Summary

An offshore technique time domain EM method that uses vertical, transmitters and receivers is studied. We look at three different aspects of this method: the effect of induced polarization, the sensitivity to a given target and finally edge detection of a reservoir. A case study from the Troll gas field is presented. Measurements from different offsets can be accounted for by fitting parameters for induced polarization, and for offsets more than or equal to 750 m, the induced polarization signal is small compared to the non-polarized signal level. The sensitivity of a reservoir resistivity and overburden thickness and resistivity are studied using a simple 1D model. The sensitivity decreases with increasing overburden resistivity and thickness as expected. The time of maximum sensitivity increases with depth and reduction of overburden resistivity. Finally, we analyze the difference between the responses from a 1D model and a model with a cylindrical shaped reservoir. Numerical simulation shows that, for the same reservoir and background resistivities, the amplitude of the reservoir signature is largely controlled by the reservoir width-to-depth ratio. As could be expected, the signature of the reservoir increases with an increase of the diameter of the reservoir, though changes are relatively small when the diameter is increased significantly beyond the reservoir depth. Typically, the maximum signal contrast due to the reservoir remains within 20% of its asymptotic (1D) value when the width goes above twice the depth below sea bottom. Thus, our modeling shows that in many cases a 1D model gives a good estimate for the electric field response, and often is sufficient for pre-survey forward modelling.

Introduction

Over the last decade, several CSEM method has been developed (Weiss and Constable, 2006). An offshore, time domain EM method that uses vertical, stationary transmitters and receivers has been developed by the Norwegian petroleum exploration company Petromarker (Holten et. al, 2009; Børven and Flekkøy, 2009). Short offsets in the range of 500 to 1500 m are used to probe the electric near-field that results from turning off a source current. The current source is a pulsing system that consists of two pulse generators working in parallel. Each transmitter dipole has a current capacity of 2500 A and consists of two electrodes attached to the vessel by cables. The lower pulse electrode is positioned on the seabed and its position is determined by an acoustic transponder. Once the lower pulse electrode has been installed on the seabed the upper electrode is lowered 30-50m below the sea surface and, by moving the vessel, placed within a meters accuracy above the lower electrode. Square pulses, with polarity varying according to an 8 bit Thue-Morse sequence (+--+-++-) are used to energize the transmitter cable. There is a silent period after each individual pulse. The vertical electric field is measured during silent periods. The fact that the horizontal field component from a horizontal transmitter greatly exceeds the vertical field component from a vertical transmitter, imposes rigid limitations on the permissible tilt angles of the transmitter and receivier.

This content is only available via PDF.
You can access this article if you purchase or spend a download.