It happens. The new screened well is pumping too much sand, and you have the hopeless feeling that the screen slot size selected was just a little too wide to hold back the sand from the formation.

Besides abandoning the well, there are two choices to remedy the problem. First, you could continue to develop the well aggressively by surging and pumping and hope that the majority of the small grains will pass through the screen and leave behind enough larger grains to seal off the flow of the sand from the formation. The key is to aggressively purge the well by forcing water in and out of the screen in order to “shuffle” the smaller grains toward the screen and into the well for removal during its development. Development could take hours, or days in some cases, to remove the fines near the well screen.

A second plan of attack may be to insert a screen inside of the screen fitted with smaller slots to hold back the smaller grain sizes. In order to obtain the maximum well productivity, samples of the sand should be obtained and sieved in order to correctly determine the optimum new screen slot size. The slot size selected should be as wide as possible but small enough to retain 70 percent to 90 percent of the grains according to the sieve analyses. After the well has been cleaned inside to its original total depth, a screen diameter should be selected that will fit inside of the screen. In larger diameters (6 inches or greater), a nominal 2-inch smaller screen normally is selected (e.g., an 8-inch screen inside of a 10-inch well, a 4-inch inside of a 6-inch, etc.), however, inside of a 2-inch well, a 1-inch or 1.25-inch screen is recommended.

For wells that will be pumped continuously, a wire-wrapped screen (PVC or stainless steel) is recommended as opposed to a slotted screen, which is more suitable for a monitoring or observation well. The top of the inserted screen should be sealed with a neoprene “K” packer or similar device to seal the annular space between the screens, or the screen insert could have a riser that extends to land surface, which then would have the appearance of a well built inside of a well.

The lesson learned here is to take the time and care up front to obtain representative samples of the formation sand and conservatively select the correct screen slot size or, if in doubt, build the well sufficiently large enough to retrofit a smaller size screen if the need arises.