Website: fayette-tx.tamu.edu. WATER WELLS. C. Wayne Keese*. The design and
construction of high-capacity water wells, whether they provide water for.
Fayette County 254 N Jefferson Rm 900 La Grange TX 78945 Ph: 979-968-5831 Fax: 979-968-5295 E-mail:
[email protected] Website: fayette-tx.tamu.edu
WATER WELLS C. Wayne Keese* The design and construction of high-capacity water wells, whether they provide water for irrigation, fish farming or some other purpose, is important to overall efficiency. Careful construction will increase the life of the well and reduce pumping costs. Selection of appropriate pumping equipment for a specific well also helps lower water production costs. Definition of Terms and General Principles Static water level is the level (distance from the surface of the ground to the water) of the water in the well when the well is not being pumped. Pumping water level is the level of the water in the well when the well is being pumped. Drawdown is the difference between the static water level and the pumping water level. Pumping water from a well always produces drawdown, which increases as the pumping rate increases. Drawdown is approximately proportional to the pumping rate up to a point, after which drawdown increases more rapidly than pumping rate or well yield. With non-artesian wells (water table wells or wells in which the static water level does not rise above the top of the water-producing strata) it is seldom practical to pump with a drawdown greater than 70 percent of the maximum possible drawdown. Such wells produce, on the average about 92 percent of the maximum yield when the water level is at 70 percent of its original depth. To produce the remaining 8 percent of the possible yield of the well requires an additional 30 percent drawdown. In most cases, the extra pumping cost is out of proportion to the increased yield. Specific capacity is the ratio of well yield to drawdown, and is usually expressed as gallons per minute (gpm) per foot of drawdown. Measuring specific capacity allows the operator to compare the performance or efficiency of different wells. The specific capacity of a well is determined by aquifer characteristics, but well construction can adversely affect the specific capacity of a particular well. Total dynamic head is a measure of the energy added to the water by the pump. It is the sum of: 1) the static water level and drawdown (pumping level or “pumping lift”); 2) the head loss of friction in the pump column; and 3) any pressure or “head” added at the surface to push the water through a pipeline, move it to a reservoir at a higher elevation, etc. Total dynamic had is one of the values needed to select an efficient pump. The effect of well diameter on well yield is illustrated in the table below. Many people incorrectly assume that well yield is doubled if the well diameter is doubled. The table shows the theoretical increase in yield that would result from enlarging the well diameter, all other factors remaining the same. Educational programs of Texas Cooperative Extension are open to all citizens without regard to race, color, sex, disability, religion, age, or national origin. The Texas A&M University System, U.S. Department of Agriculture, and the County Commissioners Courts of Texas Cooperating.
35” 48” 135 143 121 128 111 120 108 110 100 106 *Extension Agricultural Engineer, Texas Cooperative Extension, The Texas A&M University System, College Station, Texas 77843 Yield Ratios
6” 100
Well Diameters 12” 18” 111 119 100 107 100
24” 125 112 105 100
Well yield, as well as specific capacity, increases approximately 10 to 12 percent when the well diameter is doubled. Depending upon the specific situation and the cost involved, it may be worthwhile to increase the well diameter to get more water. In may cases, the minimum well diameter is governed by the space required to accommodate a pump that will produce the desired yield. Recommended casing diameter (at least down to the point that the pump will be set) is two sizes (inches) larger than the pump bowl size. The table below shows the nominal pump bowl size and casting size for different pumping rates. Pumping Rate (gpm) Nominal Pump Optimum Casing Smallest Casing Size Bowl Size (inches) Size (inches) (inches) Less than 75 4 6 ID 5 ID 75 to 175 5 8 ID 6 ID 150 to 400 6 10 ID 8 ID 350 to 650 8 12 ID 10 ID 600 to 900 10 14 OD 12 ID 850 to 1300 12 16 OD 14 OD 1200 to 1800 14 20 OD 16 OD 1600 to 3000 16 24 OD 20 OD Drilling Although groundwater is available in most areas of the state, thee is no assurance that a well yielding a specific amount of water can be completed at a specific site. General information about the availability of ground water in a certain area may be obtained from drillers familiar with the area, from neighbors with operating wells or from reports issued by the Texas Department of Water Resources, P.O. Box 13087, Austin, Texas, 78711. Test holes often are drilled to determine the potential productivity of a well in a specific location. Samples of the aquifer material may be collected during test drilling to determine screen length, screen slot size and gravel pack size, if gravel pack will be used. A water sample can be analyzed to determined whether the water is of suitable quality. A rough estimate of well yield can be made from information gathered during test drilling. Common methods of drilling large-capacity wells are rotary, reverse rotary, and percussion rotary. Each method has advantages and disadvantages. The object in drilling is to obtain a straight, plumb hole of the appropriate size and depth to produce water from the aquifer. Unless the aquifer is very thick or contains water of unacceptable quality in the lower portions, wells are usually drilled to penetrate the total thickness of the aquifer. Casing and screen are set to keep the bore hole open and allow free passage of water into the well (except possibly in consolidated formations such as limestone). In non-artesian (unconfined),
homogeneous aquifers, screening the bottom third of the aquifer will usually provide optimum yield. Screening of more than the bottom 40 percent is seldom of any value. In homogeneous formations, the pumping water level should not be below the top of the screen. Screen above the pumping water level serves no purpose. For artesian (confined), homogeneous aquifers, 70 to 80 percent of the aquifer may be screened. For stratified (non-homogeneous) aquifers, permeable strata should be screened with blank casing between screen sections. In any case, screen should be selected to retain 40 to 50 percent of the aquifer material. Where such a requirement results in slot openings of less than 0.01 inch, a gravel pack may allow larger slot size while still retaining formation sand. Generally speaking, the 70 percent retained size of the gravel pack should be four to six times the 70 percent retained size of the aquifer material. Screen should provide the maximum possible open are per square foot of screen surface in order to reduce inlet velocity and drawdown. Continuous screen costs more than slotted pipe but provides the greatest open area for any size slot opening. Screen slot openings should be V-shaped and widen inwardly to reduce clogging. Development Proper well development, which includes cleaning the drilling mud, clay and fine sand from the aquifer formation around the screen, is an important part of the well construction process. Proper development increases the yield of the well and reduces “sand pumping.” It should result in increased yield, longer well and pump life and reduced maintenance. The process of designing, constructing and developing a well is not complete until the water is sand free and the well yields the required water as efficiently as possible. Surging is a common development technique which involves raising and lowering a “surging block” in the well to alternately force water in and out of the formation. Mud and sand moved into the well during the process are removed by pumping (not with the new pump to be installed in the well) or bailing. Air-lift pumping is another common development method. A large volume of compressed air is released from an airplane at the bottom of the well. As the air rises it carries water and fines out of the well. Alternate periods of pumping and not pumping creates a surge effect which helps clean the formation. Jetting with high-pressure water is one of the most effective development methods. Water is jetted at high velocity from the jetting tool through the screen and into the formation to loosen and break down the fine materials. The tool is moved up and down inside the screen and rotated to achieve complete development. Jetting and air-lift pumping can be used simultaneously to remove loosened sand and mud as they enter the well.
Well Testing
The well should be test-pumped to determine the yield-drawdown relationship. This information is needed in order to select a pump and power unit that will operate efficiently with minimum cost. The well driller, or a well or pump service company, should do the test. Begin the test by measuring and recording the static water level. Then pump the well at least enough to draw the water level down to the top of the screen. The pumping rate required to do this is the maximum rate at which the well should be pumped. The longer the well is pumped at this rate, the more accurate the test. Adjust the pumping rate so that the water level does not go below the top of the screen. After pumping at this rate continuously for 24 hours, measure the pumping rate and the pumping water level and record both values. Reduce the pumping rate to about 80 percent of the “maximum” discharge rate. After the discharge rate and drawdown have remained steady for 30 minutes, record these measurements. Repeat the process at a pumping rate of about 60 to 40 percent of the ‘maximum.’ Record the data. With the information gained during the pumping test-well yield corresponding pumping water level-the final pumping rate can be selected system design can be completed, and the total dynamic head which must be generated by the pump can be determined. You can then go to your pump dealer and purchase a pump to operate at high efficiency in your well (turbine pumps can operate at efficiencies of 75 to 80 percent.) Disinfect the Well The driller should completely disinfect the well to destroy any bacterial contamination introduced during the drilling process. For example, iron bacteria are often transmitted by drilling tools. Iron bacteria do not cause disease but can plug the well screen and aquifer formation. A chlorine solution of approximately 100 parts per million mixed thoroughly in the water in the well and left overnight will disinfect the well.
