Acoustic Amplitude Ratio Logging

Abstract

A calibration system is obtained that permits correlation of amplitude data from borehole to borehole regardless of composition of hole fluid, hole diameter, or other both-derived parameters. Two-receiver attenuation logging provides better interpretation of acoustic cement bond logs because of this calibration system. Recent advances in acoustic amplitude logging have left undone the requisite of all physical measurements, that is, calibration to a known acceptable and consistently repeatable standard unit. In our study of two-receiver amplitude ratios we show that the ratio scale may be precisely stated in a common term of attenuation rate-decibels per foot. Results of amplitude ratio logging in various representative areas including both eased and open holes are given. Curves are presented showing ratio relations for various spacings. A nomograph for determining hole factor kh from the ratio and one single-receiver amplitude (recorded simultaneously) has been developed. This indicates the possibility that new information concerning acoustic energy translation at the wellbore-formation interfaces may be obtained. Shear wave and compressive wave attenuation rates are discussed as a means to assess fluid mobility.

Introduction

The objective of a logging service company should be to provide accurate measurement of any and all physicochemical characteristics of rock formations in situ. Historically it has been shown that any bonafide physical parameter will be found useful in providing primary or supplemental information regarding the economic evaluation of formations. An excellent example is found in the extensive use of acoustic wave amplitude recording. From the inceptions of acoustic logging, researchers in this field have studied amplitude response of the various wave types existent in the acoustic wave train. In the work of Vogel, Wylie and others, numerous speculative references to the value of amplitudes of the various waveforms have kept alive our interest in making available an accurate amplitude measurement in the borehole. More recently Pickett and Morlet have reported on the qualitative value of attenuation measurements. The qualitative use of acoustic amplitudes is now well established in estimating the effectiveness of cement bond to pipe and to formation in cased holes, as well as interpretation of amplitude anomalies in terms of numbers of acoustic interfaces intersecting the signal path of uncased holes. The following analysis of hole and formation factors which lie in the signal path, and which must be taken into account, provides a basis for calibration to a known and acceptable unit. A scale can be derived for the amplitude ratio between any two receivers, and can be stated in terms of attenuation rate, decibels per foot (db/ft). Although the user benefits from the information derived from single-receiver amplitude logs, it is apparent that a number of corrections need be applied. These are quantities which vary from well to well and include type of fluid in the hole, fluid path length and degree of acoustic coupling from fluid to formation. By use of the two-receiver amplitude ratio logging technique described here, these parameters fall by the way and the user has available a record of the effective attenuation rate of signal in the formation opposite two receivers.

Present Acoustic Amplitude Log Forms

A typical acoustic tool for amplitude logging contains an acoustic pulse transmitter and receivers at several distances from the transmitter. The main signal paths and typical resulting wave trains are shown in Fig. 1. Recording the amplitude of the first-arriving compressional wave has been most useful in bond logging. Logs of the later arriving shear wave have been used extensively for fracture location in open hole. The amplitude information in any one receiver may be recorded in several forms: 1. A curve showing electronically-measured amplitude of a particular portion of the waveform, such as the shear wave; 2. A sequence of photographs of the full wave train where the receiver signal is available at the surface equipment-information from this full "character" log, although considered quantitative, does not allow rapid comparison on location with other logs run at the same time; and 3. A variable-density presentation photographed from an oscilloscope as a function of depth-the microseismogram type of log of the signal allows rapid qualitative analysis of the acoustic wave from one receiver (Fig. 2).

JPT

P. 1243^