Abstract
We present the design and test of a magnetic actuator that is used to hold and release a ballast and switch buoyancy of a miniaturized autonomous well-logging tool from negative to positive. The actuator makes use of a low coercivity aluminum-nickel-cobalt (AlNiCo) magnet paired with a high coercivity neodymium-iron-boron (NIB) magnet to create and cancel a magnetic pull force on a steel plate by changing the polarization direction of the former magnet. Brief current pulses sent through a solenoid wrapped around the AlNiCo magnet by charging and discharging a capacitor. The resulting resistor-inductor-capacitor (RLC) circuit is analyzed to understand the polarization process of the AlNiCo magnet and state change of the actuator’s pull force. Our numerical solution using a nonlinear inductor model and test results showed that the repolarization can be achieved in steps using a relatively small 0.1 mF capacitor thanks to stored remnant field at each step. This made it possible to save space and energy for the miniaturized autonomous logging tool. Effects of NIB and steel weight piece on the AlNiCo polarization are experimentally analyzed. Magnetic circuit geometry is optimized using finite element analysis to maximize the pull force on the weight. The actuator occupies less than 2 cm3 and provides more than 2 kg-f to carry a ballast that weighs about 60 grams.
Highlights
Wells constitute the core of oil and gas industry and collecting timely downhole information, such as temperature and pressure, is crucial to keep them operational with optimized production rates
Because of similar remanence of the magnets, when the polarizations are in the opposite directions, the magnetic field lines cancel each other within a short distance, diminishing the pull force
AlNiCo-5 has a coercive force around 50 kA/m and an H-field larger than 100 kA/m is necessary to fully polarize the material to reach its maximum intrinsic magnetic field that is around 1.1 T
Summary
Wells constitute the core of oil and gas industry and collecting timely downhole information, such as temperature and pressure, is crucial to keep them operational with optimized production rates. With the advent of miniaturized low power sensors and electronics, it became possible to make a palm-size autonomous logging tool as illustrated in Figure 1(a) decreasing the operation footprint several orders of magnitude.2–4 This tool, once dispatched into a well, can travel down to a programmed depth, releases a weight to alter its buoyancy, and returns to surface. Moving parts can be jammed by solids or high viscosity fluids present in downhole environment such as sand particles, asphaltenes, grease, or tar In this manuscript, the design considerations, numerical analysis, and experimental results of the magnetic weight release mechanism will be presented. Because of similar remanence of the magnets, when the polarizations are in the opposite directions, the magnetic field lines cancel each other within a short distance, diminishing the pull force This mechanism has been proven useful for several applications such as self-configuring robots and microfluidic valves..
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