Study of the Annular Restriction Effect Caused by Magnetorheological Fluids in the Presence of a Magnetic Field

摘要

Magnetorheological fluids (MRF) have their rheology modified in the presence of a magnetic field. The MRF consist of a base fluid, magnetizable particles, and a viscosifier that supports these particles. The magnetizable particles align in the direction of the magnetic field, creating a barrier, and thus modifying the rheology of the mixture. When a large enough magnetic field is applied to the MRF, the fluid's yield stress increases drastically in a matter of milliseconds. Due to this fluid response, a tunable back-pressure can be generated when the MRF's rheology is modified downhole with the use of a permanent magnet assembly as part of the BHA. One potential application of this technology is to facilitate drilling operations in narrower mud windows. As an analogy, the technology proposed in this paper works under the same principle of the conventional MPD systems, but with the additional capability of being able to apply a backpressure downhole where the magnetic assembly is located, and therefore only modifying the pressure below the tool location. As a result, abnormally pressured formations could be safely drilled without fracturing the formations above, due to the pressure profile above the magnetic assembly remaining unchanged.As a proposed way to demonstrate this principle, the shear stress variation at different magnetic field intensities is evaluated for different samples of MRF in a special rotating bob rheometer equipped with an electromagnet. A higher magnetic field applied to the MRF produces a higher shear stress, and thus a larger pressure-drop. Upscaling this effect, some of these MRF samples were circulated in a large-scale flow-loop with two concentric pipes that resemble the drill pipe and the annulus. On the flow-loop, some electromagnets and permanent magnets were strategically located to apply a magnetic field to the circulating fluid. The pressure changes (back-pressure) along the system was monitored to evaluate the effect of the magnetorheological response on the pressure variation. As an additional application, a potential electromagnetic surface choke that "activates" the MRF could create a flow restriction, and therefore a back pressure. In that sense, this electromagnetic choke could provide a finer aperture of the choke when the magnetic field is modified accordingly and could be less susceptible to erosion of mechanical parts from solids contained in the mud.