Reliability Analysis for Preliminary Forecasts of Hydrogeological Unit Productivity

摘要

The aim of this work is to find a probabilistic characterization of the productive capacity of a well in a geological formation hosting an aquifer. Such characterization in terms of productive capacity may allow a preliminary assessment to be made of the probability of success for a required productivity (i.e. target point). This evaluation is usually carried out by statistical analysis of a geological dataset, which is likely to be influenced by many parameters. Such datasets are often incomplete or unreliable. Therefore, a method for evaluating potential productivity, using probabilistic hydraulic conductivity data, is proposed. The hydraulic characterization of hydrogeologic units is based on the collection of information obtained mainly through pumping tests and their interpretation. The results, expressed in terms of hydraulic conductivity, are summarized in a range of variability that is strictly dependent on the number of performed tests and their spatial distribution in the unit itself. If this range is known, an estimate of well’s yield can be made on a deterministic basis, through Thiem’s relationship for steady state conditions, by setting a value of hydraulic conductivity that corresponds to the average value of the range. The proposed reliability analysis enables to overcome the limitations of the deterministic approach by correlating each calculated flow rate, which is taken to be a design flow rate exceeding the critical flow rate of the hydrogeologic unit, to its probability of failure. Therefore, this approach aims to evaluate the probability of failure of the water system. The preliminary result is to associate the values of aquifer exploitation with a probability failure function. This outcome can then be used to define the potential solutions in the optimal allocation of the withdrawal by means of reliability analysis that takes into account the uncertainty of the system.