Downhole motor speed measurement method

Abstract

The operating speed of a rotor in a progressive-cavity Moineau-type pump is determined on a real-time basis using frequency analysis of vibration or pressure data to ultimately compute the rotor speed. Vibration and pressure or bending moment and axial acceleration data can be used to compute the rotor rotational frequencies. A high-amplitude peak in the frequency domain in any of the data sets which corresponds to the motor whirl frequency given by .omega..sub.m N.sub.r, where .omega..sub.m represents the motor frequency in radians per second and N.sub.r represents the number of lobes in the rotor, can be isolated. The motor rpm therefore equals .omega..sub.m /2.pi..60. In addition, modulated frequency peaks such as N.sub.r (.omega..sub.m +n.omega..sub.s) and N.sub.r (.omega..sub.m -n.omega..sub.s), where the modulating frequency is the pump stroke frequency .omega..sub.s, can also be observed. There is a coupling between the two measurements, such as a linear coupling between the bending moment and axial acceleration, as well as between fluid and the motor. Using dual-channel analysis of the data, and employing a known technique of computing the coherent output power of the two signals, the method causes an enhancement of common frequencies in the two signals and an elimination of noise. The whirl frequency and the modulated frequency components are isolated so that the motor speed can be easily computed from the isolated whirl or modulated whirl frequencies on a real-time basis.

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