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^
Pulsed sonic transducers commonly used in acoustic logging emit energy over a frequency spectrum. In addition to being dependent upon the spectrum of the emitted energy, the character of the recorded waveform is dependent upon the environment through which the waves travel, characteristics of the receiving transducer and characteristics of the electrical amplification and transmission system. Difference observed in formation-borne waves while surveying cased and cemented boreholes using impulse sources having different frequency spectra have led to an investigation of frequency dependent effects. By suitable selection of the frequency spectral characteristics of acoustic logging tools, formation-borne wave arrivals can be enhanced and some interfering wave arrivals can be suppressed to render an improved formation character log through cemented pipe. In particular, spectral selectivity has been used to reveal formation wave arrivals obscured by the low velocity waves which propagate near the velocity of sound in the borehole fluid. Spectral selectivity has been used in logging open holes to improve resolution of fractures in the formation. In open holes, high frequencies are observed to cause sharp first arrivals. Spectral selectivity techniques have significantly extended the utility of small diameter acoustic tools designed for passage through tubing. Introduction Interpretation of acoustic Micro-Seismogram logs as an aid in evaluating the quality of cement bonding has been described by Anderson, Winn and Walker. They found that in cases where pipe Was not bonded to the cement a large amplitude periodic signal resulted on the log, hut in cages of good cement bonding at the pipe-cement interface and good acoustic coupling at the cement-formation interface, formation-borne waves were observed. These formation-borne waves, when present, were shown to correlate with those observed on a log of the hole prior to completion (Fig. 1). In early small diameter acoustic instruments designed for passage through tubing, the pulse energy was concentrated at relatively high frequencies. Presence of these high frequencies is due partly to the relatively high resonant frequency of the small diameter acoustic transducers available for these instruments. Logs made with these instruments adjacent to strongly attenuating formations, such as poorly consolidated sands and shales, often exhibited an absence of formation-borne wave arrivals in intervals of good cement bonding to the pipe. This condition could be interpreted as a lack of acoustic coupling due to poor contact at the cement- formation interface; however, logs from the same hole made with instruments having the pulse-energy concentrated at lower frequencies revealed formation-borne wave arrivals indicating good contact at the cement formation interface. To minimize errors in interpretation of the cement-to- formation contact. it became necessary to eliminate the high frequencies present in the pulses used for logging. It is well known that a periodic wave train, much as the train of acoustic pulses emitted from an acoustic logging transmitter can be composed of sinusoidal waves of many different frequencies. JPT P. 407ˆ
ment are included to illustra'te the effectiveness In acoustic velocity logging the phenomena of cycle skipping has been used to indicate the presence of fractures, etc. Since the cycle skips of interest are the result of attenuation of the acoustic signal, a measurement of formation amplitude is a more positive means for this purpose. In the field of acoustic bond logging the amplitude of the formation signal depends to a great extent upon the quality of bonding of cement to pipe and formation. By comparing the open hole acoustic amplitude to that in cased hole, the over-all effectiveness of the cement job may be more clearly distinguished.Test well data is included ·to show the degree of attenuation of the acoustic signal at solid, liquid interfaces as would be present in fractured formations. Field examples of this and other applications of amplitude measure-of the application.
A,calibra:fon systetn is obtained that permits correlation of amplhude data from borehole to borehole regardless oj composition of hole fluid, hole diameter, or other hole~derived parameters, Two-receiver attenuation logging provides better interpretation of acoustic cement bond logs because o# this calibration system. Recent advances in acoustic amplitude logging have left undone the requisite of all physical nzeasurements, that is, calibration: toa known acceptable and consistently repeatable standard unit, In our study of two-receiver amplitude ratios we show that theratio scale tnay be precisely stated in a common term of atteituation rate-decibels per foot. Results of amplitude ratio logging in'various representative areas including both cased and open ho~es are given. Curves are presented showing ratio relations for various spacings. A nomograph for deiernthting hole factor kf rom the ratio and one single-receiver amplitude (recorded sitnultaneoasly) has been developed. This indicates tile possibility that new information concerning acoustic energy translation at the weilbore-fortnation interfaces maybe obtahted, She~r wave and compressive wave attenuatiori rates are discussed as a means to assess fluid mobility. .:. ,
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