Alice Springs town basin, review 2003
Read, R. E.
E-Publications; E-Books; PublicationNT; Report ; no. 42/2003
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Department of Infrastructure, Planning and Environment
Report ; no. 42/2003
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6.1.3 Current usage Extraction rates for 1999 to 2001 are shown in Table 9 . Figures for 2002 and 2003 are incomplete. 6.2 Bores & wells 6.2.1 Bore hydraulics The relationship between drawdown in production bores and screen length and aperture was investigated in Appendix I. The following conclusions were drawn: Screen length was not significant, that is sufficient screen length has been used in all cases. Generally 2 m of screen is adequate. Screen aperture was strongly related to well-efficiency, though this may largely be due to the fact that larger apertures can be used in coarser more transmissive aquifers. There are a number of bores with 2 mm aperture screens that are not particularly efficient. Both the linear and non-linear head losses, were very strongly correlated with transmissivity, that is good bores are in good aquifers. The relationship with transmissivity provides a benchmark for assessing the standard of completion of individual bores. 6.2.2 Design and construction of production bores Over the years various approaches have been used for the design of production bores. Quinlan and Woolley (1969) noted that there had been problems with the collapse of aquifers and recommended the following method: Place screens with oversize slot openings (which will pass up to 80 % of the aquifer) against the full thickness of a bed of sand with a minimum of disturbance, even though part of it is of very low permeability. Some disturbance of the alluvium and increases in well loss will occur during development. This can be kept to a minimum if the bore is developed by pumping and backwashing with an axial flow turbine pump. Surging to remove clay and silt from aquifers can be disastrous, particularly if screen openings are less than 1 mm. McDonald (1985) described a method of rotary drilling using polymer mud, reaming to 430 mm and installing 350 mm ID casing and in-line screens. Little development was required to produce a sand free discharge. Similar methods were used for the construction of further bores (Stevens 1985 and 1986). Stevens (1986) states that Most of the bores which have failed are constructed of perforated 150 mm casing, with or without a gravel pack. He further cites the performance of RN 14433 and RN 14839 as evidence of the success of the method. In Appendix I the performance of these large diameter bores was compared with that of four 150 mm diameter screened bores for which data was available. The large diameter bores did not seem to have noticeably better performance. D. Miller (pers. comm.) considers that bores in the river bed can be constructed by jetting in oversize temporary casing, installing the permanent casing and screen, and then withdrawing the temporary casing, allowing a natural pack to form around the screen. The key factor in constructing a high-yielding bore is selection of an area of high transmissivity aquifer. 51