Cause and prevention of clogging of wells abstracting groundwater from unconsolidated aquifers

“Groundwater serves many purposes: as raw material for the public and private drinking water supply, for the production of beer and beverages, and in agriculture for irrigation and watering cattle. Groundwater abstraction serves for stabilizing structures in civil engineering, like dams and tunnels, for keeping dry building excavations, and for reclamation of polluted groundwater. At the moment, groundwater is also increasingly used for energy storage (Aquifer Thermal Energy Storage, ATES). Groundwater is usually abstracted with the help of wells. At present there are probably millions of drilled wells, either permanently or temporarily. In unconsolidated sediments these drilled wells consist of a well screen with on top a well casing. The annular space between the well screen and the well bore is filled up with gravel, the so-called gravel pack. In order to prevent short-circuit groundwater flow, the gravel pack is sealed on top with clay. For the same reason pierced clay deposits are also restored with clay. After construction, the well has to be developed in order to remove the drilling mud, which is plastered on the well bore and invaded a short distance into the aquifer. But remarkably, the criterion for a developed well consists of the absence of sand in the abstracted groundwater at a discharge rate usually twice the design capacity, and not of a successful removal of drilling mud.
After some time, drilled wells may become clogged. Clogging is characterized by a decrease in the specific capacity: the discharge rate (m3/h) divided by the
drawdown (m), where the drawdown equals the difference in water level in the well during operation and rest. In well clogging, two causes may be distinguished: clogging of the slots of the screen (screen slot or chemical clogging) and clogging of the well bore (well bore or mechanical clogging). Both types are easy to distinguish, as screen slot clogging is characterized by an increasing difference in water level during abstraction inside and outside the well, and well bore clogging by the absence of an increase of this difference.”

(Citaat: Beek, C.G.E.M. van – Cause and prevention of clogging of wells abstracting groundwater from unconsolidated aquifers )