AQUIFER PARAMETER ESTIMATION FOR FRACTURED BEDROCK USING MULTI-FREQUENCY OSCILLATORY FLOW INTERFERENCE TESTING

Multi-frequency Oscillatory Flow Interference (OFI) testing was conducted on isolated intervals of wells to estimate transmissivity, storativity, and diffusivity values of discrete fractures in siliciclastic and carbonate aquifers. Tests were conducted at a nest of 3 piezometers near a municipal pumping well, and on isolated intervals in a pair of open boreholes using straddle packers. The experiment goal was to delineate fracture flow characteristics at the two sites.

OFI testing is a method that uses the propagation of a sinusoidal pressure signal from a stimulation zone to observation points to characterize flow within an aquifer. Repeating the test using a range of different signal periods creates a multi-frequency OFI test. During experimentation, a pneumatic pump pressurized the air column in a sealed test well, and then delivered sinusoidal fluctuations in air pressure into the air column of the test well. This caused water level fluctuations in the well on the order of meters to tens of meters. Pressure transducers in adjacent observation wells recorded pressure fluctuations propagating from the test well into the aquifer. Detected signals ranged in amplitude from one or two millimeters to tens of centimeters. This method required no net water removal or injection from the aquifer, and was conducted in the presence of large transient background signals.

The amplitude decay and phase lag between pressure signals in the test and observation wells were used to estimate aquifer transmissivity, storativity, and diffusivity values. Aquifer parameters can be estimated using analytical solutions that predict amplitude and phase lag at a monitoring location in response to oscillatory stimulation (oscillatory pressure changes or flow rates calculated from pressure changes and cross sectional well area) at a pumping well. Phase lag and amplitude decay were computed by transforming data into the frequency domain, and then extracting the Fourier coefficients of the frequency corresponding to the stimulation frequency. OFI tests were repeated using multiple frequencies, and estimates of aquifer parameters at each frequency, and with each analytical solution were compared.