The problem that rigs face in any drilling operation is that of pressure encountered during drilling, in essence a rig is drilling into a pressurized container and must keep that pressure under control untill the well is drilled and a well head is installed.
There are a number of ways in which the
pressure is controlled but the primary and most important method used is to increase the specific gravity of the drilling fluid so that hydrostatic pressure at the bottom of the hole can contain the pressure within the rock formation, this is currently done using a substance called "barites" which is suspended in the drilling fluid and due to it's high weight is usually very effective at it's job but not always as witnessed by the many U-tube vidios of burning rigs.
The above is just a bit to help you understand where this idea comes from.
Ok so now onto the idea, replace barites with iron nano spheres in oil based drilling fluids known in the industry as "Invert" and presto you now have a ferrofluid that can be activated by passing a high amperage DC current down the drill string and back up the surface casing by varying the current density one can vary the viscosity from total solid to what it was before the current was applied.
when a rig has gas coming to surface, known as a "kick", then current is applied and the pumps then raise the pressure on bottom using hydraulic pressure to prevent more gas from entering the well bore and the gas already in the well bore can then be circulated out and flared off in a much more controlled fashion than would be possible using barites.
Lastly iron weighs much more by volume than barites enabling one to use less in order to achive a specific drilling fluid weight or mud weight.