The water drawdown in pumping wells in a fully penetrated confined aquifers is defined by the following equation (Jacobs, 1947):
S =BQ+CQ2; where S is the drawdown inside the well (L), Q - pumping rate (L3/T), B - aquifer (laminar) head-loss coefficient (T/L2) which is attributed to the aquifer and varies with pumping duration, and C - nonlinear (turbulent) well-loss coefficient (T2/L5) caused by resistance to water flow into and inside the well. The common method to analyze the two components of the drawdown - BQ (aquifer-loss component) and CQ2 (well-loss component) is by a step drawdown test using the graphical Hantush-Bierschenk solution (Kruseman and de Ridder, 1990). The parameters determined by this test include hydraulic values such as transmissivity, and several performance and engineering values, such as aquifer-loss, well-loss, specific capacity and well efficiency (the ratio of the aquifer loss to total/measured drawdown). In wells that are technically well-constructed and with no clogging, the CQ2 term is expected to be low and most of the drawdown is due to the aquifer’s characteristic as represented by the BQ term.
Step drawdown tests were conducted in two deep wells in the northern part of Israel: HT-1 (878 m) and SH-2 (1,397 m). The two wells are located 5.2 km apart, and tap two different fractured Jurassic limestone formations. Well SH-2 is perforated along 196 m at its lower part, while well HT-1 was left as an open hole (68 m). The aquifers in both wells are confined with water heads of 12-13 atm. above surface.
The artesian discharges measured at the end of the drilling was ~1,400 and ~1,000 m3/hour in wells HT-1 and SH-2, respectively. During subsequent production, discharge rates decreased as the artesian pressure dropped. Despite the high artesian flows and according to the tests, most drawdown in both wells is due to well-loss (81-92% and 96-99% in HT-1 and SH-2, respectively). Furthermore, their well efficiency is very low (9-18% and 1-3.5%, respectively). However, according to Walton’s (1962) classification of the degree of well deteriorations, the relatively low C values in both wells (962 and 1,744 sec2/m5, respectively) imply properly designed and developed wells. Since there is no clogging in the wells, we believe that the combination of great depths and long and fast artesian vertical flow to the surface in narrow casing are the main reasons for the unique high CQ2 in these high discharge wells. The sharp decrease in the specific capacity (Q/S) over the wide range of the applied pumping rates during the tests (36.5 to 10 m3/hour/m in HT-1 and 21.8 to11.6 m3/hour/m in SH-2) confirm this interpretation.
We seek to learn whether such extremely high CQ2 values were encountered in other production wells worldwide.