Slant Well Intake Systems: Design and Construction

Abstract

In a recent amendment to the Water Quality Control Plan for Ocean Waters of California (Ocean Plan), the California State Water Resources Control Board staff recommended that subsurface intakes are the preferred technology for seawater intakes. A number of desalination projects in the planning and feasibility stage along the Coast of California are considering subsurface intakes for feed water supply. Some of these projects have product water requirements >190,000 m3/day, with feed water requirements exceeding 380,000 m3/day. Angled or slant wells, withdraw water from permeable deposits beneath the ocean floor and receive a high percentage of recharge from both vertical leakage (through the seabed) as well as horizontally in the subsea aquifers. Environmental advantages include no impacts to fish or marine life and minimal impacts to onshore water resources. Aquifers beneath the ocean also provide natural filtration from suspended organic matter and sediment, particularly during storm surges and heavy precipitation reducing or eliminating the need for pretreatment. Slant wells are drilled using the dual-rotary method of drilling in an angled cradle and have angles below horizontal typically ranging from a few degrees to a few tens of degrees. A telescoping design allows construction of angled wells up to 305 m completed with an artificial filter pack typically yielding 10,000–16,000 m3/day. A slant well layout can be comprised of one well or a group of wells at a single wellhead location (i.e. pod). Multiple pods of slant well arrays are constructed until the cumulative total discharge rate meets the feed water supply demand. For slant wells, there is no theoretical limit on the maximum number of wells or the reliability of the source of supply (i.e. ocean). The only limitation is the permeability of the nearshore and offshore aquifers and areal and vertical extent of these deposits. Interference between wells and well pods governs the number and spacing of wells and geologic and coastline land availability governs the limitation on spatial and vertical extent of the well fields. Limitations may also occur due to potential or unforeseen impacts to near shore and onshore habitat and water resources. Slant well angles can vary depending on site conditions to allow targeting specific aquifer thicknesses. Shallow angled slant wells also have higher discharge rates than vertical wells for the same formation loss drawdown and geohydrologic conditions. Water level distributions around a single or multiple pumping slant wells can be easily calculated from a discrete number of point sinks placed within the vertical projection of the well screen and utilizing the principle of superposition. Variable density groundwater flow and solute transport models calibrated to site specific historical water levels and geochemical data provide a reliable predictive tool for planning and operation of subsurface slant well feed water supplies.