Unsteady Flow Analysis of Underdamped Slug Test Responses Observed in Fractured Sandstone

Straddle packer testing equipment was developed for contaminated site investigations in fractured rock with the ability to perform a comprehensive suite of hydraulic tests (rising and falling head slug tests, constant head step tests, and constant rate injection tests) to obtain the best possible T values for calculating hydraulic apertures. Slug tests were conducted at a field site in southern California with this test equipment in 10 different boreholes in fractured sandstone, ranging in depth from 75- 250 m, at specific depth intervals (1.5 m length), in 133 different test zones. Underdamped responses, identified by clear oscillations of the formation pressure, were observed in 30% of the zones tested. Using concepts from field and laboratory studies, a mathematical solution was developed for unsteady oscillatory laminar flow in pipes. This solution illustrates that the radial velocity profiles of flow in the pipe are not parabolic and frictional losses are significantly larger. It is shown that friction is out of phase with the average flow velocity indicating that the radial velocity gradient is not proportional to the average velocity, and has a component that is in phase with the acceleration.