Vertical dipole CSEM - technology advances and results from the Snohvit field

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EM & Potential Methods

Vertical dipole CSEM: technology advances and results from the Snøhvit field Stefan L. Helwig,1* Abdul Wahab El Kaffas,1 Terje Holten,1 Øyvind Frafjord1 and Kjetil Eide1 present the ‘vertical–vertical’ approach to marine controlled source electromagnetic surveys and discuss its potential benefits compared with conventional horizontal dipole methods.

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ince its first test survey over the Troll field in 2006 the time-domain, marine, vertical source, vertical receiver method has seen dramatic improvements in signal to noise ratio and is emerging as a tool for high quality CSEM measurements. While in theory it is evident that the vertical electric field is highly sensitive to thin resistive structures, requirements on the equipment, especially on the verticality are not easy to meet. We present an example measured with the latest generation of vertical source, vertical receiver technology, and discuss the benefits and drawbacks of the vertical–vertical EM approach.

The field example focuses on integration of vertical EM data with other exploration tools in de-risking expensive drilling projects. The data was gathered over the Snøhvit (Snow White) field in the Barents Sea. The response of the 2300 m deep field is clearly visible in the raw transients.

Lateral boundaries can be derived with simple processing and fit the known boundaries. By its diffusive physical nature, the depth resolution of EM data is small compared to seismic data. Using depth converted seismic horizons as constraints during the inversion process is the key to ensuring a high quality result.

The sensitivity towards thin electrical resistors makes marine CSEM interesting for hydrocarbon exploration. This has led to a vast amount of research on the topic in the last 15 years. Horizontal dipole CSEM in the frequency domain, often called seabed logging (Ellingsrud et al., 2002), has been used extensively and, according to Constable (2010), it appears to be on the path to long-term acceptance and integration into the exploration toolkit.

The potential for using vertical dipoles as transmitters was described early in Russian literature and in the West for example by Kaufman & Keller (1983), Edwards et al. (1985), and Pellerin and Hohman (1994). Goldman (1990) shows the sensitivity of vertical transient measurements towards resistivities in between resistive air and a resistive basement.

Scholl and Edwards (2007) suggest the use of vertical-vertical arrangements in borehole measurements.

An offshore method using stationary vertical receivers and transmitters has been introduced to the market (Barsukov et

1 *

al.,2007) and tested on Norwegian fields (Holten et al. 2009).

Contrary to horizontal dipole EM, both receiver and transmitter are stationary during operation and data can be stacked to increase signal to noise performance. As the distance between transmitter and receiver is much smaller than in conventional CSEM, the method averages over a smaller volume of the subsurface resulting in a more focused sensitivity distribution and less distortion in 3D structures. The following sections explain the benefits and drawbacks of this technology and show a field example over the Norwegian Snøhvit field.

Advances in technology Holten et al. (2009) give an overview on the technology. To investigate the subsurface resistivity structure, receivers with vertical E-field antennas are placed on the seafloor and a vertical transmitter dipole, with an upper electrode close to sea level and a lower electrode on the sea floor, is put into position. Then a transmitter puts up to 6000 A through the antenna, and at time zero the current is rapidly switched off.

The change in the exciting field causes eddy currents in the subsurface. Their decay depends on the resistivity distribution. In a conductive environment the decay rates are slower than in a resistive one. In general the behaviour is similar to transient electromagnetic systems used in land applications.

However, due to the vertical transmitter the emitted field is a true TM mode which makes it useful for hydrocarbon exploration. A detailed discussion on modes in marine EM exploration is beyond the scope of this article and can be found in Chave (2009) or in Cuevas and Alumbaugh (2011), where a comparison between vertical and horizontal dipole techniques is also discussed.

Technically there are two main challenges when measuring vertical electric fields. Firstly, the fields decay rapidly and field values become very small quickly. Secondly, the verticality of the equipment needs to be maintained with great precision as small misalignments will cause the antenna to pick up horizontal field components. As these are several orders of magnitude stronger than the vertical components at late times (Chave and Cox, 1982), the requirements on verticality are quite stringent.

PetroMarker, Notberget 12, N-4029, Norway.

Corresponding author, E-mail: stefan.helwig@petromarker.com

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EM & Potential Methods of 6000 A through the water. Large cylindrical electrodes are used to ensure a low resistance coupling with the sea water.

Despite the huge current, the switch-off times are within a couple of ms.

At this point 45 vertical-vertical EM surveys have been carried out for 14 different clients or client consortiums. In the last 20 surveys, the fifth generation equipment was used. Three cases that had been predicted to be dry, have been drilled. Well 6609/10-2, well 6407/12-2 and well 33/6-4 were all found to be dry. Another prospect is going to be drilled this spring.

Snøhvit survey

Figure 1 Two fully assembled receivers and one receiver base on land. The units are built entirely from non-metallic material. A dampened pendulum with a dead weight (yellow cylinder) keeps the central rod vertical.

Originally, a receiver system was used that consisted of a bottom unit on the seabed with electrodes and a data logger, and a second unit with electrodes connected by a wire.

This second unit was held vertical by a buoy with strong buoyancy. While this resulted in a large antenna moment, the continuous movement of the upper unit resulted in strong noise and continuous misalignments in regards to verticality. To overcome these problems the original receivers were replaced by tripods (Kjerstad, 2009). The current fifth generation (Figure 1) ensures verticality within 0.1°.

They feature a 10 m antenna with electrodes on both ends. The verticality is ensured by a dampened pendulum mechanism that automatically aligns the centre rod. To prevent the centre rod from moving due to ocean currents, the pendulum is enclosed by a fabric curtain that allows electric fields to enter but shields the receiver from water movements.

Figure 2 shows the deployment of a receiver.

Unlike conventional CSEM receivers, the tripods are put into position by a crane. Acoustic telemetry in combination with DP2 class vessels is used to ensure placement with precision accurate enough for monitoring experiments.

The pendulum is not visible as it is hidden by the fabric curtains used to prevent ocean current induced movements.

These fifth generation receivers are more than two orders of magnitude less noisy than the previous generation.

The rapid amplitude decay is counteracted by a strong transmitter. The deck unit is capable of feeding a maximum

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The Snøhvit field is the world’s most northerly offshore gas field located at 70°N in the Barents Sea, approximately 140 km northwest of Hammerfest. Although already discovered in 1984, it was not put into production until 2006. Its total estimated reserves are 172.8 billion m³ gas, 8.7 million tonnes NGL, and 21.8 million m³ condensate. There has also been a thin layer of oil detected. The field is about 4.5 km wide and has a maximum elongation of almost 30 km.

The EM profile crosses well 7120/6-1 where hydrocarbons have been found in the Jurassic sands of the Stø formation in an interval of 84 m. The top of the Stø formation was encountered at 2385 m depth, more than 2000 m below the mud line, according to the Norwegian Petroleum Directorate (NPD) fact pages. Structurally the reservoir is in an EW striking horst structure. Figure 3 shows the survey layout on top of the field outline and the depth to the Stø formation. The southern boundary of the field coincides with a fault and a vertical displacement of the sediments of approximately 200 m.

Another displacement is evident in the reservoir section and the northern boundary shows a displacement of about 120 m.

The source rock is believed to be in the Tubåen formation and the base of the Nordmela formation. The approximately 20 m thick Fuglen formation overlays the Stø formation. Its top is

Figure 2 Deployment of a tripod receiver. The curtains that prevent sea currents moving the pendulum are installed on all receivers. The receivers are lowered to the seafloor with a crane. With acoustic telemetry they can be placed onto a predefined position with sufficient precision for repeated measurements.

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EM & Potential Methods represented by a rise in seismic impedance and an associated peak (Isaacson and Neff, 1999). While the structural changes are quite pronounced in the seismic data, only a subtle gas AVO response is noted at the top of the Stø formation (Isaacson and Neff, 1999).

Figure 4 shows a comparison of two raw transients from the northern part outside of the reservoir and from the inner part of the reservoir.

The red transient measured over the reservoir exhibits a faster decay at intermediate and late times. The eddy currents generated by the EM pulse dissipate slower in a conductive medium. In the extreme of a perfect conductor they would continue to flow unhindered. On the other hand, the eddy currents can’t flow unhindered in a resistive environment. So the faster decay over the reservoir is an indication of a resistive structure.

The majority of data in this survey was recorded with transmitter (TX) – receiver (RX) offsets of 850 m and 1250 m. The near zone transient approach makes it possible look much deeper than the offset between transmitter and receiver. This is a great advantage compared to horizontal dipole CSEM, where the maximum of sensitivity towards a deep resistive structure appears at large offsets. Constable and Weiss (2006) show that 3D responses of reservoirs of limited lateral extent are well approximated by 1D responses if horizontal source and receiver are above the reservoir and within the lateral limits of the structure. A similar statement can be made for vertical transmitter, vertical receiver time domain EM. However, as the TX–RX distance is much smaller, vertical–vertical EM surveys can see smaller reservoirs and provide a higher resolution. In the case of the Snøhvit field the target is a bit more than 2 km below the mud line and the field is about 4.5 km wide. In the same article Constable and Weiss show that a resistive feature becomes hard to resolve if the lateral extent is less than twice the depth of burial. While the field is very long, its lateral width is only slightly bigger than twice the depth of burial. A tow line perpendicular to the fields strike direction would put it in the category of hard to detect for horizontal towed CSEM. In the following sections we will show that the vertical–vertical CSEM method provides an excellent lateral resolution over the Snøhvit reservoir.

Figure 5 shows a cross section of the contrast. It is used as a simple check for a resistive anomaly and defined as Contrast =

Figure 3 Location of the Snøhvit survey. Green dots mark transmitter positions, red dots mark receiver positions. The yellow dot on the profile marks well 7120/6-1. The colour scheme shows the depth to the Stø formation.

Figure 4 Comparison of transient EM responses from the northern part outside of the reservoir (blue) and from the central part of the reservoir (red). At intermediate and late times the red curve over the reservoir shows a faster decay consistent with a more resistive subsurface.

EBG–EAN .

EBG

Here EBG is the vertical electric field at an off target background location and EAN is the electric field of the anomaly.

In our case, a transient north of the Snøhvit field (to the left in Figure 5) was used as background reference. At early times the contrast is very low. All transients are very similar to each other indicating a constant resistivity in the upper layers. At times greater than one second, however, a clear change in

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Figure 5 Contrast cross section. On the abscissa position along the survey line, on the ordinate time after pulse is shown. The colour code represents percent difference between a reference transient north of the reservoir and all transients along the section. The blue lines show the reservoir boundaries according to NPD information.

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Figure 6 Result of 1D unconstrained inversion. The colour code represents resistivity in Ωm. The black bars represent the lateral boundaries of the reservoir.

contrast can be observed in the mid part of the profile. The two blue lines represent the lateral boundaries as known from the reservoir outline found in the fact pages of NPD. They are in good agreement with the outer limits of the horst structure (Figure 3). A clear step in contrast can be observed at both lateral boundaries. At the maximum the contrast is above 50%.

Such contrast plots can be generated from the raw data without the use of inversions. The strong contrast over the reservoir creates confidence in the significance of the data.

Inversions are useful to further analyze the data but evidence of the existence of a resistive anomaly is clearly present in the raw data itself.

The 1D resistivity section in Figure 6 shows an interpolated representation of unconstrained 1D inversion results.

The two black bars show the lateral extent of the reservoir.

In the upper part of the section, an area of increased resistivity is visible. The response of the reservoir is evident from about 2000 m. Like all EM methods, transient vertical source, vertical receiver CSEM is based on diffusive physical phenomena. For thin layers low frequency EM methods are mostly sensitive to the product of layer resistivity and thickness, often referred to as transverse resistance, and can’t independently resolve resistivity and thickness of a thin resistive layer (Constable and Weiss, 2006). In the unconstrained inversion result shown in Figure 6, this results in the typical ‘smeared out’ view of the anomaly. A well-known approach to overcome this limitation is to constrain the inversion with a priori information from other sources, for example well logs or seismic data.

In case of the Snøhvit survey we have been in the fortunate situation that detailed data on the depth and thickness of the geological formations have been available.

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Figure 7 shows five horizons that have been used in the constrained inversion. The resistivity of the sea water column is routinely profiled in all these surveys. The data from these measurements are used to create a layered water column with fixed resistivity values. The high resistivity values visible in the upper section of the unconstrained results are caused by a calcite layer between 800 m to 850 m in depth. In the constrained inversion the boundaries of this layer are assumed to be straight lines. The top and bottom of the hydrocarbon-bearing Stø formation have been traced from the same seismic data set that was used in Figure 3.

The result of the constrained inversion is presented in Figure 8. The thickness of the resistive layer is now determined by the horizons. The resistivity becomes higher as the resistivity thickness product needs to stay the same in order to fit the data in the inversion.

The high resistive zone correlates extremely well with the lateral boundaries of the reservoir from the seismic data, and lower resistivity has been found on both sides of the reservoir. According to the operator, it is reasonable that the resistivity values south of the reservoir are higher than in the north.

Figure 9 shows a section of the composite log from well 7120/6-1. The reservoir section stretches from 2385 m to 2469 m. Within, resistivities of 100 Ωm are reached in the medium resistivity log (right panel, red curve) and at some points in the deep resistivity log (right panel, black curve).

Elevated medium depth resistivities are also encountered in the Hekkingen formation (2285 m–2367 m). Direct comparison of the resistivity values of the log and the vertical–vertical CSEM technology are not possible due to differences in scale and in registered component. While the vertical–vertical CSEM method registers primarily vertical resistivity and vertical resistivity values as shown in Figures 6 and 8, the

Figure 7 Horizons used as constraints in the inversion. Top and bottom of a calcite layer and of the reservoir bearing Stø formation were used to constrain layer thickness.

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EM & Potential Methods observed when the layer in question was placed at the depth suggested by the seismic interpretation. Changing the depth resulted in significantly lower misfits in the inversion.

Despite its generally poor depth resolution, EM can help to improve the seismic velocity model in such cases. However, more sophisticated workflows or joint inversion approaches (Colombo and De Stefano, 2007) may be needed. In any case, close collaboration between seismic and EM interpreters is necessary to achieve high quality results.

Conclusions

Figure 8 Cross section of the constrained inversion result.

Figure 9 Section of the composite log of well 7120/6-1. The top of the reservoir is encountered in 2385 m depth. The hydrocarbon section is 84 m thick.

The right panel shows medium resistivity log (red) and deep resistivity (black) log responses.

log registers horizontal resistivity. The well is located close to the northern edge of the reservoir and we were not allowed to measure close to the well. Therefore some interpolation features may influence the comparison. In general, higher maximum values are observed in the CSEM data. This is entirely possible as the formations of the Barents Sea are known to be anisotropic. It is also possible that due to the inversion constraints, resistivity values in the Stø formation have been over-estimated while values in the surrounding formations like the Hekkingen have been under-estimated.

In this particular case the prerequisites for a meaningful constrained inversion have been very good as there is detailed seismic data of high quality available. It is also of high importance that the velocity model used to convert the seismic data from time to depth domain is of good quality.

Again, the prerequisites have been good in this case as there are several wells in the area. Therefore the seismic data was tied to real depth. In areas with less a priori information, the situation might be somewhat more complicated. In a recent case in a different area, large inversion misfits were

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Improvements in vertical receiver technology have led to a very sensitive and trusted measuring system for vertical electric fields. In combination with a 6000 A strong transmitter, transient vertical source, vertical receiver CSEM has proven to be a very useful tool in hydrocarbon exploration in over 40 surveys. Due to the small transmitter receiver offset, the method has high lateral resolution and is less affected by 3D distortions than horizontal dipole CSEM. If permitted by the general geological setting, simple contrast plots can show lateral boundaries of resistive zones and give confidence in the significance of the acquired data.

A survey over the Snøhvit field found the reservoir boundaries with high lateral resolution at the correct location. The top of the hydrocarbon bearing Stø formation is more than 2000 m below mud line. Still, large contrasts of more than 50% have been observed over the reservoir.

Subsequent inversions of the data with seismic constraints show maximum resistivity values for the vertical resistivity of about 250 Ωm. While the maximum resistivity values observed in the 1984 logs of well 7120/6-1 show only maximum values of about 100 Ωm. This is no contradiction as the logs record horizontal resistivity and the formations of the Barents Sea are known to be highly anisotropic.

Acknowledgements The authors would like to thank PetroMarker for the permission to publish the results. Statoil provided the seismic horizons that have been used in the inversion and gave valuable contributions in the discussion of the results.

Shape files of the field extent, stratigraphy of well 7120/6-1 as well as the composite logging information are freely available information on the fact pages of the Norwegian Petroleum Directorate and were downloaded on 20 February, 2013.

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