Research Predicts Improved Cement Bond Evaluations With Acoustic Logs

Abstract

Abstract Laboratory investigations and test-well oscilloscope pictures were made to determine the application of acoustic signals for evaluating the effectiveness of casing cementing. This work indicated the possibilities of measuring the amplitude of acoustic signals from the pipe and from the formation, both in open and cased hole, to indicate the bonding to pipe and to formation. Field tests have shown the applicability of the system, but experience and further model tests have shown that a number of variables affect the interpretation. Some of these variables such as type of cement, quantity of admixes, sheath thickness and time of logging after cementing-are being investigated, and the results to date are included. These results indicate that it will be possible to evaluate the effectiveness of cementing under most conditions. Introduction When production tests do not agree with log, core-analysis or other formation evaluation data, the effectiveness of casing cementing is usually questioned. Even in wells which have been produced for some time and then begin to produce water or excess gas, the isolation by cement behind the casing has been found to be incomplete by a squeeze and reperforation. The answers to these questions in the past have been quite expensive and often damaging to formation productivity during the process of repair. Considerable research effort recently has been expended by service and producing companies on the cement bond log, which offers the possibility of evaluating the effectiveness of cementing. The process involves measuring the amplitude of an acoustic signal, with the quality of the cement bond (plus some other variables) affecting the signal amplitude. At this point a definition of the term "bond" should be established. For the purpose of this paper, a "bond" is defined as a coupling which joins either the pipe and cement or the cement and formation in such a manner that no intervening space is present and no density discontinuity exists except from steel to cement or from cement to formation. Acoustic Cement Bond Logging Experimental Investigation To completely evaluate the possibilities of determining the effectiveness of cement isolation with acoustic signals, a series of test holes was set up with various cement conditions using ideal Portland 15-lb/gal cement. Using an acoustic logging tool, oscilloscope pictures were taken of various spaced-receiver signals under a number of conditions. As expected, it was found that the signal through the pipe depended upon the cement bond to the pipe. Also, the amplitude of the formation signal depended upon the cement bond to the formation. The pipe signal can be considered as a vibration of the pipe which has an arrival time of 57 microseconds/ft, the bonded pipe having a very low-amplitude signal because of the damping effect of the cement sheath. The amplitude of the formation signal depends upon the presence of a bonded path through which the acoustic energy can travel. If the cement is not bonded to the formation, the signal must cross four additional solid-liquid interfaces, with the resulting loss of energy at each. Of course, some energy will be lost because of the density variation existing at the cement bond-to-formation interface, but this loss will be much less for the unbonded condition. Some of the scope pictures from this study (3 ft between the transmitter and the receiver) are presented in Figs. 1 and 2. Fig. 1(A) shows the high-amplitude pipe signal from uncemented pipe, with a very low amplitude formation signal super-imposed on the ringout of the pipe vibration. Views B and C of Fig. 1 show the same low amplitude in bonded pipe, although the section of pipe in View C was sandblasted to remove mill scale, etc. In both B and C, the high-amplitude formation signal indicates the good bond to the formation. Views D, E and F of Fig. 1 show different widths of channels and the thicknesses of cement sheaths. This and other data indicate that the thickness of the cement sheath affects the amplitude up to about a 2-in. sheath (for Portland cement); but after that point, essentially no change in amplitude can be distinguished. The channels in these three pictures were rectangular, which probably is not too common in the field; the more common situation is decentralized pipe, with insufficient cement between the pipe and formation on one side (a condition which will be shown later). JPT P. 1093^