Ferrofluid applications in petroleum engineering

Date

2021-04-09

Authors

Wang, Ningyu (Ph. D. in petroleum engineering)

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Abstract

Ferrofluid is a stable suspension of superparamagnetic nanoparticles (SMNPs). Because of their small size, and with suitable coating, SMNPs stay suspended in the fluid and their retention in porous media is minimized. When exposed to an external magnetic field, the SMNPs align with the magnetic field, triggering magnetic, mechanical, and thermal phenomena. When the external magnetic field is removed, the orientation of the SMNPs restores randomness. In this study, we explored two applications of ferrofluid in petroleum engineering. First, we explored the potential of SMNPs for flow assurance, more specifically, dewaxing. Exposing SMNPs to an alternating magnetic field produces heating due to Neel’s relaxation. SMNPs can be dispersed, lodged within a coat of paint, and applied to a pipe’s interior (nanopaint). The heating effect of nanopaint in a pipe was initially explored and the idea of injecting an electromagnetic source into pipeline for dewaxing purposes was proposed by Mehta et al. in 2015. However, they performed fewer than 10 simulations and did not consider pig movement or flowing hydrocarbon. In this study, three numerical models were set up to study the heating process of an electromagnetic dewaxing system in a pipeline with wax deposition. The device that emits the alternating magnetic field is named an electromagnetic pig, and the corresponding dewaxing process is called electromagnetic pigging. Induction heat is generated in the nanopaint layer in the pipeline and is transported to the deposited wax to melt at least part of the wax to dissolve the wax back into the flowing hydrocarbon or to peel the wax off the pipeline wall. The heating effectiveness and efficiency of a simplified electromagnetic pig composed of a single solenoid coil were numerically studied in the commercial multiphysics simulation software COMSOL. Heating effectiveness was evaluated by heating zone length, heating zone depth, and maximum pig speed, while heating efficiency was evaluated by pig induction factor (PIF). Simulation results show that the solenoid coil should have a longer radius and shorter length to achieve better wax-heating performance. Shorter coil length slightly increases the heating effectiveness and heating efficiency. Longer coil radius increases the heating effectiveness and yields stable heating efficiency. The impact of coil radius on heating efficiency is complicated, while longer coil radius avoids the lowest-efficiency region. The pig speed should be faster to decrease operation time and increase heat efficiency, as long as the heating effectiveness does not drop too much and the wax can still melt and peel off. A battery of100 kW h can power the pig to melt 216 kg of deposited wax and peel off several times more wax, which is sufficient to dewax a 0.2-m inner radius pipeline of 1.78 km (1.1 mi). To melt a longer section of pipeline, a larger battery set or a fleet of pigs would be necessary. Although the system is designed to lower the hydrocarbon temperature in the pipeline, higher hydrocarbon temperature helps the electromagnetic dewaxing. Second, we explored ferrofluid-fluid displacement at the pore scale with the potential to enhance oil recovery using microfluidic experiments. Nanoparticles have great potential to mobilize trapped oil in reservoirs by reducing the oil-water interfacial tension, altering the rock wettability, stabilizing foams and emulsions, and heating the reservoir to decrease the oil viscosity. However, the direct application of magnetic forces on SMNPs in reservoir engineering applications has not been extensively investigated. Possible oil recovery by magnetic forces when the magnetic field is parallel to the flow direction was predicted by Soares in 2014, but no experiment results have confirmed the oil recovery or the mechanism. In this study, we demonstrate the enhanced oil recovery (EOR) potential of hydrophilic magnetic nanoparticles in oil production by direct observation using microfluidics, and we examine the mechanism and hypothesize new theories to explain the experimental phenomena. Ferrofluid flooding experiments were performed in a micromodel of a converging-diverging single channel and then in a micromodel of a foot-long pore network, both with varying depth (so-called 2.5D micromodel). The micromodels were made of glass, and thus, the water-based ferrofluid was the wetting fluid. Initial ferrofluid flooding experiments in single channels were performed under a static magnetic field transverse to the flow direction. This magnetic field caused oil droplet deformation, dynamic break-up into smaller droplets, and subsequent residual oil saturation reduction. Significant oil blob displacement was observed within 2 hours after the magnetic field was applied. During one flooding experiment, the oil saturation within the observation area of the micromodel reduced from 27.4% to 12.0%. This result contrasts with the prediction of the theory of magnetic forces, and we hypothesize that oil recovery is at least partly attributed to the temporary SMNP microstructures in the ferrofluid when exposed to an external magnetic field. We then designed a rotating magnetic device to examine the hypotheses and to reveal that a changing field would have an even larger effect on saturation reduction. We subsequently observed a completely different phenomenon, namely self-assembly of oil droplets, indicating the formation of hydrophilic SMNP microstructures (chains under the magnetic field). These SMNP microstructures were ever-changing under the rotating external magnetic field. While the ability of ferrofluid to rotate small blobs was interesting in and of itself, in experiments without actual flooding (and thus synergy of hydrodynamic and magnetic forces), we did not observe any additional oil recovery. Further ferrofluid flooding experiments were performed in a foot-long 2.5D micro-model in a rotating external magnetic field to study the oil recovery effect of the rotating magnetic field at the core scale. In one experiment, the oil saturation dropped from 44.6% to 33.3% after the rotating magnetic field was applied. The additional oil recovery of 11.3% shows good potential for ferrofluid flooding with a rotating external magnetic field.

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