Determining Sand Pack Size
Generally, a well functions as a monitor or as a pumping or production well. If the well is completed with a screen, as opposed to an open hole finish in hard rock, then the screen usually is surrounded by a sand pack placed by the driller.
The size of the sand placed around the screen influences the amount of water that can be withdrawn from the well, particularly if the well is a pumping well. If the sand in the sand pack is too small, friction will be large, and the pumping capacity of the well will be decreased. Also, the fine sand may enter the well screen causing sand problems. If the sand grains in the sand pack are too large, then smaller formation grains may plug the spaces between the larger grains.
The accompanying table will aid in selecting the right sand pack size grade (or mesh) for the well screen slot size. The table is a little confusing in that the grade (or mesh) size changes in the opposite direction of the grain diameter.
Ideally, the formation and sand pack would be the same size and the individual grain diameters would be uniform, with the screen slot only being slightly smaller. The table takes into account a safety factor of adding 20 percent to the slot size to ensure that too much sand does not pass through the screen. Most sand purchased has a range, such as 20-30, 40-60, etc. Remember to select the grade or mesh closest to the screen size. For example, 0.015 would correspond to 25, and then you?d pick the next available grade down the chart, which would be 20. Depending upon availability you would select 20-25 or 15-25 sand.
Other factors also impact well yield, such as the type of screen used (percentage of open area), the type of drilling fluid used and the degree to which the sand pack is developed to remove mud, clay and fine materials. Also, occasionally, the quality control may be off at the packaging plant. If in doubt, have the sand sieved at a geotechnical laboratory for your own piece of mind. n