Towards an NMR multiphase flowmeter

Method development and performance evaluation for two-phase flow measurements

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There is an increasing demand from the petroleum industry for high accuracy subsea multiphase flowmeters to make the development of difficult to access deep water and ultra-deep water fields economically viable. Nuclear Magnetic Resonance (NMR) is a very powerful measuring principle, and is one of the few techniques considered to be capable of meeting the required high performance. The work presented in this thesis is a first step in the development of a non-invasive, in-line, full-bore, nuclear magnetic resonance based, multiphase flowmeter without phase separation. The continued development of the investigated system has led to the commercial KROHNE M-PHASE 5000 flowmeter, which is based on an improved prototype. To keep maintenance as low as possible, the flowmeter consists of a permanent polarizing magnet with two measuring probes, operating at 14.1 MHz, at different streamwise positions. Pipelines with an inner diameter of up to 10 cm fit through the bore of the flowmeter prototype. Two NMR measurement concepts have been developed, each with a different underlying velocity measuring principle, to determine the liquid flowrate in a gas-liquid flow: (i) the T1 Relaxation Residence Time (T1RRT) method utilizes the longitudinal relaxation principle to derive the velocity from the intensity of the NMR signal, which is a function of the residence time in the polarizing magnetic field; (ii) the Pulsed Gradient Spin Echo (PGSE) method exploits pulsed magnetic field gradients, to obtain the fluid velocity from the phase shift of a spin echo signal. Both flowmetering concepts resolve the fractions of the liquid components identically, from the multi-exponential course of the signal intensity associated with the different polarization lengths of the measuring probes. The measurement concepts have been tested for singlephase flow in 9.85 mm, 34 mm and 98.6 mm diameter pipes and for two-phase flow in a 98.6 mm diameter pipe.