Abstract: In some embodiments, a method for reducing magnetic debris accumulation on a downhole sensor within a tubular conveyed in a borehole comprises supplying a voltage to temporarily neutralize a magnetic field of a permanent electromagnet proximate to the downhole sensor and removing magnetic debris on the downhole sensor via a flow of a first fluid in the tubular while the magnetic field is neutralized.

Abstract: A sensor device can be used to detect the presence of a cement composition in a reverse cementing operation. The sensor device can include a resistivity sensor, an acquisition and measurement apparatus, and an actuator. The actual derivative of resistivity of the drilling mud, spacer fluid, and cement composition to be used can be pre-determined. The acquisition and measurement apparatus can be pre-programmed with a slope change derivative of resistivity profile for the fluids. The resistivity sensor can measure the resistivity of the fluids as they flow past the resistivity sensor in the wellbore. When the acquisition and measurement apparatus receives a desired slope change reading within the slope change derivative of resistivity profile, instructions can be sent to the actuator to close a valve and block fluid flow through the casing or tubing string.

Abstract: A sensor device can be used to detect the presence of a cement composition in a reverse cementing operation. The sensor device can include a resistivity sensor, an acquisition and measurement apparatus, and an actuator. The actual resistivity of the drilling mud, spacer fluid, and cement composition to be used can be pre-determined. The acquisition and measurement apparatus can be pre-programmed with a resistivity profile for the fluids. The resistivity sensor can measure the resistivity of the fluids as they flow past the resistivity sensor in the wellbore. When the acquisition and measurement apparatus receives a desired reading within the resistivity profile, instructions can be sent to the actuator to close a valve and block fluid flow through the casing or tubing string.