A Theoretical Research On On-line Detecting Of Downhole Flowrate Based On Bragg Grating

In the oil production, the flowmeter is applied throughout the exploitation,storage，transportation, processing and sales of oil. The measured flow, which isone of the most important parameters, is used to determine the production of layer,identify and determine abnormal production, ascertain mixed fluid production andso on. At present, the suspended body flowmeter, turbine flowmeter andelectromagnetic flowmeter are widely used to monitor the flow underground inDaQing oilfield. However, due to many disadvantages such as bad equipment’spressure resistance, complex mechanical structure, low SNR during the transmissionof long distance along the cable, inconvenient installation and so on, as a result, it isan essential issue to monitor the flowrate accurately underground at present.Based on Bragg grating sensor principle and Karman vortex street principle, anovel fiber Bragg grating vortex flowmeter is designed independently, where theBragg grating is enfolded in a cylinder eddy generator. Subsequently, therelationships between wavelength and flow are developed. The mechanical modelof the cylinder is established in the grating vortex flow monitoring system, whichanalyses parameters on the effect of flow. In order to improve the monitoringsensitivity of the grating vortex flowmeter, the system is simplified to be a mass-spring-damper model, which is used to analyze resonance and determining the sizeof cylinder. In terms of the resolution of FBG sensor demodulation device andfactors of vortex’s formation, the measuring range of flowmeter is determined.The result from calculation by the mechanical model and vibration model of thecylinder is compared with the simulation by Fluent, demonstrating the theory modelestablished has the certain advantages. Building a single-phase flow experimentalplatform, using the higher precision electromagnetic flowmeter calibrates the Bragggrating vortex flowmeter and the linearity is within1.35%. The relative deviation，which can reach2.25%, is analyzed between experimental curve and the theoreticalcurve, and then putting forward the improvement measures to reduce the error.On the base of single-phase flow’s monitoring system, two-stage series venturitube is designed for monitoring gas-liquid two phase flow, which is combined with temperature-pressure sensor system based on grating to monitor temperature andpressure. By the large aperture ratio of venturi tube’s first stage to depressurize thefluid and it is facile to lay the grating sensor in the throat of venturi tube’s secondstage with the appropriate throat size. Through the characteristics of the gas phase’svolume flow sensitive to pressure, reducing pressure in short distances in order todistinguish the volume of gas and liquid flow, which achieves monitoring gas-liquidtwo-phase flow. The two phases volume flows achieved are calculated as the initialvalues, using the pressure drop iterative equation of venturi tube to determine thestable output of flow, and then getting some parameters such as air rate. Build themodel of two stage of venturi tube by Fluent, using VOF method to simulate theflow around in the pipe by two phase flow, and to analyze the result of contrast withtheoretical model. In addition, the plan of grating’s lay is designed to prepare for theexperiment.According to the design of the grating sensor’s structure, the mathematicalmodel of the grating flow monitoring system is set up to determine single-phaseflow monitoring accuracy, which can reach12L/h. The structure of two stage seriesventuri tube is used to distinguish volume flow between gas flow and fluid flow,which is based on the pressure drop equation iteration to get volume flow for eachphase, guiding practical production.