Opening a new stage in geophysics in the Middle East, TotalEnergies and Qatar Petroleum launched one of the largest offshore seismic acquisition campaigns ever in 2019. Known as Ocean Bottom Nodes (OBN), this process represents a major advance in the improvement of the structural imaging of reservoirs and the understanding of the hydrocarbons that they contain. Feedback on this novel methodology performed in the Al-Shaheen field from Ruben Sanchez, Geophysics Manager at North Oil Company* (NOC).
What were the objectives of the “Ocean Bottom Nodes” (OBN) program?
Ruben Sanchez : Technically, unlike other regions, seismic imagery on Al-Shaheen was of average quality. Conventional seismic imaging exercises are undertaken using eight to fourteen cables of two to eight kilometers in length, usually comprising a single type of sensor called a hydrophone. A vessel tows the cables several meters below the water surface.
In an area characterized by many complex surface and at sea-level structures, this type of seismic acquisition has a number of drawbacks: it is dependent on meteorological factors such as swell and ocean currents, there are risks associated with existing infrastructure, the biosphere and maritime traffic etc. Another technique is to deploy cables directly on the seabed (Ocean Bottom Cables) containing sensors connected to a recording vessel. This technology and method provided the potential for a significantly improved deployment scenario as well as it represented a unique opportunity to meet both technical and HSE challenges.
The primary objective of this new type of OBN program was to improve our 4D seismic tools in order to obtain better a geological and dynamic understanding of developed reservoirs. We are also seeking to better define reservoirs in production (field geometry, estimation of porosity, presence of gas etc.) whilst at the same time adhering to a tight schedule and controlling costs.
How did you go about carrying out this offshore seismic acquisition program?
R. S.: Once the area to be imaged and the strategy had been defined, a bathymetric study was carried out in 2018 to map any obstructions or debris on the seabed. The images from the ROV (Remotely Operated Vehicle) and side-scan sonar readings confirmed and clarified the existing natural and/or anthropic structures in the area.
Given the complexity of the Al-Shaheen field, we identified a contractor able to undertake the seismic acquisition whilst adhering to the HSE criterion defined by the NOC referential. A seismic fleet was created comprising a source vessel, two ships for deploying and recovering the nodes and three chase vessels. In total, more than 400 people were involved.
How does the use of nodes classify as a technological achievement?
R. S.: Each node is an autonomous recording unit containing a hydrophone and three directional geophones, together with sensors used to synchronize seismic data and monitor all of the node’s movements while it is on the seabed. As they can be located as close as possible to the source of the information being sought, the date collected by these sensors is of a very high quality.
Nodes are attached independently to a rope laid on the seabed by a dedicated vessel. The device rests on the seabed under its own power for two to four weeks, recording information every two milliseconds. Once the acquisition is complete, the team simply attaches the end of the rope to a vessel and recovers all the nodes to check their integrity and download the data before redeploying them.
It is possible to position the node lines near the extraction platforms even though a minimum safety distance of 100 meters is required. The lines can also be deployed in several segments in accordance with the restrictions dictated by existing infrastructure and equipment installed on the seabed.
During this program, we utilized almost 15,000 nodes and collected over 3,000,000 gigabits of data!
Were there any specific environmental aspects you needed to take into account?
R. S.: The Arabian-Persian Gulf is highly sensitive from an environmental point of view because it contains a rich marine fauna with many sea turtles and cetaceans. In order to preserve this ecosystem, we decided not to run this program between 15 April and 8 October, the period when the world’s concentration of whale sharks is highest.
To limit the impact of the seismic activity on the marine life, we were accompanied throughout the program by specialist observers, liaison officers working with the local fishermen and the Qatari Ministry of Fisheries.
What were the results of this OBN program?
R. S.: It ended in January 2020, having required substantial operational effort on the part of a multitude of teams within the NOC, Qatar Petroleum and Qatar Gas. The quality of the data collected was very encouraging.
Only twenty transponders, seven nodes and three anchors were lost during the acquisition operations. By the end of the campaign, almost all of this equipment (87%) had been recovered by the ROV. We can now hope to undertake other seismic acquisition programs over the coming years in part of the area already covered in 2019 (4D) and other areas with significant gas and oil potential located in the center of this region (3D).
* Total (30%) and Qatar Petroleum (70%)