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Hydrocarbon detection and monitoring with a multicomponent transient electromagnetic (MTEM) survey

The University of Edinburgh, U.K.

Corresponding author: anton.ziolkowski@glg.ed.ac.uk

The first 20% of the full text of this article appears below.

We present 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 multichannel transient electromagnetic (MTEM) surveys.

In the petroleum industry, seismic reflection is used extensively to determine subsurface structure, and to locate potential reservoirs, but it is usually unable to determine the nature of the fluid content in the rocks. Because we wish to avoid drilling dry holes, it is obviously very important to know before drilling whether a reservoir contains hydrocarbons or not.

EM methods have the potential to reduce the risk of drilling dry holes because they can discriminate between water-saturated reservoirs (low resistivity) and hydrocarbon-saturated reservoirs (high resistivity). Until now, however, decades of research and development have failed to enable this potential to yield results of much value to the petroleum industry.

The bulk resistivity of a rock depends on its porosity, pore fluid resistivity, and saturation. Consequently, resistivity well logs are used routinely to calculate the porosity and saturation of reservoir rocks. When the pore fluid within a rock changes from water to hydrocarbons, most physical properties of the rock change. Electrical resistivity is most affected. Replacement of brine by oil in a reservoir can cause a change in electrical resistivity of reservoir rock of as much as four orders of magnitude; on the other hand, it has very little effect on acoustic impedance (Figure 1).

View larger version (26K): [in this window] [in a new window]   Figure 1. The effect of changing brine saturation on resistivity and P-wave velocity (redrawn from Wilt and Alumbaugh, 1998).