pipe.
The valve is then quickly opened which allows the water to surge outward through the screen openings. Then the air line is pulled up inside the educator pipe and the process is repeated till the well yields clear sand-free water. The position of the entire air-lift assembly then is moved up to a higher portion of the screen (0.6-1.0 m) and such cycles are repeated until the entire length of the screen has been so developed.
(iii) High Velocity Jetting of Water:
A high velocity jet of water directed horizontally through the screen openings with the tip of the nozzle at about 12-25 mm from the inner wall of the screen is generally the most effective method of well development. A jetting tool coupled to the end of a pipe is lowered into the well. The top of the pipe is connected by a hose to a high pressure pump. As soon as the pump is started the jetting tool is slowly rotated and gradually raised or lowered so that the entire surface of the screen receives the jetting action.
The well is pumped by another pump during the jetting operation to maintain the hydraulic gradient so that water and the loosened fine particles will keep entering the well. The jetting tool consists of 2-4 horizontal nozzles having 6-12 mm orifices. The jet velocity should be in the range of 30-50 m/sec, which requires a pressure head of 7-14 kg/cm2. The disadvantage of this method is that considerable supply of water will be required for effective operation. This method is not effective where perforated or slotted pipe is used as a screen.
(iv) Over-Pumping and Back-Washing:
Over-pumping means pumping the well at a higher rate to create excess drawdown, i.e., higher gradients to wash out the fine particles. But since there is no surging effect or reversals of flow, bridging of sand can occur. A higher capacity pump is required, but sometimes the pump intended for regular use in the well is employed for over-pumping.
Back-washing provides a surging effect for well development but it is often found not to be vigorous enough. A deep-well turbine pump without a foot valve is required for this purpose. The pump is started and the water is lifted to the surface or to a large tank above the ground. Then the pump is stopped and the water in the column falls back into the well. This process is repeated, which produces reversals of flow through the well screen.
Another simple method of developing is by back-washing by pouring water into the well as fast as possible and then bailing out with sand pump or bailer. The head built up by pouring water causes reversal of flow through the screen, though it may not be very effective or rapid.
(v) Dispersing Agents:
Sometimes, it is necessary to add a chemical agent to disperse the clay particles in the mud cake or in the formation to avoid their sticking to sand grains, and to speed up the development process. For this purpose several polyphosphates are used like tetrasodium pyrophosphate, sodium tripolyphosphate, and sodium hexametaphosphate and sodium septaphosphate. About 600 g of the chemical is added to every 100 litres of water in the well. The mixture is allowed to stand for about an hour before starting development.
Well Completion:
After a well has been constructed, proper sanitary completion is necessary to produce water that is safe by the public health standards.
Well completion operations include:
(i) Grouting and sealing the casing.
(ii) Completion of the top of the well.
(iii) Disinfection of the well.
Grouting the casing means the filling the annular space between the outside of the casing and the inside of the drilled hole with a cement grout. The grout is a fluid mixture of cement and water of such a consistency that can be forced through the grout pipes and placed as required. Grouting and sealing the casing in water wells serves to prevent the downward seepage of sewage or other polluted surface waters along the outside of the casing, to seal of aquifer yielding water of poor quality, to make the casing stay tight in the drilled hole and to form a protective sheath around the casing against exterior corrosion, thereby increasing its life, Fig. 12.9.
The top of the casing should normally extend as least 50 cm above the general level of the surrounding surface, well above the maximum flood water levels and isolated from direct contact with accumulating drainage wastes and sudden drainage discharges. The space around the casing should be grouted to a depth of about 6 m to seal of the well from the entrance of surface drainage. A concrete platform should be constructed around the casing at the ground surface. The top of the casing should be provided with a sanitary seal consisting of suitable bushing or packing glands that makes a water-tight seal between the pump column pipe and the well casing.
Abandoned wells should be sealed by filling with puddled clay or cement grout to avoid possible movement of inferior water from one aquifer to another and conserve the water in pumped wells. This is a necessary precaution even if the well casings are perforated in only one aquifer, since casings may eventually deteriorate, permitting interconnection of ground water bodies.
Well Disinfection:
After completion of construction, the well and its appurtenances like the casing, pump and pipe systems have to be disinfected or sterilised promptly, for which chlorine solution is the simplest and most effective agent. Highly chlorinated water can be prepared by dissolving dry calcium hypochlorite, liquid sodium hypochlorite or gaseous chlorine in water. A solution containing about 100 ppm of available chlorine should be used.
This can be obtained by adding 125 g of dry calcium hypochlorite containing 70% of available chlorine to every 1000 litres of water standing in the well or about 2 litres of liquid bleach containing 5% of available chlorine to every 1000 litres. A solution is made by mixing this total amount in a small quantity of water and is poured into the well through the top of the casing, before it is sealed. The water in the well is thoroughly agitated and allowed to stand for several hours or overnight. The well is then flushed to remove all the disinfecting agents.
Flowing artesian wells may be disinfected, if found necessary, by lowering a perforated tube, capped at both ends, filled with an adequate quantity of dry calcium hypochlorite to the bottom of the well. The natural upflow of water in the well will distribute the dissolved chlorine throughout the depth of the well. A stuffing box may be provided at the top of the well to restrict the flow and reduce the loss of chlorine.
Well Maintenance:
While the expected service life of a well depends upon the design, construction, development and operation of the well, proper maintenance helps to improve the performance and increase the life of the well. Proper records of power consumption, well discharge, drawdown, operating hours, periodical chemical analysis of water and other such observations will help in devising proper maintenance procedures.
The sudden pressure drop and increase in the entrance velocity near the screen due to high pumping rates releases carbon dioxide and causes precipitation of calcium carbonate and iron deposits near the screen. The change in entrance velocities results in precipitation of iron and manganese hydroxides. The presence of oxygen in the well can change soluble ferrous iron to insoluble ferric hydroxide.
The perforations can be cleaned by adding hydrochloric (muriatic) acid or calgon followed by agitation and surging which removes the incrusting deposits. Normally, the volume of acid required for a single treatment will be about 1.5 to 2 times the volume of water in the screen. Sulphuric acid can also be used instead of hydrochloric acid but its action is a little slower and requires a longer contact time in the well, Fig. 12.10.
The yield of the well may decrease due to the deposition of incrustation of fine particles of silt and clay near the screen. This can be removed by the use of a dispersing agent such as polyphosphates. For effective treatment, 15 to 30 kg of polyphosphate is added to every 1000 litres of water in the well. 1 kg of calcium hypochlorite should be added for every 1000 litres of water in the well to facilitate the removal of iron bacteria and their slimes, and also for disinfection purposes. The solution of polyphosphate and hypochlorite is poured into the well
and a surge plunger or the jetting technique is used to agitate the water. The well may be treated 2-3 times for better results.
The perforations may become plugged with algae or bacterial growths. Chlorine treatment of wells has been found more effective than acid treatment in loosening bacterial growths and slime deposits which often accompany the deposition of iron oxide. Since a very high concentration of 100 to 200 ppm of available chlorine is required, the process is known as shock treatment with chlorine. Calcium or sodium hypochlorite may be used and the chlorine solution in the well must be agitated by using the high velocity jetting technique or by a surge plunger.
Faulty well construction such as poor casing connections, improper perforations or screens, defective gravel packs and poorly seated valves should be located and set right immediately. Sudden failure of a casing pipe or strainer, resulting in the entry of sand, will require replacement of the well as a whole.
An additional precaution against corrosion of the screen is provided by cathodic protection by suspending in the well a rod of a metal low on the electrochemical scale, such as magnesium. This rod will corrode instead of the metal perforations or the screen, and can be replaced when necessary from time to time. Development methods by compressed air or dry ice are sometimes effective in cleaning up corrosion deposits.
Depletion of ground water supply can sometimes be remedied by decreasing pumping drafts, resetting the pump or deepening the well.