Verticalized Sonic Measurements in Deviated Wellbore for Accurate Velocity Modelling and Seismic Well Tie in Offshore Malaysia

Abstract

Vertical seismic profile (VSP) or checkshot surveys are useful measurements to obtain accurate time-depth pairs for time-depth conversion in seismic exploration. However, in deviated wells, the standard geometry correction for rig-source VSPs will not provide reliable time-depth profiles because of ray bending, anisotropy, and lateral velocity variation effects. The accuracy of the time-depth profile can be improved by using model-based correction or vertical incidence VSP simulation with transversely isotropic (TI) data from an advanced sonic measurement. Elastic anisotropy parameters derived from sonic combined with VSP time-depth information are shown to accurately place a deviated wellbore within the reservoir to improve the drainage and productivity of a reservoir in offshore Malaysia. For rig-source VSP in a deviated well, the source-receiver travel path is not a vertical straight line, but an oblique, refracted path. The seismic waves from the source travel along straight paths within a layer of constant velocity. On entering another layer, they undergo refraction and the direction of travel changes. The pseudo-vertical incidence VSP is simulated with a velocity model to accurately calculate the vertical traveltime. This deviated well passes through various layers of overburden before reaching the target reservoirs. Observations from the dipole shear anisotropy, formation dip, and using dispersion analysis, indicate that these shales can be considered transversely isotropic with a vertical axis of symmetry. A single well probabilistic inversion was used to solve for the five anisotropic constants by combining the sonic measurements and prior elastic anisotropy relationships. This advanced model-based correction was the optimal solution to improve the accuracy of checkshot time-depth velocity data in combination with the anisotropic velocity model. Isotropic model-based correction showed a 6-ms time difference compared with standard VSP geometry correction. However, the sonic data in the overburden formations showed a significant amount of layering that gave rise to significant uncertainty in the existing velocity model and thus the position of the top reservoir. The anisotropic parameters were determined at sonic scale for the shale directly overlaying the reservoir. The upscaled anisotropic velocity model showed that an 18-ms time difference with standard VSP geometry correction changed the depth of the reservoir up to 45 m. The new model now placed the reservoir at the correct position and can be used with more confidence for development purposes.