Methods of conducting air-pressurized slug tests and computation of type curves for estimating transmissivity and storativity

Abstract

Air-pressurized slug tests offer an efficient means of estimating the transmissivity (T) and storativity (S) of aquifers. Air-pressurized slug tests are conducted by pressurizing the air in the casing above the column of water in a well, monitoring the declining water level and then releasing the air pressure and monitoring the rising water level. The equipment needed to conduct an airpressurized slug test is easily constructed and assembled at the top of the well. The only equipment in contact with the water is a down-hole sensor to monitor water levels. During the pressurized part of the test, small changes in the applied air pressure result in water-level fluctuations, making it difficult to estimate T and S from the declining water-level data. However, if the applied air pressure is maintained until a new equilibrium-water level is achieved and then the air pressure in the well is released instantaneously, the slug test solution of Cooper and others (1967) can be used to estimate T and S from the rising water-level data. In lowpermeability formations, it may take an extended period of time to achieve the new equilibrium-water level for the applied air pressure. The total time to conduct the test can be reduced, however, if the pressurized part of the test is terminated prior to achieving the new equilibrium-water level. This is referred to as a prematurely terminated air-pressurized slug test. Type curves generated from the solution of Shapiro and Greene (1995) can be used to estimate T and S from the rising water-level data from prematurely terminated air-pressurized slug tests. The Fortran code AIRSLUG, included in this document, is used to generate the type curves from the solution of A detailed discussion of the equipment and procedures for conducting air-pressurized slug tests is presented along with discussions of data preparation, use of the Fortran code AIRSLUG, and method of matching the data and the type curves to estimate T and S. Field examples are presented to demonstrate the applicability of air-pressurized slug tests in estimating T and S.