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First Direct Hydrocarbon Detection and Reservoir Monitoring Using Transient Electromagnetics

Anton Ziolkowski from the Department of Geology and Geophysics at the University of Edinburgh presents the results from a transient electromagnetic experiment to detect hydrocarbons and to monitor their movement within a reservoir. The method is illustrated with data obtained from two Multi-channel Transient ElectroMagnetic(MTEM) surveys carried out in 1994 and 1996 over an underground gas storage reservoir at St. Illiers la Ville in France. (co-authors Bruce Hobbs and David Wright) (Anton.Ziolkowski@glg.ed.ac.uk)

Introduction
Replacement of brine by gas or oil in a reservoir can cause a change in electrical resistivity of a reservoir rock of as much as four orders of magnitude, but has very little effect on the acoustic impedance. Therefore electrical methods are intrinsically more suitable for hydrocarbon detection than seismic methods. The data presented here were collected as part of two Multi-channel Transient ElectroMagnetic (MTEM) surveys carried out in 1994 and 1996 over an underground gas storage reservoir at St. Illiers la Ville in France. A description of the experiment is given by Hördt et al. (2000). The reservoir is a 30% porosity sandstone anticline about 30m thick at a depth of around 700m. In the summer gas is pumped in and the gas-water contact falls; in the winter gas is extracted and the gas-water contact rises. The position of the contact is known from constant monitoring at over 40 wells. The surveys had two objectives: first, to attempt to detect the reservoir directly from the data; second, to detect the movement of the gas water contact between the two survey times. A recent breakthrough in the understanding of the system has allowed both these objectives to be achieved.

Data Acquisition
The MTEM data acquisition layout used was very similar to that of 2-D seismic reflection profiling with a controlled source 'fired' at intervals of 250 m over a 7 km profile length. The response was measured simultaneously at 32 receiver positions, spanning the central 4 km of the profile. The source was a bipolar current waveform produced in a wire approximately 250m long, grounded at both ends. The current amplitude was approximately 30 amperes. The recorded transients were of 3 forms: the inline, and crossline components of the electric field, and the vertical rate of change of the magnetic field. The MTEM line with respect to the reservoir is shown in Figure 1. The source was 'fired' between 50 and 100 times at each location. For each 'shot' a record was made of each channel of 2048 samples at 1ms sampling.

Figure 1: Location of the MTEM profile with respect to the reservoir. White circles denote monitoring wells.
Results

The data were processed to obtain the impulse response of the earth for each source-receiver pair. The impulse response is positive for a positive current, but has fluctuations. To plot the response in a time-distance display, similar to a seismic section, it is convenient to differentiate. This produces a response that has positive and negative excursions. Figure 2 shows a common-offset section of this differentiated impulse response for the 1994 data. The reservoir is clearly picked out. Figure 3 shows the corresponding result for the 1996 data. A comparison of these two figures shows the repeatability of the data for hydrocarbon detection and the obvious potential for monitoring the movement of hydrocarbons within the reservoir.

Figure 2. A common-offset gather of the processed 1994 EM response over a gas storage reservoir in France.

The green anomaly at 388 ms corresponds to a resistive rock. This is the part of the reservoir that is filled with gas. At the edges at less than 3000 m and more than 4950 m the reservoir is filled with brine, which is conductive. The time break is at 384 ms.

Figure 3. A common-offset gather of the processed 1996 EM response over the gas storage reservoir in France.

Conclusions
The interpretation of transient electromagnetic data has traditionally relied entirely on fitting the synthetic response of simple models to the observations. In our novel processing approach to interpretation the complexity of the earth is revealed in much the same way as is done in the processing of seismic data. We are now in a position to develop the method for greater resolution than was obtained here.

Acknowledgements
The data shown in this paper were collected as part of an EEC THERMIE project entitled 'Deliniation and monitoring of oil reservoirs using seismic and electromagnetic methods' (Contract OG/0305/92/NL-UK). The project was a joint collaboration between the University of Edinburgh, the University of Cologne, Deutsche Montan Technologie, and Compagnie Generale de Geophysique. The project was also sponsored by Elf Enterprise (Contract CA5527). We are indebted to Gaz de France for their cooperation and provision of the site for the experiment. We acknowledge our colleagues Pierre Andrieux, Andreas Hördt, Horst Rüter, Keeva Vozoff, Kurt-Martin Strack, and Frits Neubauer. David Wright is supported by the Natural Environment Research Council, studentship number GT 04/99/ES/82.

References
Hördt, A., Andrieux, P., Neubauer, F.M., Rüter, H., and Vozoff, K., 2000, A first attempt at monitoring underground gas storage by means of time-lapse multichannel transient electromagnetics: Geophysical Prospecting, 48, 489-509.
Wright, D.A., Ziolkowski, A., and Hobbs, B.A., 2001, Hydrocarbon detection with a multi-channel transient electromagnetic survey: Expanded Abstracts 71st SEG Meeting, 9-14 September, San Antonio, p 1435-1438.

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