Improved Reservoir Characterization through Estimation of Velocity Anisotropy in Shales

Abstract

Abstract Shales make up most of the sedimentary basins and overlie many hydrocarbon bearing reservoirs. To obtain reliable information on reservoir properties from seismic data, a good understanding of the wave propagation through shales is required. The velocity in shales depends on the direction of wave propagation, as shales are intrinsically anisotropic due to the preferred alignment of clay platelets. This inherent velocity anisotropy in shales must be taken into account for seismic imaging and inversion, seismic-to-well ties, and amplitude variation with offset (AVO) analysis. Sonic logs provide formation velocities for generating synthetic seismograms, which are subsequently used in the above mentioned seismic analyses. Due to intrinsic anisotropy of shales, the sonic logs in deviated wells can differ from the vertical well response by 30% or more. This needs to be taken into account for accurate seismic analyses. Several methodologies have been developed for estimation of shale anisotropy from deviated-well sonic logs. This paper presents a method to estimate Thomsen's anisotropy parameters from sonic logs as well as density logs in deviated wells. Three different cases are presented, encompassing exploration, appraisal & development, and production scenarios. Thomsen's anisotropy parameters are estimated using both sonic logs and density logs in deviated wells. When the well deviation is 30 degrees or more, the impact of anisotropy is significant. Accounting for anisotropy provides better confidence in the subsequent seismic analyses. For example, correct VP/VS ratio as an input to AVO analysis ensures improved prediction of reservoir fluid and lithology. Similarly, the synthetic seismograms calculated using anisotropy-corrected velocity logs result in better seismic-to-well ties and seismic inversion. Field examples illustrate the importance of shale anisotropy quantification for improved reservoir characterization. The use of both sonic as well as density logs to quantify anisotropy has the advantage of taking wave propagation direction as well as compaction into account, and may provide a better local calibration in terms of porosity and burial history.