Electromagnetics finds the last drop

An electromagnetic device capable of monitoring the volume of oil and gas remaining in offshore fields is being developed in the UK. It could help the industry avoid expensive drilling operations.

Carrying out traditional surveys involves detonating explosives and then drilling into the seabed, an expensive and dangerous process.

The electromagnetic surveying system generates electric fields and monitors the effect of the seabed’s geology on these electric fields to detect the presence and amount of oil and gas present. The resistance of sub-seafloor sediment to electric and magnetic fields is determined by the fluids (including oil and gas) it contains.

The technology was originally created by a team of researchers from Cambridge and Southampton universities. The team has since formed a spinout company, Offshore Hydrocarbon Mapping (OHM), which will be floated on the AIM market this year.

OHM is now developing the technology to monitor the volume of oil and gas within the fields.

The survey technique involves towing a dipole source just above the seafloor. The dipole source transmits a low frequency (from a few tenths to a few tens of Hz) electromagnetic signal to an array of receivers that detect and record the electric field at the seafloor. By studying the variation in amplitude and phase of the received signal as the source is towed through the receiver array, the structure of the sub-surface can be determined at scales of a few tens of metres to depths to several kilometers.

The receivers remain on the seabed, detecting the geological responses, until an acoustic signal is sent down to them. On receiving this, the sensors automatically detach a weight and float to the surface for recovery, where their data is analysed.

Unlike seismic technology, the system can identify the edges of gas and oil fields within a seabed’s substructure. These edges could previously only be identified by costly drilling operations.

It is estimated that only a third of the content of existing oil fields has been retrieved, so it is important for the future of energy supplies that the industry has a precise understanding of the extent of existing fields, and how many barrels they still contain.

OHM was formed in 2001 following successful trials of the technology, in conjunction with Norway’s Statoil, off the west African coast. The company undertook a commercial survey in 2002 for a major US oil producer and now has a long-term deep-water exploration contract with the firm.