25 JUNE 2024 WorldWide Drilling Resource® Well Efficiencies with Water Well Screens Adapted from Information by Johnson Screens Efficiency has always been a major concern in the water well industry, but when it comes to well efficiency, it is often ignored due to it being difficult to quantify. There are thousands of irrigation wells operating today with efficiencies as low as 35-50%. If they would have been properly designed and constructed, they could have been operating at efficiencies of 80% or better. When a pump is turned on, lower pressure in the vicinity of the well causes water to flow toward it from the surrounding formation. The difference between the pumping water level in the well and the static water level is called drawdown. Since this diminishes with distance from the well, a cone shape is produced called the cone of depression. During a pump test, the well is pumped for several hours at a constant rate to determine drawdown in the well pump and the observation wells. The information is plotted and the shape of the cone of depression is determined. The pumping water level is obtained and the theoretical drawdown is calculated. Well efficiency is calculated by dividing the theoretical drawdown obtained graphically by the actual drawdown measured in the well. The most important factor in producing an efficient well is the screen design. The two basic purposes when designing a well screen should be to permit unobstructed entry of water in the well and allow access to the formation for development procedures. To serve these purposes, the opening should be uniformly arranged so water flowing through the horizontal lenses of the formation can enter the screen directly and the entire formation can be reached during development. The most dominant screen type used in the water well industry would be the continuous slot wire wrap screen because the spacing of individual slots can be varied during fabrication, which helps reduce drawdown. Well operational cost is directly related to drawdown, and continuous slot screens have the least drawdown during pumping. The screen type and design (diameter, length, slot size, and location in the aquifer), graduation of gravel pack, and borehole diameter affect the critically important ability to develop the well. Together, these determine the efficiency of the well as a hydraulic structure. Efficient wells will reduce operating cost over the life of the well, where inefficient wells require more drawdown to produce the same volume of water, which significantly increases the pumping costs. The principle causes of maintenance expenses and short life are corrosion, incrustation, and sand pumping. However, if a well is correctly designed and constructed, it will reduce these destructive factors to a minimum level. The extra initial investment for an efficient well will be returned many times and it all starts with the selection of the proper screen. WTR
RkJQdWJsaXNoZXIy NDk4Mzk=