Since ExxonMobil pioneered the 3D seismic, the industry standard for understanding subsurface geology and finding oil and gas deposits, in the mid-1960s, seismic technology has undergone further developments driven by advancements in data acquisition, processing, and interpretation.
In more recent times, 3D seismic has evolved into 4D seismic, introducing time-lapse monitoring into the equation, providing new opportunities for Enhanced Oil Recovery (EOR) projects.
The basic concept of 4D seismic technology is simple. Often called time-lapse monitoring, 4D seismic involves performing repeated 3D seismic surveys over the same geographical location to understand how some of the reservoir elasticity parameters change over time, particularly while under production. The technology correlates 3D seismic data acquired at different times, under as identical conditions as possible, in a producing reservoir to understand how the characteristics of the reservoir have changed during its lifetime.
Envision you performed the first seismic survey in 2005 and repeated the process in 2010 and 2015. Due to hydrocarbon production, formation water would seep into the pores after the oil extraction, creating pressure changes which would alter the seismic response in the two surveys after production. It is the difference between the data in the three seismic surveys that make up the 4D seismic survey. These subtle differences allow interpreters and asset teams to better understand the movement of fluids in the reservoir during production and discover areas within the reservoir with bypassed reserves.
The primary goal of 4D seismic technology is to understand changes in the reservoir’s elastic properties affected by lithology, fluid content, and changes in pore pressure. Those provide insight into how fluids in the reservoir move. This has made the technology a particularly valuable tool for reservoir management and Enhanced Oil Recovery (EOR) strategies in existing reserves, as it increases the ability to observe subsurface changes throughout a reservoir’s lifetime. The better you understand how fluids move during production, the better you can design new wells to access bypassed oil reserves.
Considering the significant financial gains of increasing the recovery factor of a reservoir, 4D promises much-needed insights that have considerable economic benefits for E&P companies.
However, performing calculations on information acquired on fields several kilometers deep, makes 4D seismic complicated to apply. Seismic data is inherently noisy, which makes it difficult to measure time shifts accurately. Additionally, the 4D method requires vast amounts of other accurate data and detailed, integrated computer models from different disciplines to quantify and validate small changes in fluids. Screening identified changes in the seismic data need to be validated against geological data at the wells, and the results need to be brought into agreement with existing numerical simulation models.
4D seismic technology has found a prominent place in the Ekofisk oil field, the first commercial hydrocarbon discovery offshore Norway, and one of the most important oil fields in the North Sea. The field has been in production since 1971 and is a large and challenging one to operate, mainly because of its soft chalk reservoirs, which compacts as the pressure depletes.
ConocoPhillips, the Ekofisk operator, was an early adopter of time-lapse technology as a tool. To help identify undepleted and unswept areas, the company has applied 4D seismic technology to improve production efficiency, attain higher recovery factors, and maximize operational safety in the Ekofisk oil field. Between 1989 and 2008, five marine streamer surveys were acquired over Ekofisk. From the first time-lapse comparison in 1999, the data quality has steadily improved, both due to better acquisition technology and continual improvements in processing techniques.
Along with the production of the Ekofisk field, ConocoPhillips engineers developed in parallel a series of specialized in-house software modules and methods to support their 4D operations. With the passing of the years and as these applications grew, it became evident that to ensure the maintenance and development of the technology into the future, it was necessary to hire the services of a specialized company.
This delicate task was assigned to Cegal, who oversaw integrating all these dispersed solutions within a single module. The current solution allows users to effectively integrate seismic information, geological models from numerical simulation, well data, rock physics models, and generate the necessary output to quantify information from changes in lithology parameters due to fluid movement during production years.