Sonic Log Based Velocity Optimization with Perforation Shots in Unconventional Oil and Gas Field

Abstract

Hypocenter locations are essential information for evaluation fracking area. It is necessary to build realistic velocity model to get accurate locations. Anisotropic velocity models could represent actual travel times much more than isotropic ones in shale dominant fields. Therefore, anisotropic velocity optimization using perforation shots data is important. In this case study, gamma logs and sonic logs are available. Considering 3D reflection seismic survey interpretations, geological structure is assumed as horizontal layers. In the target depth, 8 sequences are defined in gamma logs. The sequences are classified into 5 velocity layers, and linear fitting functions are applied to the velocity trends. Anisotropic velocity model optimization procedure is carried out with perforation shots data. To take into account the velocity trends, the next 3 items are set as velocity model parameters, (1) Scaling factor for Vp, (2) Vp/Vs ratio, and (3) Thomsen parameters. The total number of the parameters becomes 19 in this study. Newton’s method is applied to solve velocity model optimization problem. The Jacobian matrix is derived by finite differences of calculated travel times. The travel time residuals of the results indicate improvements of velocity model from the original velocity model. This procedure reflect much geological setting.