OXFAM GB EMERGENCY DRILLING PROGRAMME BEST PRACTICE MANUAL

Transcription

1 OXFAM GB EMERGENCY DRILLING PROGRAMME BEST PRACTICE MANUAL Oxfam GB Emergency Drilling Programme Best Practice Manual January

2 OXFAM GB EMERGENCY DRILLING PROGRAMME BEST PRACTICE MANUAL Contents 1 Introduction 2 The need for the manual & how it will be used and who will use it 3 When is it appropriate to consider Water Drilling in Emergency 3.1 Drilling through contractors 3.2 In house, Oxfam drilling 4 Management of drilling programme & monitoring requirement 4.1 Staffing requirement 4.2 Logistics requirement & logistics support for a drilling programme 5 Types of Drilling methods & Choice of technology 5.1 Percussion drilling 5.2 Rotary drilling Rotary drilling with Mud Rotary drilling with Air Rotary drilling with DTH hammer 5.3 Jetting 5.4 Hand drilling 6 Ground water exploration methods and considerations 7 Drilling & Drilling Supervision 7.1 Logging 7.2 Construction 7.3 Well Lining 7.4 Gravel Packing 7.5 Sanitary Seal 7.6 Well Development 7.7 Pump Testing Definitions of common terms related to pump testing Why conduct borehole test pumping Equipment for pump testing Example of Constant Yield test pumping Well specific Capacity and Pumping rate 7.8 Pumps and generator sets Specifying submersible pumps Sizing electric motor Sizing generator set Installation of Submersible pumps 8 Borehole Rehabilitation 9 Ground Water quality 10 Appendix Appendix A Logical Frame work for a drilling programme Oxfam GB Emergency Drilling Programme Best Practice Manual January

3 Appendix B Appendix C Appendix D Tendering drilling work Drilling contract Drilling rig Specifications PAT 201 PAT 301TP PAT 401 Appendix E Borehole completion report format References 1 Introduction Groundwater makes the bulk of the world s available fresh water for drinking, nearly 70%. In most instances it is the most preferred option for water supply because (i) it is usually present in areas where surface water is limited (ii) the quality is usually good and does not require treatment (iii) the quantity is usually dependable, and does not easily change with the levels of changes in rainfall and (iv) groundwater can be found near villages and communities and does not require extensive investment that can be associated with systems that require treatment storage and piping of surface water sources. As part of its humanitarian intervention in the area of water and sanitation, Oxfam has been making use of groundwater sources through protection and capping of spring protections, construction of hand dug wells and by undertaking drilling programmes in different parts of the world with reasonable success. This best practice manual will cover the Oxfam s experience in ground water from drilled borehole sources. The drilling work has been partly directly implemented by Oxfam, using in house drilling capacity, i.e. Oxfam deploying its own drilling rig and drilling team, and in some instances by contracting local drilling companies to do the drilling work. In some instances Oxfam drilling work employed geophysical survey for site selection by hiring local consultants and hydro-geologists, in some cases Oxfam developed its own capacity to undertake such survey, and in many instances drilling was implemented with out any prior survey. Basically Oxfam drilling work in all its different geographical programmes followed varied system depending on the local knowledge and capacity, and of course with different end results as well. Oxfam has deployed its own drilling rig and drilling crew as part of its humanitarian intervention in South Sudan, North Sudan, Angola, Chad and Sierra Leone. Oxfam has the experience of using local drilling companies (contractors) in countries such as Ethiopia, North Sudan, South Sudan, Zambia, Malawi, Uganda, Tanzania, Eritrea, and Kenya, 2 How will this manual be used and by who The manual is meant to be simple guideline document for Engineers and drilling technicians who will be involved in planning AND implementing emergency drinking water supply from drilled wells. The key issues that need to be addressed in any borehole drilling work are briefly discussed in an outline form. The manual does not go in to detailed technical issues that can be available from proper water well drilling textbooks listed in the reference section. It also provides a general basic overview of what drilling is and what is involved in drilling programme. Oxfam GB Emergency Drilling Programme Best Practice Manual January

4 2.1 The need for such a manual: To help standardise Oxfam s approach to ground water exploitation from drilled wells. To provide simple step by step guide in the planning and implementation of Oxfam drilling intervention. To discuss the Oxfam drilling rig and consumables kits Code BDR4/2 and Code BCON/1 respectively, for quick deployment as part of first phase response during an emergency. The manual discusses the approach and methodology for the drilling intervention along with readily available stock. In cases where the drilling programme is to be carried out by contractors, the manual provides all necessary documents for tendering of the contract, selection of drilling contractor including contract agreement, readily available for use. To underline the fact that Monitoring of drilling programme is part of the intervention right from the inception and planning of the drilling work. To highlight the need for documenting all Oxfam drilling experiences, for future use as lessons learned. 3 When is it appropriate to consider Well Drilling in Emergency. Generally in first phase emergency water supply intervention, drilling for water is not considered to be first priority option, because drilling requires (i) (ii) (iii) Data/Information on ground water availability Capital equipments such as Drilling Rigs and all accessory equipment and Capable /Qualified drilling crew, All of the above are not usually readily available in emergency situation. On the other hand, it is very common that the need for emergency water supply intervention happens in areas where there is no surface water at all, and it becomes very imperative to use ground water sources as the best option. This usually happens in arid and semi arid, drought prone environments in Africa, and some Asian and Latin American countries. Checklists to consider Emergency drilling programme: No other alternative water source, such as surface water that can be used Water trucking operation is unsustainable, beyond short-term emergency period. Existing borehole water sources in the area, providing history of ground water availability in the area of intervention. There is drilling capacity in the area or it is possible to bring in drilling capacity Sustainability of water lifting devices to be adopted can be assured Availability of reasonably good logistics for the supply of spare parts of hand pumps, submersible pumps and generator sets can be assured It is possible to recruit/train local drillers and drilling technicians 3.1 Drilling through contractor Oxfam GB Emergency Drilling Programme Best Practice Manual January

5 Whenever there is a capacity in the country of intervention, it is highly recommended that local drilling contractor be employed in Oxfam s drilling work. Advantages of undertaking drilling work through contractors (i) Time saving, intervention can immediately start (ii) Builds on existing capacity (iii) Sustainability of future water sources can be assured (iv) Reduces initial capital equipment cost (v) Cost effective if the number of boreholes to be drilled are few (difficult to justify the cost of capital items if numbers of boreholes to be drilled are few.) 3.2 Oxfam drilling: In an emergency where the only possible water supply source is ground water, and there is no in country capacity to undertake drilling work, or the existing capacity is not dependable enough to meet the emergency requirement for the supply of water to the affected community, drilling is executed by Oxfam. When the intervention requires the drilling of large number of boreholes, it will not be cost effective to employ a contractor for the work, and it is recommended that Oxfam employs in house drilling, unless there is a sound reason for not to use Oxfam s own drilling intervention. To get the right number of boreholes when one needs to decide in favour of inhouse drilling or the use of contractor, it is always advisable to undertake cost benefit analysis, comparing the cost of the drilling work by contractor with the cost of acquiring a complete drilling rig plus the cost of undertaking the drilling work. Cost benefit analysis quick & rough addressing the following: - the cost of the drilling work in both options number of holes - support to local capacity- based on long term plan - job creation, which is support to local economy & the possibility of local group to take over - cost of operation & maintenance and sustainability of competed systems - logistics 4 Management of drilling programme & monitoring requirement Good understanding of drilling programme time line / project cycle will provide effective and efficient approach to drilling programme management. For successful drilling programme - programme time line that need to be followed. (i) Desk study collecting available information, from existing reports if there are any, geological and hydro-geological maps, reports of any hand dug wells and boreholes, borehole logs, test pumping results including weather data, remote sensing data. Digging out any data that can provide any information on the soil aquifer type, permeability, and transitivity etc. Oxfam GB Emergency Drilling Programme Best Practice Manual January

6 (ii) (iii) (iv) (v) (vi) (vii) Initial in Country information / data collection - from existing Water Resources Departments and other National and International agencies involved in the area of water supply activity. Initial field assessment existing water sources both surface water and ground water sources. Physically assessing functional and non-functional dug or machine drilled wells including gathering information on the water levels, geology, local history, exploratory drilling and water demand. Design & contract specifications Once decision is made to undertake drilling based on the desk study and the initial assessment, preparing design and drilling contract specification follows. Well sitting local information, geophysical surveys, maps Drilling & Supervision of drilling work borehole logging, lining/casing, construction, gravel packing, developing, test pumping and finally installation of pumps. Post well construction water quality analysis, water level monitoring, borehole performance monitoring Points (i) to (iv) above, will help decide on the issues of choice of approach, whether to use in house drilling capacity or contract local drilling agencies. 4.1 Staffing Requirement for drilling programme to be implemented by Oxfam Composition of one drilling Team: Public Health Engineer with hydrogeology background OR Civil Engineer with borehole drilling experience, to lead/supervise/monitor the drilling work. One Chief driller Two Drilling technicians Four to six Casual labourers 4.2 Logistics requirement for a drilling team: Drilling rig plus all necessary accessories, such as compressor, mud pump, DTH hammer, Toyota Pickup vehicle (assuming PAT401, PAT301 or smaller rig will be used) Vehicle for use by the supervising engineer and the team in general ( 50 75% use) **The rig, the compressor, the spare parts, the consumables etc. Monitoring requirements Successful Drilling programme is highly dependent on good logistics support and the supply of all necessary equipment and consumables at the right time. Engineers, drillers and technicians will have to work very closely with logisticians, and it is preferable that logisticians working with drilling programme have some technical Oxfam GB Emergency Drilling Programme Best Practice Manual January

7 background, and are familiar with drilling work and drilling equipment and its accessories including the consumables required. List of Specifications for PAT drilling rigs and its accessories, and the important drilling consumables is given below in Annex 5 Types of Drilling Methods & Choice of technology There are different types of drilling methods, and are briefly discussed below. 5.1 Cable tool percussion drilling Cable tool (percussion) drilling is considered as the first drilling method employed to dig wells. It consists of basic concept of repeatedly dropping and lifting of a heavy metal cutting bit usually greater than 50kg in to the ground, thus breaking through the soil formation and punching a hole through to the desired depth. It is simple and cheap but reliable drilling method, that can be employed in any type of soil formation with out the requirement of drilling mud/ air or any chemical. It consists of a tool suspended on a winch cable, can be hand or machine operated. The bit which is usually blunt chisel shaped instrument can vary with the type of the earth formation that is to be drilled. (Picture of percussion drilling bit here ) Advantages of percussion drilling: Simple to operate and maintain Suitable for wide variety of formations Possible to drill to considerable depth Oxfam GB Emergency Drilling Programme Best Practice Manual January

8 Disadvantages of Percussion drilling Slow compared with other drilling methods Equipment can be heavy Water is needed for dry holes to help remove cuttings 5.2 Rotary Drilling Rotary drilling is a drilling method in which a rotating drill bit spins around the earth Components formation allowing of basic the rotary penetration drilling in system to the ground consists forming a hole. (picture The of drilling prime movers: bits here..) those equipment that provide the power to the entire rig Hoisting equipment: consists of the tools used to raise and lower the drill pipes and other equipment that need to be lowered in to the drilled hole Rotating equipment: consists of the components that actually serve rotate the drill bit, thus effecting the digging of the hole deeper into the ground. Circulating equipment: consists of the drilling fluid or air, which is circulated down through the well hole throughout the drilling process. It does also consist of the drilling mud pit Rotary Drilling with mud using PAT 301TP drilling Rig Oxfam GB North Sudan Programme (2005) Rotary drilling with Mud Oxfam GB Emergency Drilling Programme Best Practice Manual January

9 This is Rotary drilling in which Mud is used as drilling fluid. Mud pump will be employed as one of the prime movers. The main purposes of drilling mud as drilling fluid are : Cooling the drilling bit inside the hole Removing cuttings from the hole Preventing caving of the hole Lubricating the drill pipes Checking corrosion and rust Rotary drilling with Air This is rotary drilling in which the drilling fluid is air. Basically it is the same as in mud drilling except that air is used instead of mud, and compressor will be required in place of the mud pump. Drilling with air can shorten the drilling time substantially. Figure: PAT 201 drilling rig - Oxfam south Sudan Drilling Programme Rotary drilling with DTH hammer This is rotary drilling in which percussion action of cable tool drilling is combined inside the hole by a pneumatically operated drill system. The pneumatic drill DTH is used with standard rotary drilling rig by having sufficient capacity compressor. DTH drilling is generally the fastest method of penetration in hard rock. During the DTH process the bit is slowly turned (5-15rpm) Advantages of Rotary drilling with DTH hammer Drills well in hard rocks Possible to penetrate gravel Is fast drilling method Operation is possible above and below water table Disadvantage of Rotary drilling with DTH hammer Higher tool cost Oxfam GB Emergency Drilling Programme Best Practice Manual January

10 Air compressor is required Requires experience for operation and maintenance 5.3 Jetting Jetting is a drilling technique that uses flushing a jet of water into a water bearing strata (an aquifer) through a pipe. This is a technique that can be employed in loose soil formation. Suitable aquifers consist of unconsolidated sediments such as sand, silt and fine gravel. Water is pumped down the centre of the drill pipe emerging as jet, and returns up the borehole bringing with it cuttings and debris. To help the washing and cutting of the soil formation the drill pipe is rotated and provided with up and down motion. It possible to use foot-powered treadle pump or any small pump. Advantages of Jetting The equipment is simple to use Possible to drill above and below water table Disadvantages of Jetting Water will be required to undertake the drilling It is suitable for unconsolidated rocks only Boulders and bigger gravel can prevent further drilling 5.4 Hand Auger drilling Advantages of hand-auger drilling: Inexpensive. Simple to operate and maintain. Disadvantages of hand-auger drilling: Slow, compared with other methods. Equipment can be heavy. Problems can occur with unstable rock formations. Water is needed for dry holes. Survey auger Kit Oxfam code WAS/2 being used in Oxfam Chad Programme ( 2005) The choice of technology (drilling rig) will depend on the methodology to be adopted to execute the drilling work. Drilling to be executed by contractors in this case the type of the rig will be whatever the contractor will be deploying. NB: the contractor will have to pass through proper selection process, by which the drilling capacity and drilling equipment of the contractor will be assessed before the contract is given Oxfam GB Emergency Drilling Programme Best Practice Manual January

11 Drilling to be executed by Oxfam GB Oxfam has adopted direct rotary drilling using PAT drilling rig for its in house drilling intervention, which is light machine and easy to transport. Detail specification for the rig including DTH Hammer and Air Compressor is included in the Oxfam Logistics catalogue and in Appendix. of this manual. 5.1 When is it likely that the drilling intervention will be less successful? Unfavourable geological formation Drilling in dry land where the water table is very responsive to rainfall i.e there is nor recharge Unavailability of dependable geological/hydro geological information, data Limited capacity, both of manpower and appropriate equipment When drilling option is forced, due to lack of other viable alternative water supply source 6 Ground water exploration methods and considerations Well sitting for boreholes drilling is an importation component of any drilling programme and is detrimental for the success or failure of the programme. To undertake successful of potential ground water source identification, i.e. to site a possible bore well site, one has to use a combination of three different approaches, using of geological Maps, Observation and Geophysical investigation, an approach termed as geological triangulation 1 1. Maps 2. Observation 3. Geophysics Figure : Geological Triangulation Maps : In situations where there is available geological and topographic maps for the area where the drilling is being considered, using GPS determine the locations, and by overlapping the locations on the existing geological/topographical maps an indication of the basic geology of the area can be obtained. Satellite maps, Aerial photographs 1 Adopted from Simple methods for assessing groundwater resources in low permeability areas of Africa, BGS report CR/01/168N Oxfam GB Emergency Drilling Programme Best Practice Manual January

12 Observation/ discussion with community: Get the information of existing wells, steams, rivers, vegetation cover and rock types through simple observation and discussion with community. o On existing wells depth, yield, borehole diameter, water quality salinity, Geophysics: this is undertaken when the results from the maps and observation do not provide with conclusive information for successful borehole site. The two techniques used are o Electrical Resistivity Method the two commonly used are -Vertical Electrical Sounding (VES) and -Resistivity Profiling (RP) o Electromagnetic Method : commonly used EM34 Magnetic Resonance Sounding Note: If and when it is decided to employ geophysics for well sitting, it will be imperative to recruit an experienced hydro-geologist supplied with the necessary equipment or use local/international geophysics-consultant to undertake the task. This will be important whether the drilling is to be conducted by Oxfam or a contractor. 7 Drilling, Drilling Supervision and Pump installation Drilling and drilling supervision comprises of all the drilling activities starting from setting the drilling rig in position, preparing the drilling site, undertaking the drilling work, borehole development, pumping test, completing the well head work through to final installation of the appropriate pump for abstracting the water. Each activity is summarised below in sections Preparing the drilling site comprises Demarcating the drilling area using rope or line fencing, Having clear area for drilling pipes and other drilling equipment accessories, including compressor Preparing water storage facilities if and when drilling fluid will be used for drilling Preparing the drilling mud boxes Setting up the drilling rig in place Preparing sampling boxes 7.1 Logging Once drilling is started soil sample will have to be taken at every one or two meters. This will be used for the preparation of the borehole log after the drilling is completed. The borehole log will be very vital from the point of knowing where the water bearing aquifer lies, and knowing the geological formation of the soil below ground. When it comes to putting casing and screens it will be clear where to place the slotted screens and blind casings. This will also be used for pump setting purposes, it is very important that the pump is placed well above the screen slots to avoid the sucking of sandy materials by the pump. 7.2 Drilling /Construction Oxfam GB Emergency Drilling Programme Best Practice Manual January

13 Well Diameter: The capacity of properly drilled wells is much more dependent on depth of the borehole, rather than on the diameter of the well. The only way the capacity of the well will be dependent on the diameter is when it is related to the type of the pump to be installed. The well diameter limits the size of the pump that can be installed in the well. On the other hand the well diameter to be drilled will have high determining factor on the cost of the well. The volume of soil material to be removed during the well construction almost doubles by an increase of well diameter size. Points to consider when deciding on well diameter Well capacity (yield) will not depend on the well diameter The size of the pump to be installed will depend on the well diameter Depending on the grain size of the aquifer, the well will need to be provided with gravel pack and sanitary seal grouting, which will have an effect on the size of the casing/ screen to be provided. The diameter of the well will affect the volume of soil material to be extracted from the well, therefore the cost of the overall drilling work. 7.3 Well Lining After the completion of drilling work, the borehole is provided with lining depending on the geological formation of the hole. The ling material can be a steel or plastic pipe. PVC plastic is widely used for relatively average wells for drinking water supply purposes. The section which is not water bearing layer is lined with blind casing while the water bearing formation is lined with slotted well screens. The section of the borehole that is in hard rock formation will not require any lining material. Screen slot size Normally screen slot size must be determined after examining and analysing the cuttings recovered from the borehole. The slot size should allow the free flow of water into the well while holding back the aquifer material. If the slot size is too large, sand will pass through along with the water into the pump and the distribution system. This will cause excessive wear of the pump and eventually shortening the life of the pump. If the slot is too small, it will prevent the free flow of water thus giving the impression the well is running dry. This will cause the pump to work harder. The screen slot size should be equal or less than the average grain size of the aquifer. Estimated minimum screen openings for a particular flow rate is given in the table below ( adopted from Peter Ball s Drilled Wells book ) Oxfam GB Emergency Drilling Programme Best Practice Manual January

14 Litres/h Litres/m Litres/s Minimum Screen opening Cm Slotted Screen Pipe 7.4 Casing / Screen Diameter It is very important to note the approximate inside and outside diameter of the casing/screen pipes for determining the pump size to be installed in the well. For example 4 pump motor might not fit in through nominal 4 casing ( 98mm internal diameter). Nominal Outside Approx inside size Diameter in Diameter in In inch mm mm 1 ½ ½ Table : upvc pipe sizes in metric and English units extracted From Peter Ball s book on Drilled wells 7.5 Gravel Pack (Filter pack/formation stabilizer) Gravel packing is a process of placing coarse sand or fine gravel ( 2-6mm) diameter between the borehole wall and screen lining. The purposes of placing gravel pack are: To settle out fine grained particles of the aquifer that may otherwise inter the well To increase the effective hydraulic diameter of the well Oxfam GB Emergency Drilling Programme Best Practice Manual January

15 It is very essential to install gravel pack in all wells, except in those wells that are completed in rocks, gravel or coarse sand. Important points on gravel pack Size of the gravel pack should not be more than twice the size of the screen slot. Best to have hard washed and well rounded material from dry river bed or lake shore Crushed road gravel with sharp edges are not suitable and should not be used. ( Lesson learned from Oxfam drilling work in Darfur - A borehole with a good yield drilled in Zamzam, the yield was drastically reduced after crushed road gravel was ) The Volume of gravel pack required is calculated using the following formula. V = h x π x (D 2 d 2 ) where V is the volume of the gravel needed, h the height of the gravel pack, D the diameter of the borehole and d the diameter of the casing - Knowing the volume of the gravel pack is very important, and will always assist in the understanding of whether the voids are properly filled or not, once the packing is completed. - The filter pack should extend several centimetres above the top of the screen to allow for settling during development and to prevent the possibility of fine sand interring the well. Recommended choice of screen slot and gravel pack against aquifer grain size Aquifer grain size Gravel pack grain size Screen slot size 01 to 0.6 mm 0.7 to 1.2 mm 0.50 mm 0.2 to 0.8 mm 0.1 to 0.5 mm 0.75 mm 0.3 to 1.2 mm 1.5 to 2.0 mm 1.00 mm 0.4 to 2.0 mm 1.7 to 2.5 mm 1.50 mm 0.5 to 3.0 mm 3.0 to 4.0 mm 2.00 mm 7.6 Sanitary seal After drilling is completed, i.e. after a well is fitted with casings and screens and filter pack is placed, it is always imperative that annular space is provided with sanitary sealing to prevent any surface contaminant draining in to the aquifer. Usually concrete/cement or bentonite pellets is used to provide sanitary seal. In the event that Sanitary seal is provided directly above the gravel pack with out the use of formation stabiliser, it is always important that the gravel pack extends well above the screens so that any cement grout or formation sealant such as bentonite does not clog the gravel pack and prevent the flow of water in to the well. The sanitary seal should extend to the ground surface and constructed to form the well head, so that no surface water is allowed to inter into the well. Oxfam GB Emergency Drilling Programme Best Practice Manual January

16 7.7 Well Development What is well development? During rotary drilling, drilling fluid and mud is introduced in to the well and there will be instances where some of the chemicals and mud are smeared along the walls of the borehole, thus decreasing the permeability of the borehole wall. This does also happen even in auger and percussion drilling. In order to remove the mud and the fine sediments a process of clearing up and restoring the hydraulic connection within the aquifer is required. This process is called well development. It is very important that proper well development of completed boreholes is carried out before the wells are commissioned for use. What are the purposes of undertaking well development? Two broad objectives for undertaking well development: (1) to repair damage done to the formation by the drilling operation and to restore natural hydraulic properties of the aquifer (2) alter the basic physical characteristics of the aquifer near the borehole so that water will flow more freely in to the borehole. To remove any water or drilling fluid introduced into the well during drilling To stabilize the filter pack and formation materials opposite the well screen Minimize the amount of fine-grained sediments entering into the well Maximize well efficiency and inflow of water into the well Well development can be undertaken by: OVERPUMPING using a higher rate pump, one that will not be damaged by the sand in the water. MECHANICAL SURGING water is forced to flow in and out of the well screen by raising and lowering a plunger apparatus with in the well casing, using a surge block apparatus attached to PVC pipe. AIR LIFTING/ SURGING by injecting compressed air into the well thus forcing the lift of water and sediment to the surface. For surge action, the injection of air is stopped before the water reaches the surface and the water is allowed to fall in to the borehole. To carry out this, a compressor will be required. Oxfam GB Emergency Drilling Programme Best Practice Manual January

17 The earliest time for undertaking well development should not be earlier than 24 hours after placing the annular seal. Figure..: Schematic Borehole Characterstics Ground Level Sanitary Seal (Concrete/ Bentonite) Rising main SWL Draw down curve Cone of depression Total Borhole Depth H Draw down Blind Casing Dynamic (pumping) Water level v v Pump 1 v v v v v vv v v v v v v Slotted Screen Water Bearing Aquifer 3 v v v Gravel Pack 2 v v v v v Lining Dia. BH Diameter 1 The pump is placed above the slotted screen so that no fine soil/silt is allowed during pumping Oxfam GB Emergency Drilling Programme Best Practice Manual January

18 7.8 TEST PUMPING Definition of common terms related to test pumping of wells Static Water Level (SWL): This is the level at which water stands in a well 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. Pumping Water Level (PWL) or Dynamic Water Level (DWL): This is the level at which water stands in a well when pumping is in progress Draw down: This is a difference measured between the static water level and the pumping water level Maximum allowable draw-down : difference between the water table level and the lowest level it could reach during pumping without causing the well to dry up or the pumping equipment to suffer from lack of water Recovery / Residual Draw-down : After pumping is stopped, the water level rises and approaches the static water level observed before pumping began Well Yield: Yield is the volume of water per unit of time discharged from a well either by pumping or free flow. It is commonly as a pumping rate in cubic meter per day. Specific Capacity: It is a well s yield per unit of drawdown, usually expressed as cubic meters per day per meter (m3/d/m) of drawdown, after a given time has elapsed. Safe Yield: Defined as the highest possible yield that is possible to obtain from a well. At this yield the drawdown at the well will be maintained (in equilibrium) at a level slightly higher than the maximum allowable drawdown. Submersible pump: It is a single or multi-stage centrifugal pump directly connected to an electric motor in a common housing as single unit meant for operating below the water level. Terms that describe the Characteristics of the Aquifer determined by Pumping Radius of Influence, R This is the horizontal distance from the centre of a well to the limit of the cone of depression ( usually larger for confined aquifers) Coefficient of Storage, S Represents the volume of water released from storage or taken into storage, per unit of aquifer storage area per unit change in head. In confined aquifers, S is the same as the specific yield of the aquifer. In confined aquifers, S is the result of compression of the aquifer and expansion of the confined water when the head (pressure) is reduced during pumping. Oxfam GB Emergency Drilling Programme Best Practice Manual January

19 Coefficient of storage is dimensionless, for unconfined aquifers ranges from 0.01 to 0.3 For confined aquifers range from 10-5 to 10-3 Coefficient of Transmissivity, T It is the rate at which water flows through vertical strip of the aquifer 1m wide and extending through the full saturated thickness, under a hydraulic gradient of 1 or 100%. Transmisivity rages from 12.4 m2/day to over 12,400 m2/day. An aquifer with transmissivity of less than 12.4 m2/day produces low yield Why conduct a borehole test pumping Pumping tests are carried out to determine how much ground water can be taken from a well, and to find out the effects of pumping on the aquifer and neighbouring well supplies. The two important coefficients that determine the hydraulic characteristics of a water bearing formation are the coefficient of transmissivity (T) and the coefficient of storage (S). The coefficient of transmisivity indicates how much water will move through the formation, while the coefficient of storage indicates how much water can be pumped. Determining these two coefficients for a particular aquifer will help in the prediction of the well efficiency. These two coefficients are determined by undertaking test pumping of the borehole and using the following two formulae. T = (2.3/π) x (Q/ s) = 0.183Q/ s T = coefficient of transmissivity in m 2 /day Q = pumping rate in m 3 /day s = slope of the time - draw down graph expressed as the change in draw down between any two times on the log scale whose ratio is 10 ( one log cycle) S = (2.25 T t 0 )/r 0 2 S= coefficient of storage T= coefficient of transmissivity m2/day t 0 = time since pumping started in days r 0 = intercept of extended straight line at zero draw down, in m As soon as the drilling work and the lining of a borehole is completed, the yield of the well should be assessed and the maximum allowable draw down for the well is determined, to particularly establish the well s safe yield for long term productivity. Knowing the well s safe yield is very imperative for the selection of the appropriate pumping equipment for the well. The safe yield of a borehole is determined by undertaking borehole test pumping Oxfam GB Emergency Drilling Programme Best Practice Manual January

20 Three are three main types of borehole test pumping, and these are briefly discussed below. The detailed hydraulics for all three different types of test pumping is not covered in this manual and can be referred in the books listed as reference. (i) (ii) (iii) Constant yield test with no observation wells: In this test, water levels are periodically measured at the well itself while water extraction is carried out at a constant pumping rate. After pumping is stopped, recovery water levels are periodically controlled until the original water table level is reached again at the well. Rough estimates of the well s specific capacity and of the aquifer s transmissibility may be obtained through the analysis of this type of test. Constant yield test with observation wells: As in the case above, pumping and recovery levels are measured. Measurements are performed at one or more observation wells whose relative location in respect of the pumping well and the aquifer should be known as accurately as possible. This test is usually performed as aquifer test to obtain, very accurate estimate of the aquifer s transmissibility, its specific yield or storage coefficient as well as estimates of possible interference between adjacent production wells from the analysis of the data. Variable discharge tests or step-draw down tests: are performed by pumping the well during successive periods, usually of one-hour duration, at constant fractions of its full capacity. During the test, water levels in the production well are measured at frequent intervals. Specific capacity determinations are more accurately obtained through these tests, which if properly analysed, also provide very good estimates of the well s efficiency. In simple terms, these tests provide an idea on how construction and design characteristics affect the well s capacity to produce water and may be used to assess techniques and design Equipment required to conduct a Pumping-test : Electric submersible pump of constant discharge, Generator of required power to run the submersible pump, Rising and Discharge pipe of good length to pump water away from the well and avoid recharge during the test, Gate valve in the discharge pipe to adjust the flow, A borehole deep meter, A stop watch to monitor the time accurately, Appropriate pumping sheets, Ensuring that the diameter of the well is large enough to accommodate the test pump and provide enough clearance so that the water level can easily be measured, Example of Constant Yield Tests with no Observation Wells, Ethiopia i) Basic data Borehole total depth: 31 metres Static water level: 6.40 meters Pump position: 28 meters Oxfam GB Emergency Drilling Programme Best Practice Manual January

21 Discharge: 4 litres per second ii) Draw down (Pumping) Test : Shebele Borehole Jijiga Zone Ethiopia ( carried out in 2001) Time after Elapsed time in Depth to the water Draw down in pumping started minutes level in meters meters 1 min min min min min min min min min min min min min min min min min min min min min min min min min min min min min min min min min min min min min min min min iii) Recovery Time after Elapsed time in Depth to the water Recovery pumping stopped minutes level in meters meters 1 min min min min min min in Oxfam GB Emergency Drilling Programme Best Practice Manual January

22 7 min min min min min min min min min min min min min min min min min min Use of Pumping Test Data to determine the Well Specific Capacity and the Optimum Pumping Rate According to Driscoll (Ref. Groundwater and Wells/Second Edition 1986 of which original Edition was published in 1966), the well s specific capacity varies with draw down and is gained by dividing the yield by draw down when each is measured at the same time. Theoretically, maximum specific capacity corresponds to zero draw down because there is no reduction in the saturated thickness. Driscoll also proved that at 67% of the maximum draw down, 90% of the maximum yield is obtained in a well properly developed in unconfined aquifer. If the draw down is greater than 67% of the maximum, it is uneconomical to operate the well. Based on the draw down and recovery tabulation as given in the above example, the specific capacity and optimum pumping rate can be determined as follows: i) Interpretation and Analysis of the results: Borehole total depth: 31 metres Static water level: 6.40 meters Saturated thickness: meters ( BH total depth SWL) Pump position during pump test: 28 meters Discharge: 4 litres per second Drawdown duration: 24 hours Dynamic water level: meters Total draw down: 5.80 meters Total recovered drawdown: 5.40 meters ii) Specific capacity of the well Specific capacity = discharge/ total DD = 4.0 l/sec / 5.8 m = 0.68 l/s/m ii) Optimum pumping rate Percent of recovery within 8 hours after pumping stopped = Total recovered drawdown / Total drawdown at the time of pumping = 5.40 / 5.80 X 100 = 93% (higher than 90% recommended by Driscoll)) Well yield at 67% of draw dawn At 100% drawdown = 4 l/s yield Oxfam GB Emergency Drilling Programme Best Practice Manual January

23 At 67% drawdown (Driscoll) = 4 l/s X67/100 = 2.68 l/s Thus the recommended optimum pumping rate = 2.68 l/s 7.9 Specifying pumps and generator sets Depending on the yield and the test pumping results appropriate pumps have to be selected. The following will be briefly discussed Hand pumps Surface pumps Progressive cavity pumps (mono pumps) Submersible pumps Specifying submersible pump : The choice of the type and size of submersible pumps should take into account the following parameters Site conditions: temperature, altitude and humidity The nature of water to be pumped (clear, sandy, muddy, corrosive etc) The recommended pumping rate based on the results of the borehole pumping tests, The diameter of the borehole at pump setting level, The static and dynamic water levels, The pump setting depth, The length of power cable and rising main required, The total pumping head, Wherever possible, installing pumps which are supplied locally for quick purchase, installation and availability of spare parts, If the pump shroud is required Sizing the electric motor to run the pump For Sizing the electric pump to run the pump the following formula applies: P W = QρgH Pp = P W / η Pp = (QρgH)/η Pp = power output of the pump in watts (pump shaft power or water power) Q = Flow rate of water in l/sec ρ = density of water (1000 kg/m3 or 1 Kg/l) Oxfam GB Emergency Drilling Programme Best Practice Manual January g = gravitational acceleration, 9.81m/s 2 H = operating pressure head against which the pump must discharge η = overall pump efficiency (taken as 70% for direct-coupled transmission engine/pump

24 Example: Calculate the shaft power demand to run a pump delivering 10 l/sec against 20 meters operating head Pp = (QρgH)/η Pp = (10X1X9,81X20)/0.7 = 2803 watts i.e. 2.8 Kw Calculate the electric motor power input of 80% efficiency (three phase cage motor) to run the above pump Mp = Pp/η (Mp is motor input power) Mp = 2.8/0.8 Mp = 3.5 Kw (Mp = Motor power) In practice, the motor should be able to supply about 15% more shaft power than the theoretical requirement to provide a safe margin. Therefore, the motor size should be a minimum of: (3.5X0.15) = = 4Kw Sizing the generator set to run the system (a) Alternator The size and length of cable from the generator up to the pump position in the well will matter due to electrical power lost in the line. The maximum voltage drop at full load should not exceed 4% of the rated supply voltage otherwise a large loss may prevent the motor/pump set from starting. Refer to tables below for maximum recommended cable lengths versus the cross-sectional area of different cables. Thus, using the effective motor power of 3.5 Kw given in section 3, the calculation of the alternator power required is given by the following formula: Assume efficiency of the generator is 80%, Gp = (Mp/η) + p Gp = (3.5/0.8) = 4.37 Gp = (4.37 X 0.04) Gp = Gp1 = 4.54 Kw (Gp1 = Effective power needed by the motor from the Generator) Sizing cable for three phases (3X415 volts) submersible pumps (from Engineering in Emergiencies page: ) Maximum cable length (metres) and size (mm2) to allow max of 4% voltage drop Motor power (Kw) 1.5 mm2 2.5 mm2 4 mm2 6 mm2 10 mm2 16 mm Oxfam GB Emergency Drilling Programme Best Practice Manual January

25 Sizing cable for single phase (1X240 volts) submersible pumps Maximum cable length (metres) and size (mm2) to allow max of 4% voltage drop Motor power (Kw) 1.5 mm2 2.5 mm2 4 mm2 6 mm2 10 mm THERE ARE OTHER FACTORS TO CONSIDER FOR SIZING THE GENERATOR i) Peak load factor (Ppk) The alternator must also be able to cope with both the peak running load (the full load) and the increased load due to starting (starting load). If the alternator is only just big enough to run the system at full load, the extra load on starting may result in the motor/pump set failing to start. Motor type Peak factor Split ring motor 0.9 Squirrel cage motor 0.5 Using the same example given in section 3, the peak power input will be: Ppk = Gp1 X 0.5 Ppk = 4.54 X 0.5 Ppk = 2.27 Kw, then Gp2 = Gp1 + Ppk Gp2 = Gp2 = 6.81 Kw (Gp2 is the Generator power needed to resist the peak load) ii) Motor starting load factor (Ps) The magnitude of the starting load depends on the type of motor and the method of starting. Oxfam GB Emergency Drilling Programme Best Practice Manual January

26 Motor type Starting method Starting load factor (K) Split ring motor Rotor resistance 1.5 Squirrel cage motor Star Delta Electronic soft-start Auto-transformer Direct-On-Line (DOL) Assume the motor/pump starts in DOL mode Ps = Gp2 X K Ps = 6.81 X 7 Ps = Kw, then Gp3 = Gp2 + Ps Gp3 = Gp3 = Kw (a) - (Gp3 is the Generator power needed to resist the peak load & starting load) Assume the motor/pump starts in Star Delta mode Ps = Gp2 X K Ps = 6.81 X 2.5 Ps = Kw, then Gp3 = Gp2 + Ps Gp3 = Gp3 = Kw (b) - (Gp3 is the Generator power needed to resist the peak load & starting load) Interpretation: It can easily be seen that the selection of the starting mode is of great importance. In the above example, using a DOL mode require more than the double power from the alternator (generator) if the same pump was started using Star Delta mode. Selection of the alternator/generator In practice, the alternator should be able to supply about 15% more power than the theoretical requirement to provide a safe margin. Therefore, the total generator s power required should be = (23.82X0.15) Gen tot power = = 27.39Kw Conversion of KW into KVA Generator s power is normally expressed in KVA (Kilo Volts Amps) i.e. Apparent or True Power Power in KW is called Active Power and represented by Pa Pa (in KW) = UXIX1.73Xp.f P (in KVA) = UXIX1.73 Therefore, Pa = P X p.f. P = Pa/p.f. P = 27.39/0.8 (assume power factor is 80%) Oxfam GB Emergency Drilling Programme Best Practice Manual January

27 P = KVA Looking at the standardised generator s rated powers, a generator of KVA may not be available. In that case a generator having a power directly superior to that one will be selected Installation of Submersible pumps Before installing the pump, check the following: - If you have all the necessary tools and material like: sufficient rising main pipes, pipe lifting tools, pipe clamps, tripod/or crane, chain block, pipe wrenches, wire brush, grease, etc - If the voltage, frequency and phases indicated on the pump s nameplate are compatible with those of the generator or the main power supply (if available), - If the insulation of all cables (motor cable, main supply cable) and confirm if they are adequately sized - If the low-level cable and connection to the electrode to ensure continuity - If the borehole has a large diameter all the way to the pump setting depth to accommodate the pump, the riser pipes and the joint between the motor and the the supply cable, Pump installation - Insert the pump, riser pipes and cable into the borehole using the lifting tackle and pipe clamps (never clamp the pump body) - As the pump is lowered into the borehole, attach the supply cable, the dip tube and the low-level cut-out electrode cable to the riser pipe at 3metres intervals using cable clip, - When the pump is submerged, repeat the insulation test between the cores of the supply cable and the earth. - After reaching the pump setting depth, clamp and support the rising main aat the top of the borehole (do not install the pump at the very bottom of the borehole, where water may enter the pump from above/top without flowing past the motor for cooling purpose. If the pump has to be located where the flow over the motor may not occur, such as below the screen, in surface water source or large diameter well, the pumpset must be surrounded by a pump shroud. - Headwork including installation of the pressure gauge, installation of the flow meter installation of a non-return valve and the discharge valve in this order starting from the final rising pipe; - Covering the top of the borehole to prevent the ingress of any contaminants - Connecting the pump set (motor) to the starter and the starter to the power supply from mains or generator, - Installing the earth wire, - Adjusting/setting the motor protection system in the starter/control panel and generator dash board (overload relays, circuit breakers, fuses) appropriately, - Install a pipe to enable to measure/monitor the water level (swl and dwl) of the borehole with a dipper tape. Operating/Testing the borehole pump installation - Before connecting the pressure mains to the borehole rising pipes, start the pump, measure the flow rate, to adjust the pressure through throttling the discharge valve and Oxfam GB Emergency Drilling Programme Best Practice Manual January