ABSTRACT

Wireline openhole formation evaluation has been the standard in petrophysics and quantitative reservoir evaluation for many years. During the last five years we have witnessed a migration of some of the basic, high quality openhole services to cased hole logging applications. One major challenge remains: measuring the formation-bulk density in cased hole environments. Several feasibility studies have indicated that under favorable casing-cement conditions density measurements are sufficiently sensitive to the formation bulk density in cased holes. Some experimental cased-hole logging tools were developed to exploit this sensitivity; however, none of these experimental prototypes has been developed into a commercial service. This study analyzes the response of a three-detector wireline density tool to formation density in cased wells. The tool response was experimentally measured in density blocks and in controlled test-tank conditions for a variety of formations, casing, and cement parameters. This new formation-density measurement has successfully been used in a number of cased wells. Under favorable casing-cement conditions the data agree with the openhole density logs within their accuracy. Several log examples illustrate this good agreement. Logging operations and log-quality-control flags are adapted to the more difficult cased hole environment. The logging speed is reduced to compensate for the reduced count-rate statistics. The photoelectric-effect (Pe) log cannot be used for lithology identification, but serves to estimate the casing thickness. The thin-bed resolution is slightly impaired. Yet, the resulting formation-bulk-density log has proved to be a quantitative formation-evaluation measurement that is readily combined with the other cased-hole formation-evaluation services to give a comprehensive formation description. Sometimes the cement thickness behind the casing may exceed a cut-off threshold. In this case, complementary nuclear-porosity logs can be used to substitute the density measurement with a porosity-inferred density. Wireline openhole formation evaluation has been the standard in petrophysics and quantitative reservoir evaluation for many years. During the last five years we have witnessed a migration of some of the basic, high quality openhole services to cased hole logging applications. One major challenge remains: measuring the formation-bulk density in cased hole environments. Several feasibility studies have indicated that under favorable casing-cement conditions density measurements are sufficiently sensitive to the formation bulk density in cased holes. Some experimental cased-hole logging tools were developed to exploit this sensitivity; however, none of these experimental prototypes has been developed into a commercial service. This study analyzes the response of a three-detector wireline density tool to formation density in cased wells. The tool response was experimentally measured in density blocks and in controlled test-tank conditions for a variety of formations, casing, and cement parameters. This new formation-density measurement has successfully been used in a number of cased wells. Under favorable casing-cement conditions the data agree with the openhole density logs within their accuracy. Several log examples illustrate this good agreement. Logging operations and log-quality-control flags are adapted to the more difficult cased hole environment. The logging speed is reduced to compensate for the reduced count-rate statistics. The photoelectric-effect (Pe) log cannot be used for lithology identification, but serves to estimate the casing thickness. The thin-bed resolution is slightly impaired. Yet, the resulting formation-bulk-density log has proved to be a quantitative formation-evaluation measurement that is readily combined with the other cased-hole formation-evaluation services to give a comprehensive formation de

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