PneuSine Well Test – Pneumatically Generated Sinusoidal Hydraulic Testing & Analysis
HydroResolutions has developed an exciting new aquifer test as a complement or alternative to traditional aquifer characterization – the PneuSine well test. Analysis of a PneuSine test response results in aquifer information comparable to that obtained from a constant-rate test (industry standard pumping test), but doesn’t require a pump or a rig (unless packers are installed), doesn’t produce any water, and the only thing in contact with the water is one pressure transducer. In addition, the PneuSine test produces an unambiguous signal that is easily detectable at observation wells, it propagates considerably farther into the hydrogeologic system being tested than a standard slug test, and modulated input frequencies can be used to assess heterogeneities at different scales.
The PneuSine well test uses pneumatic pressure to create a sinusoidally varying pressure signal in a test well by manipulating the water level in the wellbore. By positioning pressure transducers both below the water level and in the head space above the water, we can monitor the total pressure acting on the formation and the changes in water level. From the changes in water level, it is possible to calculate the flow rate into and out of the well. The PneuSine test can be analyzed numerically as a variable rate test, or analytically using techniques developed specifically for sinusoidal tests.
HydroResolutions’ custom-designed wellhead is attached to the top of the casing and gas pressure in the casing above the water column is then controlled via HydroResolutions’ PneuSine flow-control system. Real-time parameters and responses can be viewed on the control system screen.
A PneuSine test begins by increasing the gas pressure in the source well. The system is then allowed to equilibrate for some period of time as the height of the water column decreases, allowing the total pressure to return to static pressure. It is the total pressure that affects the aquifer. During this equilibration period, a constant gas pressure above the water column is maintained. This test included both pneumatically induced sinusoidal and slug responses to compare signal propagation at the observation wells.
Note how much more strongly the observation wells respond to the initial pressurization and sinusoidal portions of the test as compared to the slug test. No worries about testing only the gravel pack with a PneuSine test.
Formation flow (i.e., no wellbore storage) can be calculated directly from the changes in water level, using the known casing diameter and the fluid density.
Using the calculated flow rates, the sinusoidal responses can be analyzed numerically as variable rate tests to determine aquifer properties. HydroResolutions uses the advanced numerical well-test simulator nSIGHTS (n-dimensional Statistical Inverse Graphical Hydraulic Test Simulator) for analysis.
Rasmussen et al. (2003) developed an analytic solution for a test with an observation well that uses the phase difference between the flow rate in the source well and pressure in the observation well to estimate the diffusivity (D=T/S), and uses the ratio of the amplitude of the pressure in the observation well to the amplitude of the flow rate in the source well to estimate T. Knowing D and T, S can then be calculated. Renner and Messer (2006) developed a similar solution for a source well with no observation well.
Aquifers act as low-pass filters. In the source well, the amplitude of the 30-min period was about 2.2 times larger than that of the 16-hr period. After the signal had traveled just 40 ft to the nearest observation well, the higher-frequency signal was attenuated 20 times more than the lower-frequency signal.
Multi-frequency tests can be used to assess the heterogeneity of the aquifer and determine if flow boundaries are present. Increasingly larger extents of the aquifer can be investigated as the sinusoidal frequency is lowered.
The ratio of the pressure to flow-rate amplitudes for different periods (P/Q amplitude ratio) can be plotted to determine if aquifer properties are homogeneous or heterogeneous. In this test, nine different periods ranging from 1 minute to 2 hours were used to determine that the aquifer behavior deviated from an infinite-acting radial flow (IARF) response.
Like the P/Q amplitude ratio, the phase difference (QP) between the flow rate and the pressure is also affected by heterogeneity/boundaries. Utilizing different input frequencies at the source well, this powerful diagnostic capability can be used to assess heterogeneity/boundaries in the flow system at different scales.
A bandpass filter can be used to isolate signals with a specific period (the known source-well period) in order to aid in the detection of low-amplitude signals. After filtering the observation-well signal, the 10-minute period input signal from the source well is clearly visible in the observation well data. It is indiscernible in the unfiltered data.
Barometric and earth-tide corrections are another method that can be applied to reveal small-amplitude signals in distant observation wells. Given that the source-well input period is known, the signal at the observation well can be unambiguously identified.
References:
Rasmussen, T. C., K. G. Haborak, and M. H. Young (2003), Estimating aquifer hydraulic properties using sinusoidal pumping at the Savannah River Site, South Carolina, USA, Hydrogeology Journal, Vol. 11, 466–482.
Renner, J., Messar, M., 2006, Periodic pumping tests, Geophysical Journal International (2006) Vol. 167, 479- 493
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