9.4.4: Well Construction
- Page ID
- 44596
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)In constructing and developing a successful well, several features require careful consideration. These features include the method of drilling, well alignment, depth of well, casing material, casing perforations, gravel packing, well development, and well testing. Driscoll (1986) presents significant detail on well design and specifications, as well as well drilling methods. The specifications for wells and the certification of well drillers is often regulated by states in the U.S. Also, it is not uncommon for states to regulate the spacing of wells to prevent interference with neighboring wells.
Most irrigation wells are drilled with cable or rotary tools. With cable tools, a heavy bit is repeatedly dropped onto material at the bottom of the well. Crushed material is removed periodically with a bailer. Wells up to 5,000 feet deep have been drilled with a cable tool. The most common method of drilling, however, is by rotary tools. A bit is rotated by a drilling pipe and a mud slurry is pumped through the pipe to the bit to carry cuttings up the outside of the pipe to the soil surface.
An irrigation well should normally penetrate the water-bearing formations as deeply as possible. The deeper well will usually provide a greater yield of water per foot of drawdown of the water table. It is imperative that the well is vertical for the installation and replacement of pumps. Particularly for wells that have a gravel pack, misalignment because of gravel wedging somewhere below the surface causes the casing to be pushed out of alignment. With proper drilling, well development, and maintenance, a well should last several decades.
Casing materials for wells include steel, wrought iron, concrete, plastic, and fiberglass. Many states have specifications for casing depending on the type of well. Well casings are perforated, slotted, or screened near the bottom of the well to facilitate entering groundwater. Properly sized perforations prevent subterranean particles from flowing into the well with the water.
Wells drilled in unconsolidated material with rotary drills are usually gravel packed. The selection of the right gravel material is crucial to prevent particles from moving into the well. The gravel needs to be large enough to permit sufficient water flow but with small size pore spaces to prevent solids from moving. The gravel packing material cannot be more than five times the average size of the substrata material if the well is to be stabilized. With these size restrictions, many times a specially manufactured well screen is used rather than the gravel pack. Screens are constructed of brass, bronze, galvanized steel, stainless steel, plastic, or fiberglass to resist corrosion.
Immediately after a well is constructed, it is normally “developed.” The purpose of well development is to make the well sand-free and maximize the flow of water from the aquifer. To prevent pump damage, materials like clay, drilling mud, silt, and sand are removed from the vicinity of the well casing that is screened. There are several methods to develop a well. They are all designed to loosen fine particles so they can be pumped from the well before the permanent pump is installed. The pump used during well development is designed specially to be tolerant of fine particles. The most common method to develop a well is surging. The pump is turned on and then off to allow water to surge back into the well thereby drawing fine materials into the well to be removed by pumping during a repeat of the surge cycle. Another surging technique uses a surge block. The surge block is a tool fastened to the end of the drill. As the drill is moved up and down, it produces a pumping action to draw fine particles into the well.
After the well is developed, a temporary pump is installed for a pumping test. During the pumping test the flow rate (discharge) and drawdown are measured simultaneously. This information is required to select the proper size of pump. Sterrett (2007) provides a practical reference for planning and installation of water wells.