Add a new possible use for downhole casing: It can serve as broadcast antennae. Saudi Aramco recently reported that it has successfully tested a method for mapping oil and water underground using electromagnetic waves generated by running an electrical current through the steel casing in a well (IPTC 17845).
The method could represent a cost-saving step forward for the Saudi Arabian national oil company’s long-term effort to monitor changing fluid flows in its reservoirs with electromagnetic energy to study how water injected into its fields is sweeping out the remaining oil, and see what it is missing.
Previously, Saudi Aramco created the electromagnetic field needed for this imaging method by using electrodes deep in the well to send an electrical current to electrodes on the surface. Arrays of up to 1,000 field sensors as far as 4 km away from the well gather data on how fluids in the reservoir respond to the energy field.
Water and oil respond differently when stimulated by an electromagnetic field. The high salinity water in Saudi reservoirs is an electric conductor, while oil is not. The difference in resistivity shows up in images, which also note differences in induced polarization caused by electromagnetic energy.
The goal is to map fluid distribution and how it changes over time “to identify and describe zones of hydrocarbons bypassed by water injection. This could be used to increase recovery factors and avoid premature water breakthrough,” said Alberto Marsala, a petroleum engineering specialist for Saudi Aramco production research and development. He presented the paper at the recent International Petroleum Technology Conference in Kuala Lumpur.
Using methods and equipment from two US companies, GroundMetrics and Berkeley Geophysics Associates, the new method creates a similar electromagnetic field by running a relatively low electric current from a connection to the casing at the surface, avoiding the need to insert an electrode in the well.
The approach is promising because “this allows a similar way to convey electromagnetic signals with a big advantage on cost,” Marsala said.
If this method proves able to produce useful images, it could reduce the cost and production interruptions associated with inserting equipment into a well, thus enabling the wider use of the method, which has been used in Russia and China. Using a casing to create an electromagnetic field is also easier to do in horizontal wells where tractors must be used to pull devices to the bottom of the hole.
Recent tests did not reveal any practical barriers to the new method.
The tests of the casing-based method found that the 40 Amp current used creates a voltage that appears predominantly at the surface electrodes. Marsala said “it is safe to hold the casing during transmission.”
While electric currents can speed corrosion, shortening the life of the pipe, the test concluded that this application would cause only minor metal loss, allowing long-term monitoring.
As for imaging, the paper said the top-hole casing method, like the previously development approaches, has “the potential to detect fluid movements in hydrocarbon reservoirs using sensors deployed on the surface.”
A large-scale field test was planned for early this year, Marsala said. Electromagnetic testing would add to the data sources used by Saudi Aramco for reservoir modeling. The company is working on others, including a seismic method for tracking fluid flows, and downhole gravity measures that are sensitive enough to distinguish the difference between oil and water. The barrier to in-well gravity testing is creating an affordable tool that is slim enough to fit into wells and perform reliably in hot places where it is likely to be banged around.
