A Digital Computer Application to the Investigation Of Aquifer Properties

Abstract

Abstract The objective of this work was to use computational techniques advantageously in planning aquifer tests, and evaluating test data for the quantitative description of geologic and hydrologic parameters affecting well and aquifer behavior. A digital computer program was developed to compare the actual pressure performance of aquifer systems to the calculated pressure behavior based on analytical analogs, and to select the analog which best fits the test data; and predict the pressure response of an aquifer discharging through one or more wells to help design proper lest procedures and to monitor test activities in the field. Computer processed and evaluated test data from two aquifers are presented to demonstrate the usefulness of well tests and to show that automatic data interpretation is an excellent time- and money-saving tool in the exhaustive search for the best available mathematical models for the aquifer systems. Introduction This paper develops a digital computer program, based mostly on the concepts of analytical mathematics as used in describing the phenomenon of fluid flow in porous media to facilitate aquifer test planning and test data evaluation. The basic objective of aquifer tests is to evaluate the occurrence, availability and quality of groundwater. When analyzed with appropriate equations or computing techniques, aquifer tests also yield information relative to the geologic and hydrologic characteristics of the aquifer systems. There are many different types of aquifer tests. The constant terminal rate discharge tests followed by pressure build-up tests are of particular interest to engineers who develop natural gas storage pools in aquifers or those who work with producing gas and oil fields subject to water drive or encroachment. These tests are performed by pumping water from one or more wells completed in the aquifer and noting the unsteady-state pressure response of the discharging well and a number of observation wells that are strategically located at various distances from the discharging well. Due to the low compressibility of an aquifer system, pressure transients created by groundwater flow toward the discharging well can be transmitted considerable distances during relatively short periods of time. Thorough knowledge of the behavior of aquifer-aquiclude systems under static and dynamic conditions is indispensable when such systems, associated with structural and/or stratigraphic traps, are considered for the storage of natural gas. Generally, gas injection tests are used as a pilot project to study the confining ability of the aquiclude (confining caprock). The object of the pump tests is to ascertain whether there is leakage through the aquiclude, and to investigate the effectiveness of the porous and permeable strata in receiving, retaining and releasing the gas. However, the element of risk is reduced considerably if the gross characteristics of the aquifer-aquiclude system are dynamically tested by pump test data is much more to gas injection. Also pump test data is much more susceptible to rigorous analysis than gas injection test data. Classical methods of analytical mathematics can often be used to analyze pump test data, and to determine in situ parameters and geometry of the aquifer-aquiclude systems. Such analytical methods apply to relatively uniform systems with simple geometry. A large number of calculations based upon the available structural and stratigraphic data are made for careful planning of pump tests to prevent misuse of test methods and equipment. Analysis of pump test data also requires many computations before a quantitative description of the hydrologic characteristics of an aquifer-aquiclude system is possible. Digital computer methods of planning aquifer tests, interpreting test data and constructing ideal aquifer models of the real aquifers are excellent time- and money-saving tools at the disposal of engineers in their study of aquifer gas storage prospects and producing hydrocarbon fields subject to water drive or encroachment. Typical costs of running computer analysis for a set of well test data are given in the program application section. JPT P. 827ˆ