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Stephen Pharez, Petroleum Geo-Services (United Kingdom)
Anton Ziolkowski, Petroleum Geo-Services (United Kingdom)
Guy Hall, Petroleum Geo-Services (United Kingdom)
Chris Anderson, Petroleum Geo-Services (United Kingdom)
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The MTEM method
The Multi-Transient Electro-Magnetic (MTEM) method implements a current bi-pole source with a sequence of receiver stations that measure the resulting voltage. Source and receiver stations are located in a straight line similar to 2-D seismic. Onshore and offshore acquisition systems have been developed providing continuous coverage of the subsurface including the transition zone. The earth's impulse response is obtained for each source receiver pair by deconvolving the received voltage for the input current. The source signal is a Pseudo Random Binary Series (PRBS) that combined with vertical stacking allows us to maximize S/N. The subsurface resistivity is evaluated from the very shallow sediments down to the target depth by continuously optimizing the acquisition parameters. This involves adjusting the length of the source bi-pole, the bandwidth of the source PRBS, and the sampling rate of the recorded signal to be optimal for each offset range. The method allows for real time monitoring of the signal and real time assessment of the subsurface resistivity. The final deliverables are 2-D depth sections inverted to resistivity along 2-D profiles
Reservoir characterization
The MTEM data can be interpreted as a standalone product but the full value is realized when the inversion results are overlain on seismic 2-D depth sections showing the structure and stratigraphy. The reservoir(s) can then be evaluated easily for N/G, thickness and saturation. In well constrained cases where the storage capacity of the reservoir is well known, the STOOIP can be estimated.
Time-lapse monitoring
Seismic has proven the value of time-lapse or 4-D monitoring and is now uniformly recognized as an accepted technique. However, only half of all the reservoirs are suitable for seismic 4-D evaluation, whereas a large part of the remaining half should be well suited for MTEM 4-D. Resistivity monitoring has some significant strengths compared with seismic impedance monitoring. For example, many reservoirs require significant withdrawal of hydrocarbons before a detectable seismic 4-D signal is achieved. On the other hand, the resistivity changes most dramatically at maximum in situ hydrocarbon saturation facilitating 4D evaluation of possible by-passed volumes early in the production cycle. Resistivity will then provide a much larger time-lapse change for a given saturation change, but EM suffers a loss of signal when the lateral extent of the by-passed volume is small in relation to depth of burial. Each case has to be evaluated individually by means of modelling.
MTEM offers a strong alternative to seismic in many situations where seismic cannot reveal in situ characterization of fluid type and saturation, nor any 4-D changes in saturation. The two technologies complement each other and provide a very strong solution when combined. Seismic can provide the structural and stratigraphic information and MTEM reveal the hydrocarbon saturation, hence de-risking proposed drilling targets.
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