Periodic pumping tests

Abstract

SUMMARY We performed pumping tests with a periodic succession of injection and production intervals in a field of three shallow boreholes penetrating a jointed sandstone formation. Tests were conducted at periods ranging from 10 to 5400 s and pumping rates between 5 and 20 l min−1. Two types of analysis for the periodic pumping tests are presented. Injectivity analysis rests on the determination of the amplitude ratio and phase shift between the periodic flow rate and pressure records from the pumping well. Interference analysis is based on an evaluation of attenuation and phase shift between the periodic pressure signals at the pumping and monitoring wells. A periodic excitation permits to employ standard signal processing tools, such as fast Fourier transformation, facilitating the evaluation of weak signals. The observed amplitude ratio and phase shift values are compared to analytical predictions of 1-D and radial flow models. Results of interference tests are particularly diagnostic for the applicability of a subsurface model. Our test results do not fully agree with any of the considered analytical models suggesting a heterogeneous subsurface. We suppose that the two analysis methods yield averaged hydraulic properties of different volumes of a heterogeneous subsurface. Periodic injectivity tests screen the subsurface to increasing penetration depth if pumping period is systematically increased, while the subsurface volume between pumping and monitoring well dominates the outcome of an interference test at all periods. Periodic pumping provides several operational advantages compared to conventional testing, such as drawdown, pulse or shut-in tests. Periodic testing can be superposed on a long-term operation reducing operational conflicts and permitting continuous monitoring of changes in subsurface properties. Processing of periodic signals provides results even in a field with strong transient changes in fluid pressure related, for example, to operations in nearby wells. Periodic pumping can be performed in a closed loop minimizing the total fluid volume involved.