![]() ![]() Corrosion by-products also can cover portions of the screen and cause higher entrance velocities through the remaining open area, thus increasing head loss across the screen.Ĭhemical Encrustation Chemical encrustation is the deposition of minerals on the well screen or gravel pack which act to restrict the movement of water into a well. The corrosion of well casings or screens can cause holes to develop in a casing and cause the screen slot size to increase, allowing sand and/or gravel pack to enter the well. The migration of fine materials acts to reduce the effective porosity and restrict water flow, increases head loss in the immediate vicinity of the well, and often results in sand pumping. These same materials may not migrate at pumping rates that maintain laminar flow throughout the formation. Overpumping a well will cause turbulent flow in the formation near the well screen and promote the movement of fine-grained materials. Also, a well screen could be placed opposite layers of sand that are significantly smaller or more graded in particle size than the other aquifer materials. An improper well design may include the selection of a gravel pack, which is too large to effectively filter fine formation materials, or selection of an inappropriate screen slot size. The movement of fine particles typically is caused by improper well design or by overpumping a well. The second type is caused by corrosion by-products of the metal portions of the well, which act to cover the openings of a well screen. The first type involves the movement of fine-grained soil materials from the natural formation to the borehole face or the face of the screen. Mechanical Blockage There are two types of mechanical blockage, which can act to restrict the movement of ground water into a well. The most frequently encountered problems are discussed below. If well rehabilitation is attempted, the proper technique must be selected. If redrilling is chosen, the previous design and construction methods should be evaluated and improved upon, if possible. Whatever the solution, it is important to identify and understand the cause of well failure. However, the performance of many wells experiencing these problems can be improved by well rehabilitation techniques. Redrilling of a problem well sometimes is the most cost-effective approach. In the past, loss of well yield and/or sand pumping often was tolerated until continued operation of the well was no longer economical, and the well then was redrilled. Factors 3, 4 and 5 can be limited by proper well design, construction and operation, and are the primary topics of this paper. Pump problems can be identified by inspection and/or a properly conducted step-drawdown test. Static water levels should be monitored over time to detect trends, so that loss of yield caused by regional static water level declines are not mistaken for well maintenance problems.
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