Testing Gassmann Fluid Substitution Using Sonic Logs and Core Measurements in a Sandstone Reservoir

We have developed a workflow to test Gassmann fluid substitution by comparing saturated P-wave moduli computed from dry core measurements against those obtained from sonic and density logs. The workflow has been tested on 43 samples taken from a 45m turbidite reservoir from Campos Basin, offshore Brazil. The results show good statistical agreement between the P-wave elastic moduli computed from cores using Gassmann equation with the corresponding moduli computed from log data. This confirms that all the assumptions of the Gassmann are adequate within the measurement error and natural variability of elastic properties. These results provide further justification for using Gassmann theory to interpret time lapse effects in this sandstone reservoir and in similar geological formations.