Determining Acoustic Velocity of Subsurface Formations From Well Log Data

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For engineering, geophysical, and geological purposes, accurate formation velocities must be secured in areas where acoustic parameters have not been defined. Methods and techniques are discussed for correcting inaccurate well log data and for estimating velocities reliably as a function of other parameters . When acoustic values are available, cross sections and maps parameters . When acoustic values are available, cross sections and maps that require such information can be constructed. Introduction A special session on geophysics and petroleum engineering was held at the 1977 SPE-AIME Annual Fall Technical Conference and Exhibition. As the synergism between these disciplines grows, the use of subsurface information to calibrate and interpret seismic information will increase. Of particular interest are the quantity and quality of velocity information available from well logs. Acoustic velocity information is required when predicting the seismic character of subsurface formations and for translating the position of subsurface zones from depth (the engineering frame of reference) into time (the geophysical frame of reference).Many difficulties are encountered. Both well logs and seismic records vary from the measurement of absolute values and, as previously stated, their references are not the same. Altered rock types, damaged wellbores, and equipment-related spacing problems are obvious immediately and have been problems are obvious immediately and have been noted often. Unfortunately, continuous acoustic velocity log coverage exists for only a small percentage of all the wells ever drilled. Acoustic logging did not come into common use until 1957. The major service companies estimate that even today acoustic logs are run on only 40% of new wells, and no more than a small fraction of these have top-to-bottom coverage. By 1965, the gamma-gamma density log had become a routine service in the industry, and because it proved to be a superior porosity log in many provinces, the use of acoustic logs for evaluation (and consequently the gathering of velocity information) actually may have declined. Consequently, the determination of accurate formation velocities is not always straight-forward, even when subsurface information is available.Although seismic and wireline borehole methods use different sampling techniques and examine different volumes, the various measurements can be resolved to provide applicable comparative results. Formation velocities can be obtained from well log data by several methods. Acoustic Well Logs Conventional Acoustic Devices Standard spacings and configurations are engineered to minimize extraneous effects and to provide an optimum response for deriving petrophysical properties (porosity, shaliness, the percentage of oil properties (porosity, shaliness, the percentage of oil or gas, etc.) of formations in a liquid-filled borehole. For seismic comparisons, the logs must be edited and corrected to eliminate those effects that do not provide meaningful data. The lack of such data has provide meaningful data. The lack of such data has necessitated the development and use of alternative approaches for seismic problems. One alternative is the long-spaced sonic log. JPT P. 1453