1-Analysis of Pumping Test Data on Large Diameter Wells Using Optimization Technique.

Abstract

The pumping phase data on large diameter wells, particularly in hard rock terrane, usually show negligible aquifer response indicating that most of the water is pumped from the well storage. Therefore, the recovery data should also be taken into account to estimate the hydrogeological parameters. The observed time-drawdown recovery datais matched with the analytically or numerically drawn theoretical type curves to estimate the hydrogeological parameters. In case of a numerical method, forward modeling is carried out wherein the parameters are progressively varied so as to obtain a close match with the observed time-drawdown/recovery curve. However, a visual match is naturally subjective and biased. Besides, the process of ploting and matching is a long drawn one. An alternative approach is presented here which ensures an unbiased, objective, and fast interpretation of the pumping test data on a large diameter well. The technique essentially consists in the minimization of the drawdown function which is the sum of squares of difference between the observed and computed drawdowns. The non-linear drawdown function is linearized around initially guessed hydrogeological parameters. The partial derivatives of this function with respect to hydrogeological parameters result into a set of simultaneous equations giVing new values of these parameters. The guess values of parameters are replaced by these newly estimated parameters. This iterative process is continued till a pre-set convergence criteria is achieved. This technique eliminates the need for a manual matching of field and computed time-drawdown curves. An illustration for interpreting the pumping test data on a large diameter well in a eranitic terrane is described in detail.