QUANTITATIVE INTERPRETATION OF TIME-LAPSE SEISMIC MONITORING UTILIZING VSP DATA FROM FARNSWORTH FIELD UNIT: ROCK PHYSICS AND WAVEFORM MODELING

This study examines the impact of CO₂-WAG (water-alternating-gas) cycle on the seismic properties of a saturated reservoir rock by adopting a systematic fluid substitution workflow. The selected study site is the five-spot injection pattern within an active CO₂-enhanced oil recovery process at the Farnsworth field unit, Texas. Actual time-lapse Vertical Seismic Profile (VSP) measurements were acquired from the field to monitor the movement of CO₂ in the reservoir by looking at the changes in the seismic properties of the rock-fluid system.

In this work, we present the current status of the reservoir fluid and rock physics studies which are fundamental to the interpretation and integration of time-lapse seismic data for reservoir surveillance. A Petro-Elastic Model (PEM) was developed to relate the changes in the elastic properties to the petrophysical properties of the saturated rock. A comprehensive fluid analysis and rock physics modeling was performed using the site characterization data and results from the reservoir modeling studies. The results of the fluid studies were combined with the output from the rock-physics analysis to forward model the seismic velocities using Biot-Gassmann’s equation. Measured time-lapse compressional velocities, inverted from the field VSP, formed the observed datasets for the study. An assisted history matching workflow was applied to calibrate the simulated velocities using the field time-lapse seismic velocity measurements. The mismatch between the predicted and measured time-lapse velocities were minimized iteratively using a trained neural network proxy (ANN) coupled with a Particle Swarm Optimizer (PSO). The results from the simulation study indicated that the changes in the seismic velocities were negligible. Gassmann’s fluid substitution modeling results showed that the rock framework excessively masked the subtle changes in the reservoir fluid properties. The inclusion of the rock matrix in the Fluid substitution resulted in a P-wave velocity change below 1%. As a result, seismic waveforms were generated to evaluate the impact of CO₂ injection on the reflection amplitudes and traveltimes. The waveforms generated for the simulated and the actual time-lapse VSP revealed minimal amplitude differences for the baseline-monitor survey pairs.