This project seeks to improve current methods of time-lapse seismic reflection modeling for carbon dioxide sequestration and miscible CO₂ floods in oil and gas reservoirs and to develop new strategies to invert such data to estimate changes in pressure and CO₂ saturation over time.

4th Wave Imaging Corp.
Aliso Viejo, CA

Time-lapse seismic monitoring during CO₂ injection is still in its infancy. Seismic monitoring requires knowledge of the rock and fluid properties of the oil reservoir to track changes in CO₂ saturation and pressure over time. Despite the growing importance of CO₂ for EOR projects and geologic storage and sequestration, very little is understood about its physical properties in oil-water/porous rock systems. Further understanding is needed to remotely monitor the injected CO₂ and to estimate injected CO₂ saturation distributions in subsurface aquifers and reservoirs.

Project Results
This project has resulted in new algorithms to model accurately time-lapse seismic changes during CO₂ injection, and to invert these data to estimate changes in reservoir properties, such as pressure and CO₂ saturation, that cause the seismic anomalies. Both modeling and inversion algorithms rely on rock physics relations to estimate seismic parameters, such as velocities and densities, as a function of CO₂ saturation and pressure. 

The major achievements of this project include:

• Investigations of new ways to compute fluid properties of oil-water-CO₂ mixtures.
• Development of a 1-D time-lapse well-log modeling program.
• An algorithm to generate time-lapse seismic attribute changes as a function of changes in CO₂ saturation and pressure.
• New tools to invert time-lapse seismic anomalies to yield estimates of CO₂ saturation and pressure changes and the calibration of these tools on a synthetic dataset.
• Quantification of time-lapse seismic anomalies from different vintages of a 3-D data set from Sleipner gas field in the Norwegian North Sea.

Benefits
Miscible CO₂ flooding has become an increasingly important enhanced oil recovery (EOR) technique in the U.S. for recovering residual or bypassed oil. For example, roughly half the CO₂ floods in the world are located in the Permian Basin, producing more than 20% of the area's total oil production. A broad application of CO₂ EOR methods to thousands of U.S. reservoirs may contribute an additional 43 billion bbl of reserves, which would provide significant revenues to state treasuries, provide thousands of additional domestic jobs, and improve the U.S. trade balance by reducing imports. By developing an accurate approach for tracking CO₂ fronts during EOR operations, this project is expected to help improve recovery rates, optimize well patterns, locate bypassed oil, and minimize the cost of injected CO₂. Project results also will benefit the public by addressing the potential contribution of CO₂ to postulated manmade climate change. By improving current methods for monitoring reservoir leaks and verifying the location and quantity of sequestered CO₂, this project will help minimize the impact of CO₂ on the environment.

Project Summary
The three major objectives of this DOE/NETL research project are to:

• Research and develop improved methods to quantitatively model the rock physics effects of CO₂ injection in porous reservoir/aquifer rock systems.
• Develop a means to quantitatively model the seismic response to CO₂ injection from well logs (1-D) and from digital earth model volumes (3-D and 4-D).
• Research and develop new technology to perform quantitative inversions of time-lapse 4-D seismic data to estimate injected CO₂ distributions and pore pressure changes within subsurface reservoirs and aquifers.

In Phase I of the project, researchers investigated new ways to calculate fluid properties of oil-water CO₂ mixtures under varying reservoir conditions using both EOS (equation of state) methods and molecular dynamics modeling. An EOS formulation has been developed that can calculate bulk fluid properties from multiple liquid and gas phases for supercritical CO₂ mixtures. In Phase II this EOS has been used to perform 1-D time-lapse seismic modeling to predict changes in seismic data during CO₂ injection by calculating how well-log velocities and densities change under varying CO₂ saturations and pressures. In addition, progress has been made in building a 3-D seismic modeling tool that can be used to predict time-lapse seismic anomalies in 3-D field data. In Phase III, a method was developed to invert time-lapse seismic anomalies to yield maps of CO₂ saturation and pressure changes over time. This method, which was applied successfully to a synthetic dataset, is based on the generation of seismic attribute changes as a function of CO₂ saturation and pressure, including changes in both miscible and free CO₂ levels. Many of these seismic attributes have been extracted from the Sleipner 3-D time-lapse seismic dataset for use later in the inversion algorithm.

The work has resulted an improved and efficient ability to remotely monitor injected CO₂ for safe storage and enhanced hydrocarbon recovery.

(June 2006)
The project is complete. All the tasks of Phase 1, 2, and 3 are completed. The final report is submitted to DOE.

$624,000

$156,000 (20% of total)

NETL-Purna Halder (hurna.halder@netl.doe.gov or 918-699-2083)
4th Wave-Mark Allan Meadows (mark.meadows@4thwaveinmaing.com or 949-916-9787)