The extreme well-loss component of drawdown in two deep artesian wells in Israel

The water-level drawdown in pumping wells is the sum of two components: aquifer loss and well loss. The latter results from mostly turbulent and nonlaminar flow in and around the well. In a properly designed well, the well-loss component is usually much smaller than the aquifer loss. Analyzing step-drawdown tests of two deep (1,397 and 878 m) artesian wells drilled in a fractured carbonate aquifer in Israel, revealed exceptional proportions between the two drawdown components. Despite the high artesian flows and the fact that the two wells are properly constructed, most of the drawdown (96–99% and 82–90% of the total drawdown) is attributable to well loss. Accordingly, the well efficiencies are very low and decrease as flow increases. The anomalous values of the well-loss component are also reflected in the wells’ hydrographs; each opening and closing of the artesian flow results in an immediate jump in the head pressure. As far as is known, such unusual proportions have never been encountered in other water wells. The vertical flow velocities within the casing of both wells are very high, and the Reynolds numbers confirm turbulent flow. The combination of flow in fractures and high frictional head loss within the well pipes are the factors that enable this exceptionally high well loss and low efficiency in these high-discharge wells. The high frictional head loss, calculated by applying the Darcy-Weisbach equation, is the result of great well depths and turbulent rapid vertical flow up to the surface in a narrow and long casing.