Ingress of sand usually occurs in wells abstracting from sandstone aquifers that are poorly cemented. Impacts can be serious and may add significantly to production costs. They include loss of yield and continuity of supply, a reduction in the efficiency and operational life of wells and pumps, more frequent maintenance of wells and ultimately well replacement. The design of high-capacity sand-free wells in sandstone aquifers is often problematical. This is partly because a substantial proportion of the flow is through fractures, which means that well design criteria of screen slot size and artificial pack grain size and grading that have been developed for unconsolidated granular aquifers are not generally applicable. Also, many sandstone aquifers are multi-layered. Interbedded fine-grained and friable sandstone layers or loose sands are often responsible for sand entering a well. Excluding them is often difficult in practice without blocking productive fracture zones, and, therefore, risking a reduction of well yield. Well design alone is not always capable of controlling sand pumping. Pumping at a lower discharge rate, regulating flow at start-up, and the use of sand traps at the ground surface or inside wells are other measures that can assist in alleviating the problem.

The Palaeocene Basal Sands of the London Basin are usually in hydraulic continuity with the underlying Cretaceous Chalk aquifer. They are a minor aquifer rarely utilized for water supply. Instead, production wells are completed in the Chalk aquifer, which provides the bulk of the water. The conventional well design is for cemented casing to extend into the top few metres of the Chalk, with the rest of the Chalk section below left unlined (open hole). Although this has generally proved to be successful, there have been occasions where failures have occurred owing to sand entering the wells from the Basal Sands. The pathways are not always evident, but can include fractures in the Chalk, damaged casings or poor grout seals. Setting casings deep into the Chalk does reduce the risk of sand ingress, but yields may be diminished. In some cases, screening of the Basal Sands and the very top part of the Chalk has been used as an alternative. Detailed data from geological and geophysical logs can greatly assist in the design process, and the risk of sand ingress can be reduced by following good drilling and construction practice and ensuring that casings have been properly grouted. In this paper the terms wells, production wells, boreholes and water supply boreholes are used interchangeably.

No abstract