Multi-Frequency EM Method for Hydrocarbon Detection and for Monitoring Fluid Invasion During Enhanced Oil Recovery Carlos A. Dias* (LENEP/UENF), Hédison K. Sato (CPGG/UFBA) and Olivar A. L. de Lima (CPGG/UFBA)

Summary

In this work we discuss the results of an experimental study performed with a multi-frequency electro-magnetic method over a mature oil field in Recôncavo basin, Bahia – Brazil. Five 1.8 km transects 200m apart and extending over a block of the oil reservoir were surveyed. The processed EM data are represented as cross-sections of apparent resistivity and induced polarization parameter, using a consistent plotting procedure developed by Dias and Sato (1981). All the sections, controlled by seismic and well log data, although showing some distortions in the IP-resistivity configurations, allow to recognize the following geological features: (i) the oil sandstone horizons and their trapping shales; (ii) the oil-water interface and some zones of steam invasion; and (iii) lateral electric contrasts re-presenting fault zones. These results suggest the real possibility of the use of the spectral EM method in the direct detection of hydrocarbons, as well as for monitoring the efficiency of the artificial fluid injection used for secondary recovery.

Introduction

The technological developments of the last decades have produced a notable progress in the performance of EM methods. Today we have available: (i) very precise and versatile instrumentations; (ii) powerful computers to register and process a large amount of data; and (iii) adequate techniques for processing and plotting, as well as to model and to invert EM data. Most of the limitations usually attributed to EM resolution and environmental and cultural noises can be surpassed by a dense data acquisition, an adequate processing and an effective integration of the available geological and geophysical information. Due to these developments, in the last years we are testifying a large effort to demonstrate the feasibility of the EM methods in oil exploration and development (Wright et al., 2002; Eidesmo et al., 2002; Cardador et al., 2003). All these reported applications, however, involve transient techniques

used both in land and offshore. The present work is the first attempt to test the EM spectral multi-frequency technique both for the detection of hydrocarbons and to monitor fluid invasion effects related to enhanced oil recovery.

This research has also an innovative character. From its beginning as a Ph.D. work (Dias, 1968), the EM spectral multi-frequency method, proposed to measure inductively both IP and resistivity, has undergone a large development related both to its scheme of data acquisition, and graphical representation and interpretation as developed by Sato (1979) and Dias and Sato (1981). In 2001 the Canadian company Phoenix Geophysics Limited constructed a 5th generation prototype under contract for the LENEP/UENF, incorporating all the previous-referred technological advances. This is the equipment used in the present work. A detailed description of its basic units is found in Dias et al. (2001). For now, we inform that it operates with 54 values of frequency in the range 1Hz to 10kHz.

Field Procedure and Acquisition

The selected area for the experiment is part of a geologically and geophysically well studied oil field, having good quality 2-D and 3-D seismic surveys, sample data from more than hundred wells with their geophysical logs, and a detailed geological characterization of their main oil reservoirs. The studied area encompasses a sedimentary sequence having 1000m of maximum thickness, mainly composed of interlayered shales and sandstones, overlaying a crystalline basement. The oil-bearing horizons are located between 300 and 700m below the topographic surface, being sealed above by thick shales and laterally by fault zones.

The experiment was focused over a structural block having the following lateral dimensions: width of 800m, length of 1,800m and depth of 1,000m (Fig. 1). Five transects 200m spaced were laid down along the structural dip of the sedimentary layers. In each side of a transverse we used two fixed transmitter

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positions 500m apart, symmetrically disposed. Thereceiver stations were regularly spaced by 100m, theacquisition starting at 600m from the near transmitterposition. For the inductive transmission, two largeone-turn squared loops were used. One having 200mof side for receiver stations ranging from 800m to 1700m from the transmitter. Another, 400m of side, for T-R distances between 1800m and 2700m.

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Data Processing and Plotting

Part of the data processing is performed during thefield acquisition by a statistical data treatment andpresentation made within the data acquisitionsoftware. Normally the equipment is set to repeateach measurement a number of stacks, which can be increased in case of high noise. Next, from the realand imaginary parts of the mutual impedance datameasured between the transmitter and receiver coils, it is computed the apparent resistivity and theinduced polarization parameter ( a , I/| |) following the theoretical developments of Dias (1968). Thesevalues are attributed to a sub-surface point between the transmitter and receiver positions according to a procedure proposed in Sato (1979). These processingsteps are automatically made within a Fortranprogram written by Sato. The final sections are thencontoured using the GMT package (Wessel andSmith, 1998).

Results and Conclusions

Two apparent resistivity sections and two of polarization parameter are used to show the mainresults of this experiment (Fig. 2a,b and Fig. 3a,b). In each resistivity section we superpose somegeophysical well log data and delineate the maingeological contacts in the section for checking the EM interpretation coherence. The faults were definedby inspection of the electrical sections. We report thefollowing main results: (i) the lithological interfacesdefined from well log data as well as the sub-surface faults are easily correlated with the color contouringscheme presented in the sections; (ii) The oil-saturated sandstone reservoir is depicted as a highresistivity/low polarization zone in all sections; (iii) the oil-water contact is also electrically distinguishedin these sections. In addition to these features, wemay also emphasize the following inferences fromthe EM results: (a) the oil-water interface appear tobe slightly displaced upwards from it original position; (b) in some sections it is observed aresistive anomaly within the low resistivity trappingshales, that are being interpreted as resulting fromsteam injections performed in nearby wells.

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In summary, the multi-frequency EM results allow us to identify and approximately locate the oil-bearingsandstones and their water zones, the crystallinebasement and the upper shale layers, the fault zonesand the effects of steam and water injections. Thus itis quite plausible the entrance in the oil productionand development activity of a new geophysicaltechnology based in the spectral multi-frequency EM method totally developed by Brazilian scientists.

Acknowledgments

This work is part of a project jointly supported byFINEP/CT-PETRO and PETROBRAS within the Brazilian Research Network on Geology andGeophysics of Mature Oil Fields. We would alsoacknowledge CNPq by the fellowships to C. A. Diasand O. A. L. de Lima.

References

Cardador, M. H., Cuevas, A. L., Watanabe, H., Saito,A., Wada, K., Ishikawa, H. and Okuzumi, K., 2003,

Experimental evaluation of hydrocarbon detectionwith the Long-Offset Time-Domain electromagneticmethod in the Cretaceous carbonates of the Tampico-Misantla basin, Mexico. Jour. Applied Geophys., 52, 103-122.

Dias, C. A., 1968, A non-grounded method for measuring induced electrical polarization and conductivity. Ph.D. Dissertation, Univ. of California,Berkley, USA.

Dias, C. A. and Sato, H. K., 1981, A multifrequencyelectromagnetic method for interpretation of IP and resistivity: Theory and experimental work using asystem operating in the range 21 to 43,008 Hz. 51thAnn. Internat. Meet., Soc. Expl. Geophys., Los Angeles/CA, Exp. Abs., 1, 27-28.

Dias C. A., Sato, H. K., Yamashita, M., Carrasquilla,A., Sampaio, E. E. S., Lima, O. A. L., and Loures, L. G. C. L., 2001, A multi-frequency inductive 5th generation EM system for geophysical exploration.7th Internat. Cong. Soc. Bras. Geof., Salvador/BA,Exp. Abs. TS-24, 56-59.

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 directidentification of hydrocarbon filled layers indeepwater areas. First Break, 20, 144-152.

Sato, H. K., 1979, Electromagnetic method for induced polarization and resistivity interpretationusing a multi-frequency system prototype. MasterDissertation in Geophysics, Fed. Univ. of Bahia,Brazil (in portuguese).

Wessel, P. and Smith, W. H. F., 1998, New,improved version of Genetic Mapping Toolsreleased. EOS Trans. Amer. Geophys. U., 76 (33),579.

Wright, D., Ziolkwski, A., and Hobbs, B., 2002, Hydrocarbon detection and monitoring with a multi-component transient electromagnetic MTEM survey.The Leading Edge, 9, 852-864.

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EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2005 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. Multi-Frequency EM Method for Hydrocarbon Detection and for Monitoring Fluid Invasion During Enhanced Oil Recovery REFERENCES Cardador, M. H., A. L. Cuevas, H. Watanabe, A. Saito, K. Wada, H. Ishikawa, and K.

Okuzumi, 2003, Experimental evaluation of hydrocarbon detection with the long-offset time-domain electromagnetic method in the Cretaceous carbonates of the Tampico-Misantla basin, Mexico: Journal of Applied Geophysics, 52, 103-122.

Dias, C. A., 1968, A non-grounded method for measuring induced electrical polarization and conductivity: Ph.D. dissertation, University of California, Berkley.

Dias, C. A., and H. K. Sato, 1981, A multifrequency electromagnetic method for interpretation of IP and resistivity: Theory and experimental work using a system operating in the range 21 to 43,008 Hz: 51st Annual International Meeting, SEG, Expanded Abstracts, 27-28.

Dias C. A., H. K. Sato, M. Yamashita, A. Carrasquilla, E. E. S. Sampaio, O. A. L. Lima, and L. G. C. L. Loures, 2001, A multi-frequency inductive 5th generation EM system for geophysical exploration: 7th International Congress, SBGF, Expanded Abstracts, 56-59.

Eidesmo, T., S. Ellingsrud, L. M. MacGregor, S. Constable, M. C. Sinha, S. Johansen, F. N. Kong, and H. Westerdahl, 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.

Sato, H. K., 1979, Electromagnetic method for induced polarization and resistivity interpretation using a multi-frequency system prototype: Master dissertation, Fed. University of Bahia, Brazil (in Portuguese).

Wessel, P. and W. H. F. Smith, 1998, New, improved version of Genetic Mapping Tools released: EOS Transactions AGU, 76, 579.

Wright, D., A. Ziolkwski, and B. Hobbs, 2002, Hydrocarbon detection and monitoring with a multicomponent transient electromagnetic MTEM survey: The Leading Edge, 9, 852-864.

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