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Electromagnetics via towed streamer

Friday, July 15, 2016

Seismic company PGS has developed a technology for recording marine electromagnetic data with a streamer towed behind a vessel, writes Joshua May of PGS.

PGS' controlled source Towed Streamer electromagnetic (TSEM) system is designed to operate in shallow water environments.

As a rule of thumb, resistivity can be confidently recovered down to a depth of 3,000m below mud line in water depths of up to about 500m.

To ensure client confidence in the suitability of the technology to their specific prospect or geophysical challenge, PGS conducts feasibility studies free of charge, often resulting in a positive outcome on a target which may have previously been considered marginal from an EM imaging perspective.

The EM streamer and associated equipment are specifically designed to look and handle, where possible, like a towed streamer seismic system. This not only ensures safe deployment and recovery by highly skilled offshore crews, but also efficient rigging of vessels new to TSEM acquisition and reduced HSEQ exposure by minimising the use of non-standard equipment.

Integrating TSEM and seismic

In order to ensure maximum understanding of the subsurface, PGS encourages the integrated interpretation of TSEM and seismic data.

To achieve this, PGS can acquire either TSEM data over existing 2D or 3D seismic data in 'EM only' mode, or 2D GeoStreamer and TSEM data simultaneously from a single vessel.

When acquiring 3D EM surveys, PGS operates with a single EM streamer with a line spacing of less than 1.5km, which enables both '2.5D' resistivity sections to be produced as well as 3D resistivity volume(s). Data acquired with a line spacing of above 1.5 km is considered '2.5D' (ie partway between 2D and 3D) and produces resistivity sections rather than a volume.

PGS has acquired EM data over a large number of known hydrocarbon accumulations with publically available well logs, ensuring that its clients can remain confident in the technique even in frontier areas, as well log resistivity values compare well to TSEM data.

One advantage of the TSEM system is the high density of data on both the source and receiver side, ensuring accurate, high resolution inversion results, with a particular uplift in resolution in the shallow subsurface.

In one survey, by combining this dense data with the efficiency of towed streamer acquisition, it was possible to acquire over 200 km2 of high density EM data in a single day, in the Barents Sea in 2014.

While TSEM can be used to great effect to de-risk large frontier areas such as the Barents Sea or the Fastnet Basin offshore Ireland, other applications should also be considered.

For example, estimation of gas saturation of 3D seismic identified shallow gas prospects, imaging the overburden of a producing field from a drilling hazard perspective, and even monitoring how the distribution and saturation of this shallow gas changes over time.

Near field exploration is particularly relevant in these current challenging times, when we are all looking for ways to reduce cost and improve value. TSEM may be able to provide the key to unlocking reserves close to existing infrastructure, potentially reducing the production cost per barrel and delaying decommissioning.

To improve the resolution and structural conformity of resistivity data, seismic horizons can be used to guide the inversion.

This guiding is softer than the more traditional constrained inversion. The inversion is able to anticipate a significant change in resistivity at a specific horizon, but remains free to populate the cells above this horizon in a manner which best fits the model.

While guided inversion can improve the resolution, unconstrained inversion remains a high value product in itself, especially when interpreted in conjunction with dual-sensor broadband GeoStreamer data.

Unconstrained inversion can highlight potentially prospective structures identified on the seismic and add an independent attribute for an improved understanding of the subsurface.

Seismically guided anisotropic 2.5D inversion of Towed Streamer EM data significantly improves the lateral and vertical resolution of resistivity anomalies, adding further value to the complementary seismic and EM data through integration of the two.

Returning to the theme of data density, PGS' standard EM streamer setup (8,700m in length) has 72 receiver pairs which vary in length from 200m at the nears, to 1,100m at the far offsets.

These overlap, and ensure a dataset rich in both frequencies and offsets, it's this rich, densely sampled dataset which provides such uplift in resolution when compared to traditional EM acquisition methods.

The high density 3D EM data acquired in the Barents Sea, for example, can also be used in more detailed reservoir level workflows, adding further value to the TSEM data.

Sensitivity at depth is also improved through this dense data, if the shot spacing for example is reduced from PGS' TSEM standard of 250m to 1,000m there is a notable decrease in sensitivity at depth in the subsurface.

PGS has invested in a significant MultiClient EM data library in the Barents Sea over the last three years.

This started from the proof of concept simultaneous EM and GeoStreamer acquisition over known discoveries during 2013, leading to the large scale high density 3D EM surveys conducted during 2014 and 2015.

When acquired, interpreted and integrated with seismic the maximum value can be extracted from TSEM data.

This makes it a highly cost effective method to de-risk frontier areas, improve well location decisions, provide drilling hazard identification and monitor changes in gas saturation over time.

The key differentiators of Towed Streamer EM over traditional acquisition methods are acquisition efficiency and the dramatic increase provided in data density; resulting in cost effective and accurate mapping of subsurface resistivity.

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