Total is a leader in 4D seismic and for many years has leveraged this monitoring technique to optimize its field developments. Backed by its numerous successful deployments of 4D seismic monitoring, notably in the Gulf of Guinea, Total’s Exploration & Production is now pushing the envelope to extend its scope of application. Frédéric Cailly, geophysicist and Head of the 4D Seismic Monitoring department at Total, shares the current status and the outlook for 4D seismic monitoring.
What role does 4D seismic play in Total’s reservoir management?
A starring role! And the more-than forty 4D seismic surveys we have performed since the 1990s on our operated acreage are there to prove it. Approximately 25 of our operated fields have benefited and continue to benefit from this geophysical monitoring technique, which we apply routinely to our deep offshore fields. In addition to our operated fields, we have been and continue to be involved in 4D seismic acquisitions and interpretation for several of our non-operated fields, in a wide range of geographical and geological contexts.
How does 4D seismic actually work?
This type of monitoring is based on comparing seismic images captured at different points in time over the life of a field. By analyzing the differences between two identically-designed 3D seismic surveys acquired at two different times, we are able to understand subsurface changes such as fluid displacements, pressure changes or variations in geomechanical properties that have taken place during the time lapse between surveys as a result of production, in both the reservoir and the overlying layers or overburden. The explanation is that production-related changes within the reservoir (including fluids distribution and pressure field modifications) change the contrasts in wave velocity and density at the layer interfaces. These changes in turn affect the reflectivity of the subsurface that is captured by the seismic imaging.
What are the main advantages of this type of monitoring?
By highlighting bypassed or poorly-drained sections of the reservoir, time-lapse (also known as 4D or repeat) seismic images enable us to evaluate the effectiveness of the existing wells as well as of artificial lift techniques (e.g., waterflooding or gas injection). This then gives us a valuable basis for optimizing the siting and trajectories of infill and additional wellbores. We have made abundant use of this approach to optimize the successive phases of our deep offshore developments in Angola.
Time-lapse seismic can also reveal a potential need for a well workover – such as a water shut-off on a producing well of a reservoir horizon highlighted by the 4D seismic – or the need to remedy an injectivity loss on an injection well. Better yet, time-lapse seismic can also help us avoid interventions if the imaging shows them to be unnecessary.
Moreover, the value of 4D seismic is not confined to the reservoir alone. We also use it for HSE purposes: Elgin-Franklin, one of our operated fields in the UK sector of the North Sea, is one example. In this case we are monitoring the overburden to detect any geomechanical changes induced by compaction of the subsurface reservoirs. The essential aim here is to improve the quality of our geomechanical model to reduce uncertainties surrounding the risks associated with changes in the subsurface strain field.
What regions of the world have benefitted the most from the use of 4D seismic?
Angola, on the Total-operated fields of the deep offshore Block 17, is where we have given the most eloquent demonstration of the power of 4D seismic. The superb quality of the repeat-seismic images acquired beginning in 2002 on the Girassol field, followed by Dalia, Pazflor and CLOV, highlighted fluid changes in the turbidite channels with remarkable resolution.
We now use 4D seismic to monitor all our field developments in the Gulf of Guinea, both in Nigeria and Congo. We have also used it to advantage in our fields in the North Sea (UK, Norwegian and Danish sectors), the Persian Gulf (Al Khalij, Qatar), Southeast Asia (the Yadana carbonate formation in Myanmar), and the Americas (Matterhorn, USA and Carina, Argentina).
Is there any chance of expanding the geographic and technical scope of 4D seismic?
Yes, we believe we can surpass the limits that currently constrain the regional scope of application for 4D seismic. Our ambition is to make it an effective solution for monitoring the oil-bearing carbonate formations of the Middle East. Some of these giant fields have reached maturity and are encountering new problems such as water control, identifying the best sites for new infill drilling, etc. But their seismic signature is often blurred due to the heterogeneity inherent in carbonate structures and their geological environment, which typically generates “multiples” – parasite seismic reflections – in the near-surface sedimentary layers that interfere with the primary seismic reflection waves from the deeper layers.
We have already scored success in our efforts to overcome these difficulties by developing ad hoc 4D processing algorithms that deliver interpretable 4D images. Some of this work was carried out in partnership with ADNOC on a pilot 4D acquisition over the shallow Umm Shaïf (Abu Dhabi) field, and with Qatar Petroleum on the giant Al Shaheen offshore field.
Another future prospect, which we tested recently on a Total-operated onshore field in Argentina, is to use 4D seismic to monitor unconventional developments. In this case, the goal during the pilot development phase is to compare seismic acquired both before and after the hydraulic fracking of the wells, to quickly obtain a better estimate of the stimulated rock volume and thereby optimize the design and spacing of the development wells.
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