Implementation of Integrated and Practical Workflow for Fluid Substitution to Improve Seismic Inversion Results: a Case Study from a Carbonate Field in the UAE

Abstract

The field under study is structurally and stratigraphically complex, which increases the uncertainties associated with field development. To optimize reservoir characterization using seismic data, seismic inversion (Deterministic, Stochastic and Simultaneous) was planned to be performed using the 3D seismic data covering the field. Given that well log data provide constraints and calibration in the inversion workflow, their quality has a direct impact on the robustness of inversion results. This paper focuses on corrections made to sonic and density log data to make them suitable for seismic inversion.Sonic and density logs have shallow depths of investigation and are affected by borehole irregularity and filtrate invasion. Errors due to borehole irregularity in the section overlying the reservoir impact two key stages of the inversion process: wavelet extraction using well-seismic ties, and construction of the low frequency or a priori model. Invasion within the reservoir impacts the accuracy of porosities obtained from seismic impedance data. Typically porosity estimation is done using a regression relationship between porosity and acoustic impedance derived from well log measurements. The log impedance data (both velocity and density) are affected by saturation changes related to filtrate invasion which must be corrected for, to obtain a reliable porosity-impedance relationship.A robust and practical workflow to address log quality issues begins with log data quality QC: identifying zones with missing logs or borehole affected log readings. Shear sonic logs are unavailable for many wells, which limits their use for Simultaneous inversion, and for invasion correction using a rock physics model. We constructed shear sonic logs from compressional sonic logs using a rigorous, iterative process. Gassmann's equation was then used to correct for filtrate invasion effects. Invasion effects were found to change impedance values within reservoir intervals by as much as12 %, which has implications for 4-D seismic monitoring of the reservoir.