Vízügyi Közlemények, 1960 (42. évfolyam)
4. füzet - VI. Képek a Föld különböző részeinek vízépítési munkáiról
(16) ECONOMICAL SPACING OF WELLS By J. Juhász (For the Hungarian text see pp. 61.) An unsolved problem of water works supplied from drilled wells was the spacing to be adopted for the wells. Geologists favoured in the majority of cases distances sufficiently great so that the wells should not, or at least not to any great extent affect each other. Relying on practical experience and depending upon the permeability of the water bearing layers involved distances of 200 — 300 and 400 m were specified. The method considered as most reliable was to carry out extended pumping test on pilot wells, and to determine on this basis the well spacing regarded as practicable. However, all these investigations were concerned at the best with hydraulical aspects of well spacing, neglecting the not less important economical aspect of the problem. This point of view will be discussed in the present paper, which accordingly comprises two parts. Yield conditions of a group of wells, corresponding to given geological conditions were determined in the first part. As a first step the geometrical pattern (e. g. row, square-, rhombic well patterns, eLc.) of the contemplated supply wells is established. Subsequently a group comprising 4 to 6 wells is selected from this field and — considering them independent from the water works system — their individual yield is determined. Their total yield is obtained as the sum of these values. The yield per unit area (specific yield) is determined from the total area occupied by the wells, and from the total yield. (In the case of well rows the yield per unit length is determined.) The specific yield is plotted for various drawdowns, well distances and discharges in a graph. The hydraulically permissible highest discharge is determined for the given stratigraphical conditions. This value is entered as a limit into the former graph. The maximum well spacing that can still be applied at the discharges withdrawn, can thus be determined. The rate of recharge is hereafter determined for the area. Data obtained by test pumping, or practical experience may be used for this end. By entering the recharged discharge into the graph those well distances are obtained, at which the entire recharged water quantity is withdrawn from the area. The point where the recharge curve and the curve of the maximum hydraulically available discharges intersect defines the maximum well spacing at which the entire recharge to the area is still withdrawn. In the case of deep wells, where drilling costs are high, this distance ist at the same time the optimum one. If, however, the drilling depth is small and the yield high, because of the economy in energy costs closer well spacings and the use of lower drawdowns may be found economical. In such cases first costs, maintenance costs and depreciation costs of the well, the well head, the mechanical and electrical equipment, as well as energy costs pertaining to various yields and drawdown values should be estimated. Relating these to the water quantity withdrawn, the cost per unit water volume gained can be established. Similar investigations carried out for various well spacings, with the results plotted graphically yield the optimum well spacing as the minimum of the curve. If the unit-cost curve attains no minimum value, the maximum well distance should be considered as most economical. In the future the distance of wells located on the basis of various considerations should preferably be determined in advance by the aid of a few exploratory drillings. (Authors summary translated by Z. Szilvássy struct, eng.)
