Harmonic Testing for Continuous Well and Reservoir Monitoring

Abstract

Abstract Harmonic testing for obtaining dynamic reservoir information was first proposed some thirty years ago. Although not much used in the oil industry, interest in the method is revived periodically, mostly for the determination of skin effect and near-wellbore permeability. This paper looks at the practical aspects of using periodic rate variations for testing oil wells. It is shown that such tests can provide the same information as conventional well tests and can be interpreted in the same way. Their main advantage is that they do not require fluids to be brought to surface in exploration or early appraisal testing, or wells to be shut-in in production testing. They also provide data that are less affected by measurement errors and wellbore effects such as multiphase flow or phase redistribution. The main limitation is that, for the same radius of investigation, harmonic tests are significantly longer than conventional tests. Consequently, they cannot be used for reservoir characterization in exploration and appraisal wells. They appear well suited, however, for monitoring reservoir changes from production wells. Introduction The concept of harmonic testing was first proposed by Kuo1 in the early 70's, as an extension of pulse testing2.Pulse tests aim at generating interference data between two wells through a sequence of alternating production and shut-in periods in order to obtain intra-well reservoir properties and were believed to be more practical than interference tests. Kuo's suggestion was to use a periodic production history in a single well to determine the well near-wellbore properties. If the well is produced at a sinusoidal (or at least periodic) rate, the resulting pressure drop is also periodic, after early transients have died out and a pseudo-steady regime is established. As in pulse tests, the amplitude and phase lag of the pressure relative to the flow-rate can be measured and matched with the response of an interpretation model to obtain the corresponding reservoir parameters. The analysis is performed in the frequency domain instead of the time domain. Rosa and Horne offered a comprehensive review of previous publications on the subject. Many aspects of harmonic testing were studied in the late 70's and early 80's by Jouanna and co-workers3–7 at the university of Montpellier, France. They developed a number of well test interpretation models in the frequency domain and identified some of the difficulties inherent to the method. They also designed several testing devices and performed experiments in the laboratory and in shallow water wells. They concluded that harmonic testing could provide reservoir parameters such as skin factor, damaged zone depth, permeability and the group fct. They pointed out, however, that inertia effects and fluid-solid coupling should be included to fully understand the experimental results. Pan9 recently investigated harmonic testing in the medium frequency range (0.01Hz-10Hz), where more than just Darcy flow is involved. Her work focused on the identification of pore structure properties and the upscaling of these properties to infer the macroscopic behavior of the porous medium. She also suggested that wells could be stimulated by increasing effective reservoir permeability and porosity with elastic wave periodic excitation, thus providing higher flow-rates. Another example of frequency-domain analysis of pressure behavior has been presented by Firoozabadi and Chang10, who showed that reservoir compressibility and permeability can be estimated from the analysis of pressure data influenced by tidal effects. The input signal is a fixed gravitational potential variation instead of an imposed flow-rate variation and their analyses use two frequencies only (diurnal and semi-diurnal). An important development in harmonic testing came from Mercier11, who showed that the derivative of the pressure modulus in the frequency domain had a behavior similar to that of the pressure derivative in conventional well test analysis11. She therefore concluded that the interpretation methodology developed for conventional tests12 could be applied to harmonic tests.