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A world first for the oil and gas industry, the innovative seismic acquisition solution we have deployed at one of our deep offshore projects in West Africa is a low-cost way to improve image quality in particularly complex subsalt environments. The rewards? Improved well positioning and better reservoir monitoring for optimized hydrocarbon production.

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Constantin Gerea

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David Goyallon

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Gary Geddes

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Pierre-Louis Pichon

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Vladislav Lesnikov

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Getting better reservoirs images in the context of subsalt oil fields - Best Innovators 2016 - Exploration Production - Total

The successful development of deep offshore subsalt fields is a major challenge. The propagation of seismic waves in a subsalt oil field is disrupted by the body of salt, thus impeding visibility of the reservoirs beneath. Under such conditions, conventional seismic waves are insufficient for providing reliable imaging. The results obtained from 4D seismic acquisitions are, of course, well proven, but this method is still extremely expensive.

In order to accurately locate the dips in rock layers and track changes in subsalt reservoirs at the lowest possible cost, we have developed a cost-effective and highly effective alternative solution. Our method consists of installing permanent seismic sensors in a development well. This innovation means we can hear the wavefronts in the immediate vicinity of the reservoirs, without the disruption from the salt barrier that usually affects conventional readings. The resultant image quality is astonishing.

Our approach has been assessed, tested and optimized using 3D digital simulations of seismic wavefronts in elastic environments, combined with the computing power of the Pangea supercomputer installed in our research center in Pau, France.

Effective Seismic Acquisition at 20-50% of the Cost

This solution makes it possible to obtain a 3D seismic acquisition without the need for a costly drillship. The information thus gathered will not only improve the installation of subsalt wells, but will also improve the management of operational reservoirs over time. Moreover, this system will reduce drilling costs linked to geoscientific uncertainties of the subsalt layer, for example by avoiding the need to drill another well if the reservoir is not found in the expected location. Once a field enters production, the system can also be used as a highly cost-effective way to monitor reservoirs without mobilizing a single drillship.

Our 3D well seismic acquisition method meets the Group’s cost control targets, and costs half as much to set up as conventional surface seismic acquisitions using OBNs (Ocean Bottom Nodes), which involves placing nodes on the seabed. In addition to this, the cost of seismic monitoring campaigns during production is 80% less costly than “regular” 4D seismic monitoring.

  • Complex wave propagation in presence of steep-dipping subsalt reservoirs requires cost-effective fit-for-purpose 3D well seismic technology to unlock subsalt stakes and optimize field production.
  • Improved subsalt imagery and reservoir monitoring with cost-effective 4-D WELL Seismic to mitigate geoscience and drilling uncertainties and optimize field development.
  • Realistic 3D elastic wave simulations demonstrated that steep-dipping and overturned subsalt reservoirs yield improved seismic imagery with the 4-D well seismic survey in an attempt to mitigate geosciences and drilling uncertainties in challenging deep-offshore subsalt context.

In West Africa, the Project Will Be Deployed in 3 Stages

The first stage will allow us to validate the project’s feasibility and design. It includes the engineering, procurement, manufacture and installation of a seismic well system composed of 115 geophones. This system will be permanently deployed once the first water injection development well is completed.

The second stage consists of carrying out a two-week 3D seismic surface acquisition, with one boat equipped with a seismic source and another stationary recording boat connected to the wellhead by an umbilical line. The seismic data recorded in the well will be sent to the surface via this umbilical line and checked in real time. This acquisition will be followed by a processing and 3D imaging stage that is specific to the subsalt conditions.

Only carried out once production has commenced, this third project phase will replicate the 3D seismic surface acquisition performed in phase 2 with two boats. Its purpose is to boost production during the lifespan of the field by optimizing management of the infrasalt reservoir, all the while reducing operational costs and taking HSE factors more into account. This new acquisition can be performed “on demand” without the need for intervention in the well or deployment of drillships.

This ambitious deep offshore development project is the result of collaboration between the Geosciences, Drilling, SPS and Contacts teams for the project and the subsidiary. It was jointly led with specialists from head office and the supplier of the seismic well data recording equipment, all of whom were motivated by a desire to reach the same end goal: the successful entry into production of the next FPSO[1].

The use of this innovative 3D acquisition method for subsalt wells could be expanded to other Group projects currently under development.

 

[1] Floating Production Storage and Offloading

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