WATER WELL SCREEN & CASING

Transcription

1 WATER WELL SCREEN & CASING

2 Water Well Design Well design is the process of specifying the physical materials and dimensions for a well. The principal objectives of good design should insure the following: The highest yield with minimum drawdown consistent with aquifer capability Good quality water with proper protection from contamination Water that remains sand free A well that has a long life (25 years or more) Reasonable short term and long term costs Well casing Design Choosing the proper casing diameter for the well is important because it may significantly affect the cost of the structure, depending on the type of drilling equipment used. The diameter must be chosen to satisfy two requirements: 1- The casing must be large enough to accommodate the pump, with enough clearance for Installation and efficient operation 2- The diameter of the casing must be sufficient to assure that the up hole velocity is 5 ft/sec (1.5 m/sec) or less. Anticipated Well yield (m³/day) Nominal Size of pump bowls (mm) Optimum size of well casing (mm) Smallest size of well casing (mm) Less than ID 125 ID 409 to ID 150 ID 818 to ID 200 ID 1640 to ID 250 ID 2730 to OD 300 ID 4360 to OD 350 OD 6540 to OD 400 OD to OD 500 OD to OD 600 OD

3 Casing for water wells is constructed from various types of fiberglass-reinforced plastic materials. Fiberglass casing is resistant to most forms of corrosion, is not conductive, and for its weight has the strength of steel. Casing must have: Collapse strength The following collapse formula provides the user practical tools to estimate collapse strengths of various types of casing and screen. It is important that one pays particular attention to the material and manufacturing standards for the casing and screen specified so that acceptable tolerances are considered and adequate safety factors applied. Timoshenko's Formula, shown below, is commonly accepted to be the most accurate method available for estimating the collapse strength of pipe with diameter/thickness ratios common to those used in the construction of water wells. Where: Pcr = Theoretical collapse strength of a round tube Where: Pe = Collapse pressure with ellipticity E = Young's Modulus u = Poisson's Ratio Do = Outside diameter of casing t = Wall thickness S = Yield strength e = ellipticity, frequently assumed as 0.01

4 Tensile strength The axial tension loads imparted to the casing string during installation come primarily from the weight of the suspended pipe and screen, and also from the additional drag forces that could be transferred to the string assembly during gravel packing. The weight of a single diameter casing assembly is easily calculated using equation below: Where: Wt: Total weight casing t: Wall thickness of pipe D: Outside diameter of pipe L: total length of pipe k: 2.7 for US. Customary & 0.62 SI = An equation commonly used for determining casing tensile strength is shown below (AWWA Standard for Water Wells, ANSI/AWWA A100-97, Appendix K, K.3) = 2000 Where: Cts : Casing tensile strength Do: Casing outside diameter St: Ultimate tensile strength t: Casing wall thickness

5 Column strength If the downward compressive force on a laterally unsupported casing assembly exceeds the yield strength of the material, then the casing will buckle. The axial compressive (column) strength of casing can be calculated using equation below (which also is published in the AWWA standard for water Wells, ANSI/AWWA A100-97, Appendix K, K.3). = 2000 Where: Cas: Casing axial compressive strength Srp: Material yield strength Do: Casing outside diameter t: Casing wall thickness 6

6 Well casing joint Bell and Spigot Lock Joint The Bell and Spigot Joint with 2 O-rings and Locking Key is the optimal solution for the water wells field. The bell is integral at one end of the pipe and the spigot is the other end. The hydraulic sealing is performed by means of two rubber O-rings. The mechanical coupling is performed by a Locking Key that is inserted through the bell opening into a groove. The joining system is very easy and can reduce installation s time up to 50%. While drilling through the float shoe and the cement, the casing is subject to a rotational force. This could result in a relative rotation of two consecutive sections of casings. To avoid this, the joint can be manufactured upon request with a special Anti-Rotational configuration. 8

7 Well Screen Design Waverail manufactures two types of GRP screens: Cut Slot Screens Cut slot screens are produced by cutting directly the GRP pipe wall. The cut slots are in staggered rows to reduce the negative effect on the screen s strength. The slot sizes are between 1 and 3.5 mm and can give open areas ranging from 5% to 15%. As compared to PVC slotted screens, GRP cut slots screens have much lower wall thickness and therefore have less clogging problems. These screens are recommended for: - Consolidated formations or formations with large particles sizes. - Shallow and medium-depth wells. Pipe Based Screens Pipe-based screens are used in water wells in many part of the world because of their strength and durability. The assembly for this screen is made by perforating a base pipe and then mounting a continuous-slot screen over it by slipping a separate continuous-slot well screen jacket over a pipe (screen jacket). The pipe based screen has two sets of openings, the outer continuous slot and the holes drilled in the pipe base. The open area of the pipe base generally is less the outer screen open area, and the inner pipe has a maximum open area of 10%. Hydraulic performance of the assembly depends on the open area in the pipe base.

8 The external GRP screen jacket is obtained by a special injection molding technique which gives smooth and tapered slots. The slot openings are horizontal and V-shaped inwards to prevent clogging. The wires have non-modified triangular shape with round edges to guarantee laminar flow.

9 Continuous slot screen design The first step in selection of slot sizes for naturally and filter packed well is conducting a grain size analysis.

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12 Screen strength Collapse strength of continuous slot screen is dependent on diameter of the screen, size, shape, and material type of wire used in its manufacture, and the slot size. The following formula can be used to estimate the collapse strength of continuous slot screen. Where: I = Moment of Inertia = 24 + = h! + 4 "+"! 36 "+ Where: P cs = Collapse pressure of continuous slot screen E = Young's Modulus w = Width of wire on external face s = Slot width of screen D = Mean diameter of the screen h = Wire height b = Width of minor base of wire

13 Area/Flow Calculations Well Production at AWWA Standard of 1.5 feet per second: To calculate the flow in gallons per minute per foot of screen, first you must calculate the percentage of open area of the screen, then use that result to calculate the flow. Determine the open area for the type of screen used then convert area to a percentage. Calculation of open area for continuous slot screen: Where: A = Open Area s = Slot size w = Wire width % = & ) & + * + 12) Do = Average outside diameter of screen d = Diameter of rods n = Number of rods Calculation of the flow in gallons per minute per foot of screen: Where: F = Flow - =4.8 % 12 Ap = Open Area (percent %) Do = Average outside diameter of screen = pi or