Method for Cement Evaluation with Acoustic and Nuclear Density Logs

Abstract

Method for evaluating cement quality in a cased well. A density log of the well is obtained using, for example, a GammaRay sources and detectors ( 51 ). The detector count rates are inverted to provide initial estimates of cement density and thickness ( 53 ). Acoustic waveform data are obtained from the well using an acoustic logging tool ( 52 ). The acoustic data are inverted ( 54 - 56 ), using the initial estimates of cement density and thickness obtained from the density logs, and an updated density log is inferred. Cement images are obtained from the updated density log, and cement bond quality can be estimated ( 57 ).

1 . A method for evaluating cement quality in a cased well environment using a density logging tool and an acoustic logging tool, comprising:
 obtaining density logs from the cased well and extracting from them initial estimates of cement density and thickness;   obtaining acoustic logs measured in the cased well;   using the initial estimates of cement density and thickness as an initial medium model, inverting the acoustic logs to infer an updated medium model, wherein the inverting is performed using a computer; and   using the updated medium model to evaluate cement quality in the cased well.   
     
     
         2 . The method of  claim 1 , wherein the inversion comprises using a forward-modeling algorithm to predict the acoustic logs with the initial medium model as input data, then comparing the predicted logs to the measured logs and updating the initial medium model to reduce misfit. 
     
     
         3 . The method of  claim 2 , wherein the method is iterative, continuing until the misfit is less than a preselected amount or other stopping condition is reached. 
     
     
         4 . The method of  claim 1 , wherein the cement quality evaluation is based at least partly on cement density and thickness images obtained from the updated medium model. 
     
     
         5 . The method of  claim 1 , wherein the cement quality evaluation includes evaluating cement bond. 
     
     
         6 . The method of  claim 1 , wherein the density logging tool and the acoustic logging tool are adapted to be lowered into a wellbore on a wireline. 
     
     
         7 . The method of  claim 1 , wherein the density tool comprises a gamma ray source and at least one detector. 
     
     
         8 . The method of  claim 7 , wherein the initial estimates of cement density and thickness are obtained by inversion of the density logs, said density logs comprising gamma ray count rates measured by the at least one detector. 
     
     
         9 . The method of  claim 8 , wherein the inversion of the density logs is performed on a computer using an algorithm comprising forward-modeling predicted gamma ray count rates using an assumed density model, comparing the predicted count rates to the measured count rates, and updating the assumed density model to reduce misfit.