Finding Value With Resistivity

From simultaneous EM and GeoStreamer® acquisition, to integrated inversion and interpretation, the PGS Towed Streamer EM solution provides cost effective subsurface de-risking. With demonstrated production of more than 200 sq. km of 3D EM acquired in a single day, a new standard has been set for EM acquisition efficiency.

Key Benefits

  • Saves time and reduces cost of acquisition
  • Accurate inversions from high data density
  • Complementary datasets improve de-risking at regional and prospect scale
  • Maximizes value for exploration expenditure

The PGS Towed Streamer EM system is designed to operate in water depths up to 500 m, and can accurately recover sub-surface resistivity down to 3 000 m below the mud line.

The system can be deployed in deeper water, though signal penetration and sensitivity to resistivity variations in the deeper sub-surface may be reduced.

To confirm our ability to accurately recover sub-surface resistivity in a particular setting, PGS conducts feasibility studies prior to acquisition. Typically this involves forward modeling of various water depths, target depths, target sizes and resistivity scenarios, followed by inversion of synthetic data from the model.

A typical Towed EM and GeoStreamer setupA typical simultaneous Towed Streamer EM and GeoStreamer acquisition setup

Applications areas for this technology:

  • Regional-Scale exploration in frontier areas
  • Near-field exploration in mature areas
  • Estimating and measuring changes in gas saturation
  • Detection of shallow gas hazards
  • Detection of gas hydrates (water depths up to 1 250 m)
  • Characterization of the overburden

Case Study – Barents Sea

Following three annual acquisition programs, PGS has built a growing library of MultiClient EM data in the Barents Sea. The 2013 project was a simultaneous 2D EM and GeoStreamer survey. In 2014 we progressed to a large scale high density 3D EM survey, then a further 3D EM project was acquired simultaneously with 2D seismic in 2015.

MultiClient 2.5D and 3D data in the Barents SeaMultiClient 2.5D / 3D data in the Barents Sea – acquired either over, or with GeoStreamer data

Each of these Barents Sea surveys highlights how the technology can be applied in regional, frontier, de-risking, as well as near field exploration.

An extensive volume of high density 3D EM data has been acquired. This has been used by clients, in conjunction with seismic, to de-risk applications for the 23rd round.

In 2013, simultaneous acquisition of EM and GeoStreamer data over Caurus and Skrugard clearly illuminated new resistive anomalies close to the known discoveries. These might have been missed if the seismic had been interpreted in isolation.

Unconstrained 3D resistivity volume overlain on fast track 3D GeoStreamer data from the Barents SeaUnconstrained 3D resistivity volume overlain on fast track 3D GeoStreamer data from the Barents Sea

Inversion and Integration

Frequency responses (navigation merged, de-noised field data) and unconstrained 2.5D sections generated using the parallel adaptive finite element MARE2DEM code are standard 2.5D EM deliverables.

The 2.5D inversion process starts with a resistivity value assigned to a half space. In the Barents Sea this value is typically between 5-20 Ωm. Thanks to the density of the Towed Streamer EM acquisition grid, altering this initial value does not significantly affect the final output of the inversion. Varying the half space value only really affects the number of iterations the inversion has to run before it reaches the final model. The number of iterations required decreases the closer the initial value is to reality.

By performing unconstrained inversion of the Towed Streamer EM data to determine sub-surface resistivity, PGS extracts the maximum possible value from the data prior to considering any constraints on the solution.

Unconstrained and seismically guided 2.5D inversions are undertaken by PGS' specialist EM Imaging and Interpretation group in Oslo. If the acquisition line spacing is 1.5 km or less, then the 3D unconstrained inversion can be performed in parallel by TechnoImaging in Salt Lake City, providing an independent QC of the inversion process at an early stage.

Depth slice showing unconstrained 3D inversion from HD3D EM acquisition in the Barents Sea Southeast, overlaid with 2.5D sections. The area shown is ~5 000 sq km.Depth slice showing unconstrained 3D inversion from HD3D EM acquisition in the Barents Sea Southeast, overlaid with 2.5D sections. The area shown is ~5 000 sq. km

Seismically Guided Inversion

To improve the resolution of the EM data, seismic horizons, or other geophysical data, can be used to guide the inversion. This guiding is softer than the traditional constrained inversion. The inversion is allowed to anticipate a significant change in resistivity at a certain horizon, but remains free to populate the cells in the inversion above this horizon in a manner which best fits the model.

Although guiding can improve the resolution, unconstrained inversion is highly valuable in itself, especially when interpreted in conjunction with dual-sensor broadband GeoStreamer data. Unconstrained inversion can highlight prospective structures identified on the seismic.

Seismically guided anisotropic 2.5D inversion of Towed Streamer EM data significantly improves the lateral and vertical resolution of resistivity anomalies, and by integrating the seismic and EM data we can maximize the value of the complementary coverage.

Example of a seismically guided inversion from the Barents Sea, Towed Streamer EM and GeoStreamer data acquired simultaneously in 2013Example of a seismically guided inversion from the Barents Sea, Towed Streamer EM and GeoStreamer data acquired simultaneously in 2013

Quantification of Shallow Gas Saturation

Consider an area where a large number of shallow amplitude anomalies are identified on 3D seismic. In the Southern North Sea these may be associated with potential gas fields. However the lack of data available to analyze their potential gas saturation levels means ranking these prospects is challenging. This could mean the difference between commerciality and non-commerciality of a field.

Towed Streamer EM enables significant de-risking of gas saturation levels pre-drill, by providing an estimate of gas saturation. Highly efficient acquisition with 1km line spacing would provide dense EM data which can be inverted in 2.5D sections as well as a 3D volume, safely, quickly and cost effectively.

Drilling Hazard ID and Characterization

With density of data acquired through a rich range of offsets and frequencies, it has been demonstrated that Towed Streamer EM is sensitive to a vertical resolution of down to one meter in the shallow sub-surface. This means that the technology can be employed to identify, characterize and monitor changes over time in shallow gas in the over-burden of a producing field, significantly reducing drilling hazards. 

Estimating Hydrocarbon Volume in Place

Seismic, well log and Towed Streamer EM data can be usefully combined to accurately estimate total hydrocarbon volume in place.

Towed Streamer EM data delivers maximum value when it is acquired, interpreted and integrated with seismic. In such cases it becomes 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 Bottom Line

Towed streamer acquisition methods significantly improve efficiency, and dramatically increase data density, resulting in cost effective and accurate mapping of sub-surface resistivity.