No. 200, October 2002
http://www.ext.nodak.edu/extnews/spouts
Mark Your Calendar for these Irrigation Workshops
Park Your Center Pivot in the Right Direction
Why Well Development is Important
Nov. 26, Tuesday Carrington Research Extension Center
This workshop is for new and potential irrigators. Topics will focus on the basics of getting into irrigation including water sources, water and soil quality, water permits, financing options, irrigation economics, basic water management and an introduction to common irrigation equipment.
Dec. 5, Thursday Best Western Ramkota Hotel,
Bismarck(formerly Radisson Inn)
This workshop is for experienced irrigators and will cover improved irrigation management options as well as developments in irrigation.
The North Dakota Water Users annual convention is scheduled for Dec. 4 and 5 at the Best Western Ramkota Hotel, Bismarck. An irrigation workshop sponsored by the Missouri Slope Irrigation Development Association (MSIDA), the NDSU Extension Service and the N.D. Water users will be held in conjunction with the convention. As part of the convention there will be an exposition where irrigation suppliers demonstrate their products and services.
More information about the workshops will be mailed in November. An application for Certified Crop Advisor (CCA) CEUs will be made for each workshop. If you have any suggestions for workshop topics, please give me a call or send an e-mail or letter.
Tom Scherer, (701) 231-7239
NDSU Extension Agricultural Engineer
tscherer@ndsuext.nodak.edu
When it comes to wind and sleet, the center pivot is a rather fragile machine. Center pivots that have no nearby protection such as windbreaks should be parked for the winter pointing to the northwest or southeast, not to the northeast or southwest, Figure 1. The ice storm and blizzard of April 5-8, 1997 damaged many center pivots that were parked pointing in the wrong direction.
Figure 1. For the winter, point your pivot
towards the northwest or southwest, not
the northeast or southwest.
From October to April the worst storms and highest
winds come from the northwest. Properly parking a pivot will
present the smallest surface area to the wind. Make an exception to
this guideline for center pivots that border windbreaks. In this
case, the pivot should be parked next to the windbreak.
While you are parking your pivot for the winter, do the following checks to save some work in the spring.
Flow meters appear to be equipment that many irrigators don't use, don't repair and constantly overlook when managing their irrigation systems. It is common to visit an irrigation pumping plant and find the flow meter not working. Often it has been inoperable for many years. North Dakota winters are hard on flow meters. The freeze/thaw cycles quickly cause the bearings and other moving parts to wear out.
Flow meters and pressure gages provide extremely valuable management information. Having an accurate measure of flow rate is important for proper chemigation, selection and modification of sprinkler nozzles, calculating the application rate of the pivot, checking the production of the well and tracking the performance of the pump.
Before the irrigation season starts, establish a program to fix or replace the flow meters on your wells. If your flow meter is working properly, consider removing the flow meter this fall and storing it in a warm place for the winter. It takes about 15 minutes to remove the flow meter and cover the hole in the pipe with a piece of tin. If you take care of your flow meter it will last a long time and provide accurate information on the performance of your irrigation system.
Tom Scherer, (701) 237-7239
Extension Agricultural Engineer
tscherer@ndsuext.nodak.edu
I often hear irrigators say they don't want to pay for the cost of well development because it doesn't help well performance. If the well isn't developed properly, that might be true. However, if the well is properly developed at the time of construction, it will have less drawdown, save on pumping costs and have less trouble supplying water during high water-use periods.
A well provides access to the aquifer where the water is located. During pumping, if a well is functioning properly, the water from the aquifer will enter the well screen with the lowest amount of restriction possible. Anything that restricts the flow of water into the well can affect energy costs and flow rate by increasing the drawdown.
The drawdown is the difference between the static water level and the pumping water level, Figure 1. From a hydraulic point of view, drawdown is the head (pressure) required for water to flow into the well. The greatest amount of drawdown occurs within a few feet of the well where the velocity is the greatest.
Figure 1. Side view of the flow into a well.
Most irrigation wells are constructed with a rotary drilling
rig that uses a high viscosity fluid (often called a "drilling mud")
to keep the borehole open during the drilling process.
Although necessary, the drilling mud seals the borehole and
often penetrates into the surrounding aquifer formation. The
rotary drilling process also smears the borehole surface,
compacting the natural material around the borehole. The most
common drilling mud uses bentonite, a naturally occurring clay mined
in Montana and Wyoming. If left in place after construction,
the drilling mud will seal part of the aquifer and increase
the drawdown.
After the casing and screen are set in the borehole, it is very important to remove the drilling mud left in the aquifer formation, Figure 2. This operation is even more important with gravel-packed wells because the gravel pack is a barrier to removing the drilling mud. There are several methods for developing a well. They are listed in order (least first) of effectiveness and cost.
Figure 2. Side view of the materials just outside the
well screen for a naturally developed and gravel pack well.
Air lift pumping forces compressed air through an air line
to the bottom of the well, Figure 3. As air bubbles rise, they
create a surging effect that carries water and fines out of the well.
Air lift pumping is alternated with short periods of no
pumping, which forces water out into the formation to help break up
sand bridging around the screen. Well development is only
effective if the water is deep enough in the well to get the surging
action. Air lifting does not work if lift to the surface is too great.
Figure 3. Well development with air lift pumping and agitation.
Surging alternately forces water into and out of the
formation through the well screen openings, Figure 4. A piston-like
tool moves up and down in the well to create the surging action. The surging of the water through the well screen loosens
the mud and fines in the borehole and draws them into the
well to be removed by pumping or bailing. Surging is especially
well-suited to cable tool drilling. While common for bridge
or louvered well screens, surging is not very effective with
very deep wells (over 200 feet) or those with multiple screens.
Figure 4. Well development by mechanical surging with a surge block.
The best well development method is high-pressure water jetting with simultaneous air lift pumping, Figure 5. High-velocity water jets through the screen and gravel pack into the formation to loosen and break down fine materials. The jetting tool rotates slowly as it is moved up and down inside the well screen. Air lift pumping removes the loosened sand and mud as they enter the well screen. The jet stream can be directed at any part of the formation around the well for selective development. Cage wound screen is best for jetting because its design allows the jet to impinge directly on the gravel pack or borehole. Well screens that use louvered or bridge openings do not respond to this type of development because the opening design interferes with the jet of water. Jetting is often the most costly development method.
Figure 5. Well development using high velocity water jetting.
In the late 1970s, a well development research project was conducted at a research farm in Staples, Minn. The project had 10 wells constructed using different types of screens, drilling fluids and design parameters. After construction, each of the wells was developed first by over pumping. Then they were test pumped for 24 hours. Then each was developed by mechanical surging followed by another 24 hour test pump at the same flow rate as the first test pump. Then each was developed with water jetting and another 24-hour test pump was performed.
The results for the properly constructed wells showed consistent improvement. For example, well 5 was pumped at 183 gallons per minute (gpm) for all three 24-hour pumps tests. The drawdown after over pumping was 12.1 feet, after surging it was 7.2 feet and after jetting it was 6.7 feet. Development cut the drawdown almost in half.
As water flows into a well, it carries minerals with it. These minerals can build up on the formation materials near the well screen because that is where the water velocity is the greatest. It may sound counter-intuitive but some minerals will precipitate at the high velocities. Over time, the deposition of minerals can encrust the screen and formation, increasing the resistance to flow of water into the well. By adding a weak acid to the well and agitating the resulting solution, these mineral deposits can be dissolved and removed. By combining water jetting with acid, the well can be redeveloped and often can be brought back to almost-new levels of production. Caution: This method should only be used on stainless steel and plastic screen. Some older, iron screens are susceptible to acid and may collapse.
Tom Scherer, (701) 231-7239
NDSU Extension Agricultural Engineer
tscherer@ndsuext.nodak.edu
Water Spouts, No. 200, October 2002
NDSU Extension Service, North Dakota State University of Agriculture and Applied Science, and U.S. Department of Agriculture cooperating. Sharon D. Anderson, Director, Fargo, North Dakota. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. We offer our programs and facilities to all persons regardless of race, color, national origin, religion, sex, disability, age, Vietnam era veterans status, or sexual orientation; and are an equal opportunity employer. This publication will be made available in alternative formats for people with disabilities upon request, 701/231-7881.