Getting The Most Production Out Of Screened Recovery Wells
In screened wells, the problem is primarily in two areas; 1) matching of screen and filter pack materials with size of grains in the formation and 2) proper development (aggressive) of the well after completion of the filter pack.
The biggest mistake made in production or recovery well design is selection of too large of a screen slot and filter pack believing it will increase production. Allow me to exaggerate to illustrate the problem. Placing "pea gravel" and a 0.030 slot screen against a clayey sand will decrease yield as opposed to installing 30-40 fine sand and 0.010 slot screen. The reason the larger filter pack (pea gravel) will produce less water is the formation clay and silty sand will plug the pores of the gravel and become more difficult to develop. Ideally, there should be a gradual gradation of grain sizes from the formation to screen. Considering most screen packs are only about two inches thick, there is very little space to develop the filter pack into a well-graded filter pack. The screen and filter pack should be sized to allow about 25 to 50% of the "fines" in the formation to be removed during well development. This brings us to the next point of constructing high efficiency wells, namely well development.
Pre-packed screens have been around for more than 10 years, however, the same principals apply. The formation must still be developed and gradation must occur outside the pre-pack with the formation, otherwise the interface with the pre-pack will become plugged.
Proper well development typically takes an hour to days for proper development. The length of time depends on length of screen, how aggressive development is, amount of clay or fines to be developed, variation in grain sizes and how the well was drilled. Generally, bentonite drill mud should not be used in the screened section of production wells. Bentonite does an exceptional job of plugging the formation during drilling; however, it is very difficult to "break the gel strength" of the mud cake. Hollow stem augering or organic bore-drilling muds are recommended because smearing or strength of the organic mud seal is more easily broken down than a borehole drilled by the mud rotary method.
Many veteran well drillers will agree that surging using a surge block is the best method for developing wells. Surge blocks fit snugly against the screen and are typically lifted up and down quickly to produce a washing action on the filter pack. "Washing" the water in and out allows more of the fines to be pulled into wells as opposed to straight pumping, which tends to "bridge" clay particles against the screen. Back washing breaks the bridge and allows fines to shuffle more effectively into the screen. Developing should continue until purge water is clear during purging actions. Development may take hours or even days, but it is important to develop as much of the filter pack and formation as possible, which will add considerably to the life span and efficiency of the well.