Water Well Construction
There are three general construction styles in the Garber-Wellington aquifer for municipal wells in Central Oklahoma – perforated and cemented, screened and gravel packed, and torch cut slot wells.
Screened and Perforated Wells
Any well construction involves drilling a borehole into the aquifer. In screened and perforated wells, the borehole is cut to total depth before casing is installed. It is advisable to run an open hole geophysical log to define the water producing zones. The casing is installed after the geophysical logs are run. Location of the screened casing is defined from the geo-physical log in screened and gravel-packed wells. Then the casing is assembled, welded together and placed in the borehole.
After casing installation, the void between the casing and the well bore (annulus) is filled with coarse sand or gravel. The purpose of the gravel pack is to restrict aquifer material from entering the well during water production. Drolleries can place cement or mud grout over undesirable zones encountered in well bores for screened wells. Solid casing, not screened casing, is located adjacent to these undesirable zones. It is very difficult to grout between the screened intervals in deep wells. Therefore, most wells over a few hundred feet deep are gravel-packed from the bottom to the top of the aquifer
State law requires that at least the top ten feet of a well be cemented to seal the well from possible surface water contamination.
Perforated and Cemented Wells
Perforated and cemented (“oil field” style) wells are different from screened wells. Upon completion of the borehole, solid heavy-duty steel casing is installed. This solid casing in a perforated well is then sealed in the borehole with cement. The cement completely fills the annulus and seals the well bore from the aquifer. Five to seven days are required for the cement to properly cure. Then holes are placed through the casing and cement and penetrate only into the good water zones encountered in the well bore.
The most effective perforation procedure is the use of shaped charges. The shaped charges produce a high temperature energy burst that penetrates steel casing, cement and aquifer rocks. Perforated wells are recommended for deep hard rock aquifers (Garber-Wellington, Vamoosa-Ada, Antlers, and Chickasha Sandstones) while only screened wells can be installed in soft sediment, alluvial terrace aquifers.
Torch-Cut Slot Wells
Torch-cut slot wells are a more primitive and ancient design than the other well construction styles. Most torch cut slot wells were installed in central Oklahoma from statehood to the mid 1950’s. This construction style usually produces a telescoping casing string. An initial large diameter borehole is drilled 20-400 feet in depth. Shallow casing is installed in this large initial borehole that sometimes extends as deep as the top of the aquifer. It is cemented in place like the casing in a perforated well. A smaller borehole below this first casing then penetrates the water-producing portion of the aquifer.
After determining the depth of the water producing zones using open hole geophysical logs, casing is assembled at the surface. A cutting torch (or chisel-cut in very old wells) is used to cut slots in the casing adjacent to the water zones. This second string of casing is lowered into place in the smaller borehole. The casing is designed to extend from the base of the cemented casing to the bottom of the second borehole. Drilling of smaller bore holes and installation of smaller diameter casing can continue until the well has four to six casing size reductions.
In torch cut slot wells there is nothing behind the casing to control the shales and sandstones in the aquifer. A properly constructed 800-foot deep perforated well will have less than two square feet of rock face exposed to the well bore, while an 800-foot deep, torch-cut slot well can have over 1000 square feet of rock face exposed in the well (these rock surface volumes were determined on 12 1/4-inch boreholes). Torch-cut wells can produce excellent water for a century. However, the delicate nature of the vast extent of open rock face requires careful planning prior to well maintenance.
John Harrington, P.G., CFM
Director, Water Resources Division
Most of the operational domestic wells are screened and gravel pack construction. Eight and one-half to 12-inch boreholes are drilled. Casing installation is usually a matter of driller’s preference and experience in the area – rarely are samples taken or a well log run. A typical well installation diagram is shown below.
The wells completed in alluvial sediments are 30-60 feet deep and are cased with five- to six-inch PVC casing. The bottom ten to 20 feet of the casing is slotted screen PVC. The remaining casing is solid. The well is usually gravel packed with 5/32-inch washed river gravel (pea gravel). The top ten feet of the casing is sealed with cement to reduce pollution from surface waters. These wells commonly yield 25-75 gpm. Most of these wells penetrate the entire Quaternary aquifer. The driller stops when encountering the underlying Permian redbeds.
The domestic wells completed in the Garber-Wellington Aquifer are much more varied in depth and construction. Most wells are 100-500 feet deep and cased with five- to seven- inch steel casing. The bottom 25-200 feet of the casing is slotted. The entire casing except the top ten feet is gravel packed with 15-20 to 30-40 “Colorado Frac Sand”. The top ten feet of the casing must be cemented to reduce surface water pollution. These wells yield 10-100 gpm. A vast majority of these domestic wells only penetrate the upper portion of the aquifer.
If the well is to be used for any purpose other than domestic use as defined by OWRB rule, the landowner must file a groundwater use permit with the OWRB before the contactor can complete any work (other than test drilling) (OWRB, 1999).
Construction techniques of irrigation wells are quite variable. Wells completed in alluvial sediments are 20-65 feet deep. They are screened wells cased with five- to eight- inch PVC or steel casing. These alluvial wells are completed in 9 to 12 inch boreholes. The bottom 10 to 20 feet of the casing is slotted screen and the remaining casing is solid. Such wells are usually gravel packed with 5/32- inch washed river gravel (pea gravel) or 10-20 “Colorado Frac Sand”.
The top 10 feet of the casing is sealed with cement to reduce pollution from surface waters. These wells yield 25-400 gpm. Most of these wells penetrate the en-tire Quaternary aquifer, which is usually only 60-100 feet. The driller stops when encountering the underlying Permian redbeds.
There are two construction styles found in Garber-Wellington irrigation wells. Most irrigation wells are screened and constructed like the alluvial wells. The only difference is that many of these wells have screens adjacent to every sandstone exposed in the well bore. Depths on such wells range from less than 100 feet to 530 feet. There are also a few cemented and perforated irrigation wells.
Plugging Abandoned Wells
If you abandon your well, you are required by state law to properly plug the well and ensure that it is completely sealed to prevent potential contaminants for entering the hole and infiltrating the underlying or surrounding groundwater formation. In addition, if you have an abandoned well on your property, you must ensure that it has been properly sealed to prevent pollution of your existing well or your neighbor’s wells (OWRB, 1999).