Constrained Optimization for Velocity Prediction in Rock Physics Study

A new approach named "constrained optimization" for velocity model has been developed. This model focuses to the reservoir parameters for well-log evaluation with the inversion of pore aspect-ratio, so that rock elastic properties and velocities can be predicted accurately. For implementation, removing the environmental effects from well-logs is an important and hard stage because the quality of log data may be highly variable due to borehole environments with bad well conditions during logging. The principal effects in variability were usually from borehole enlargements or rugosity which may result in poor log data within certain depth intervals. In order to minimize these effects, the method of constrained optimization has been proposed in our study for one of gas fields in Sichuan basin which showed a good agreement to compare with the acquired core, image logs and fluid testing. The constrained optimization model provides an important improvement for the application of the Xu-White velocity model in which a variable pore aspect ratio log to be introduced into the Kuster and Toksöz model to replace the assumption of fixed aspect ratio. In summary, the constrained optimization model shows a significant improvement in the predicting of velocity for AVO analysis.