The Structure and Interpretation of Noise From Flow Behind Cemented Casing

Abstract

Here is a noise-logging technique for finding behind-casing leaks. The leak source is located from a noise-amplitude log and the type of leak (single- or two-phase flow) is determined from a spectrum of noise at the source. Appropriate frequency cuts are then used to estimate leak rates. Introduction A noise-logging technique described in this paper has proved useful in searching for fluid movement in proved useful in searching for fluid movement in channels in the cement behind the casing of oil and gas wells. Such channels, of course, provide undesirable paths of communication between sands of different pressure. The idea of noise logging is old. In 1955, Enright qualitatively described a procedure for locating down hole, with an appropriate listening device, the peak noise associated with the point of origin of a leak. Korotaev and Babalov Used a noise detector to locate gas-producing zones in thick, open intervals. Stein et al. described the interesting procedure of listening opposite a productive zone in a flowing gas well for the "pinging" of sand grains against the producing string, thereby determining the maximum rate permitted before sand production begins. permitted before sand production begins. In spite of these applications, the general industry attitude still seems to be that a noise log is no better than or not even as useful as a temperature log - the traditional leak detector. This attitude is not entirely justified since there are instances in which noise logging has clear advantages over temperature logging. For example, it is seldom possible to flow a potential blowout well at a rate sufficient to give a definitive temperature anomaly. Likewise, a leak behind casing in a producing well might have a rate too small to detect with a routine temperature survey. We give in this paper experimental results that show that if one uses a detection sonde with good fidelity over the bottom half of the audible frequency range, the frequency structure of channel-type leaks has quite distinctive features. These features not only help distinguish single-phase flow from two-phase flow, but also provide a clue to a procedure for estimating an order-of-magnitude leak rate from peak noise. Several field examples demonstrate the routine operation of noise logging for detecting leaks and estimating rates. In particular, we present cases in which the noise log gives about the same information as a temperature log, cases in which the noise log gives additional information, and cases in which incorrect conclusions were drawn from a temperature log. Finally, we describe the potential of the noise logger as a flowmeter without moving parts. Here we rely primarily on experimental data, since our field data for such applications are quite limited. Frequency Structure of Channel-Type Leaks Details of construction and calibration of the detector are in Appendix A. We merely emphasize here -that the device, when driving 9,000 ft of armored cable, has its half-power points at 20 Hz and at 5,000 Hz. All subsequent spectra data are normalized to this frequency response (see Fig. 13). The experimental noise chamber, or leak simulator, is described in Appendix B. In using the simulator to study noise, we expand fluid across a porous plug and into a jacket packed with crushed marble. A 2 1/2-in. tubing string packed with crushed marble. A 2 1/2-in. tubing string through this jacket allows the detector to be positioned at the point of the leak. Variations on the basic theme are discussed in Appendix B. JPT P. 329