Synthetic Analysis of Full-Waveform Crosshole Seismic Toward CO2EOR Monitoring in Abu Dhabi

Abstract

This study aims at designing optimal crosshole seismic approach for CO 2 EOR (enhance oil recovery by CO 2 injection) monitoring in Abu Dhabi. It is not easy to perform successful reservoir monitoring for carbonate reservoirs in the Middle East because rock properties may not change much associated with fluid replacement. We investigate advantage of newly available elastic full waveform inversion (FWI) for detection of CO 2 flooded zones. To investigate advantage, applicability and future problem of time-lapse FWI for CO 2 EOR monitoring, we conducted rock physics study, elastic seismic simulation, and test application of FWI to synthetic data.We first made realistic velocity models based on Gassmann's equation and actual well logs obtained before and after CO 2 injection at carbonate reservoirs in Abu Dhabi. The in-situ light oil at the target reservoirs was replaced by CO 2 (Secondary mode CO 2 EOR). We prepared models with different width and thickness of CO 2 flooded zones and analyzed detectability of seismic waveform change. We also conducted test application of elastic full waveform inversion to synthetic data for delineation of CO 2 flooded zones.Results of the rock physics study show 1.6% decrease in P-wave velocity, little change in S-wave velocity and density associated with secondary mode CO 2 EOR. The small property changes are caused by the rigid rock frame and small density difference in light oil and supercritical CO 2 at the reservoir condition.Next we performed synthetic seismic analysis based on the rock property models, available crosshole seismic source and realistic well distances. Because of the small property change, seismic traveltime difference and waveform change are quite small when well distance is 116ft. When well distance is 700 ft or greater, guided waves, or trapped energy inside low velocity layers can be observed; relatively large waveform change can be observed in the guided waves due to CO 2 injection. Although guided waves have not been used for reservoir monitoring, they are sensitive to small Vp and thus can be an indicator of the CO 2 flooding.The elastic FWI result for a model with well distance of 1160 ft demonstrated good representation of the differential Vp with vertical resolution of 20-30 ft and some horizontal resolution. Although we need further investigation on repeatability and signal transmittability using actual field data, our synthetic study indicates possibility of identifying layers with CO 2 using guided waves and elastic FWI.