Download Illuminating Reservoirs with Electromagnetics - EAGE-SEG

Index
Table of contents
Illuminating Reservoirs with Electromagnetics
L. J. Srnka
ExxonMobil Upstream Research Company, Houston, Texas USA
Introduction
The use of electromagnetic methods for hydrocarbon exploration began early in the twentieth century.
Indeed the first issue of the journal Geophysics in 1936 contained papers on this subject. Marine
controlled-source electromagnetic (CSEM) research began more than 80 years ago with DC surveys
off the southwest coast of England by the Schlumberger brothers, but has only emerged in the last five
years as a practical tool for oil and gas applications. Many of the theoretical and practical
developments needed for this technology came from university and government researchers over the
last 25 years (Chave et al., 1991; Constable, 2006). ExxonMobil began research on marine CSEM in
1981, and since 1998 has allocated significant resources to evaluating the potential of this technology
(Srnka et al., 2006). The principal strength of marine CSEM data is that they provide new information
on subsurface geology independently from the seismic method, although at significantly lower
resolution. More than 220 marine CSEM surveys have been acquired worldwide by industry since late
2000 when the first test was conducted offshore Angola by Statoil (Eidesmo et al., 2002; Ellingsrud et
al., 2002). Subsequent pre-drill prediction success rates are reported to be generally very favorable
throughout the industry. Although the method was seen initially as a “yes/no” technique for indicating
hydrocarbon presence in targets previously identified by seismic data, marine CSEM methods are
sufficiently flexible that they can be applied to a larger set of geological problems. These problems
range from detecting gas hydrates (Weitemeyer et al., 2006) to mapping facies and structure where
seismic and other geoscience information may be inadequate to provide definitive answers. In this talk
I review examples of such applications in deepwater West Africa and Brazil, stressing integration of
CSEM with other data, and then look ahead to new opportunities.
CSEM Surveying
The most useful survey technique for detecting electrically resistive horizontal or sub-horizontal layers
beneath the seafloor, including of course the commercially important hydrocarbon-bearing rocks, uses
a neutrally-buoyant towed horizontal electric dipole source and multi-component electric and
magnetic receivers on the seabed (Constable and Srnka 2007). This technique has the theoretical
advantage of exciting both vertical and horizontal electrical currents in the earth, as well as having
operational advantages. Figure 1 shows the general configuration of a marine electromagnetic survey,
using current commercial operations. Multi-component autonomous electromagnetic receivers are
placed on the seafloor in locations selected using seismic information and/or pre-survey CSEM
modeling. The continuously towed source transmits a high-current low-voltage waveform at a low
fundamental frequency, typically from 0.1 to 0.5 Hz. Low frequencies are needed to provide adequate
signal penetration to deep sub-seafloor targets, due the well-known electromagnetic “skin-depth”
effect. Most data processing, analysis, and interpretation is performed using frequency domain
techniques, although time-domain CSEM methods are being developed (Ziolkowski et al. 2006) that
may have advantages in shallow water.
Although the early proof-of-principal test surveys over large discovered reservoirs showed robust
resistivity responses, many other reservoir targets in exploration have relatively weak CSEM
responses due to any combination of large target depth of burial, small reservoir net thickness, low
resistivity contrast with the non-reservoir “background”, and limited lateral extent. However, careful
integration of these data with other geoscience information, plus application of 3D nonlinear inversion
EGM 2007 International Workshop
Innovation in EM, Grav and Mag Methods:a new Perspective for Exploration
Capri, Italy, April 15 – 18, 2007
and imaging, can produce useful results even in complex geological settings (Green et al., 2005;
Carazzone et al., 2005). In frontier basins where prior information may be limited to only sparse 2D
seismic data and regional potential field data, detection theory can be used to design optimal CSEM
surveys (Houck and Pavlov, 2006).
Figure 1. Marine electromagnetic surveying (Srnka et al., 2006).
CSEM challenges
Two major challenges must be addressed when applying marine CSEM methods for hydrocarbon
identification. The first is to acquire data with sufficient quality (sensitivity, accuracy, and signal-tonoise), frequency range, and spatial coverage to adequately address the geological problem. This
challenge can now be largely met, at least for deep water surveys and large hydrocarbon prospects,
due to advances in source and receiver technology and the growth of CSEM receiver fleets as a result
of industry uptake. Pre-survey 3D CSEM modeling is essential to properly design most surveys.
However, there are other remaining needs for more robust components in the measured data,
particularly those components that are not aligned with the source dipole moment. Novel acquisition
and processing methods are needed to enhance subtle resistivity responses, especially for targets in
which traditional seismic direct hydrocarbon indicators (DHIs) are unreliable.
The second major challenge is in data interpretation, primarily the need to discriminate potential
hydrocarbon resistivity effects from lithology effects. This need can be extreme in basins that may
contain electrically resistive non-reservoir rocks which may not be readily distinguished in seismic or
potential field data. This is especially true in frontier areas. For example, since resistivities of
carbonate sands are often much larger than resistivities of clastics of the same depositional age, for the
same pore fluids and saturations, the resistivity responses from these lithologies can be mistaken for
hydrocarbon responses. In addition, larger resistivity variations can occur within the geological unit
due to changes in rock matrix and porosity than due to fluid type, as indicated in Figure 2. A key
mitigation technique is careful 3D mapping of the CSEM response in comparison to 3D seismic
information, when this is available. Basin analogues and rock physics modeling may also be helpful.
CSEM Application In Mixed Clastic And Carbonate Settings
CSEM has been applied successfully to verify reservoir discoveries and to predict fluid type in several
prospects offshore Brazil that contain both clastics and carbonates. Several of the survey areas contain
EGM 2007 International Workshop
Innovation in EM, Grav and Mag Methods:a new Perspective for Exploration
Capri, Italy, April 15 – 18, 2007
a shallow, pervasive, resistive carbonate/marl layer. Multiple frequency CSEM data were used to
characterize this shallow resistivity, and to evaluate targets beneath it. We also used CSEM
successfully to map Brazil offshore facies that include resistive fill in a large structural feature. High
resistivities were interpreted to represent thickening of resistive interbedded marls and carbonates
infilling a Late Cretaceous graben, based on integration of CSEM, seismic reflection, and log data.
Figure 2. Resistivity ranges and alteration processes in petroleum basin rocks and fluids.
Summary And A Look Ahead
Comparisons of multi-component marine CSEM data with realistic 3D simulations from dozens of
surveys calibrated by wells have demonstrated that the geophysical fundamentals of the method are
sound. Such excellent agreement of theory and experiment over many orders of signal magnitude for a
broad range of frequencies reflects the fact that the seafloor (especially in deep water) is an ideal
locale for CSEM measurements. As expected, the robustness of data interpretation is significantly
enhanced when these data are integrated with other geoscience information. Marine CSEM methods
may be the most important geophysical technology for imaging below the seafloor since the
emergence of 3D reflection seismology some 25 years ago. But remote resistivity determination is
definitely not a “magic bullet” for hydrocarbon identification (as well log interpretation schools warn
us!) since many geologic facies such as evaporites, volcanics, coals, cemented and/or fresh water
sands, and tight carbonates often exhibit enhanced electrical resistivity relative to their surroundings
when they contain no hydrocarbons.
There is room for much more technical progress in marine CSEM, and so it is likely that this
technology has not yet reached its full potential in Upstream applications. For example, acquisition
systems will continue to improve so as to produce more reliable vector data, whose interpretation will
reduce geologic ambiguity of CSEM interpretations. More efficient and fit-for-purpose acquisition
techniques are also needed to reduce survey costs for large areas. And since resistivity can be a
sensitive measure of fluid saturation, time-lapse (or 4D) methods may be attractive for reservoir
monitoring, possibly incorporating borehole techniques with the seafloor CSEM measurements
(Scholl and Edwards, 2007). In the final analysis judgment, the key determinant of success for this
technology will be whether the value of such information is worth the money spent, relative to what
other data can provide.
Acknowledgments
I thank ExxonMobil Upstream Research Company and ExxonMobil Exploration Company for
permission to present this talk.
EGM 2007 International Workshop
Innovation in EM, Grav and Mag Methods:a new Perspective for Exploration
Capri, Italy, April 15 – 18, 2007
References
Carazzone, J.J., [2005] Three dimensional imaging of marine CSEM data. 75th Meeting, Society of
Exploration Geophysicists, Expanded Abstract.
Chave, A. D., Constable, S. C., and Edwards, R. N. [1991] Electrical exploration methods for the
seafloor. In Electromagnetic Methods in Applied Geophysics 2, 931-966, M. Nabighian (ed).
Society of Exploration Geophysicists, Tulsa.
Constable, S. [2006] Marine electromagnetic methods - A new tool for offshore exploration. The
Leading Edge 25, 438-444.
Constable S. and Srnka, L. J. [2007] An introduction to marine controlled source electromagnetic
exploration for hydrocarbons. Geophysics 72, (Issue 2, March-April, in press).
Eidesmo, T., Ellingsrud, S., MacGregor, L. M., Constable, S., Sinha, M. C., Johansen, S., Kong, F. N.,
and Westerdahl, H. [2002] Sea bed logging (SBL), a new method for remote and direct
identification of hydrocarbon filled layers in deepwater areas. First Break 20, 144-152.
Ellingsrud, S., Eidesmo, T., Johansen, S., Sinha, M. C., MacGregor, M. C., and Constable S., Remote
sensing of hydrocarbon layers by seabed logging (SBL): Results from a cruise offshore Angola.
The Leading Edge 21, 972-982.
Green, K. E., [2005] R3M case studies: detecting reservoir resistivity in complex settings. 75th
Meeting, Society of Exploration Geophysicists, Expanded Abstracts 24, 572-574.
Houck, R. T. and Pavlov, D. A. [2006] Evaluating reconnaissance CSEM survey designs using
detection theory. The Leading Edge 25, 994-1004.
Scholl, C. and Edwards, R. N. [2007] Resolving resistive targets with an electric downhole transmitter
in a marine environment. Geophysics 72, (Issue 2, March-April, in press).
Srnka, L. J., Carazzone, J. J., Ephron, M. S., and Eriksen, E. A. [2006] Remote reservoir resistivity
mapping. The Leading Edge 20, 972-975.
Wahrmund, L.A., Green, K. E., Pavlov, D., and Gregory, B. A., [2006] Rapid interpretation of CSEM
reconnaissance data. 76th Meeting, Society of Exploration Geophysicists, Expanded Abstracts
25, 709-713.
Weitemeyer, K., Constable C., and Key, K. [2006] Marine EM techniques for gas-hydrate detection
and hazard mitigation. The Leading Edge 25, 629-632.
Ziolkowski, A., Hall, G., Wright, D., Carson, R., Peppe, O., Tooth, D., MacKay, J., and Chorley, P.
[2006] Shallow Marine Test of MTEM Method, 76t h Meeting, Society of Exploration
Geophysicists, Expanded Abstracts 25, 729-731.
EGM 2007 International Workshop
Innovation in EM, Grav and Mag Methods:a new Perspective for Exploration
Capri, Italy, April 15 – 18, 2007