Study Of Low-start Fiber Grating Flow Sensors In Oil Recovery Wells

Flow measurement plays a vital role in the field of oil production logging.Accurate measurement of fluid flow information downhole of oil and gas wells is of great significance for safety monitoring in wells and improving recovery rates.In recent years,with the continuous development of the oil production industry and the increasing demand of the people for oil energy,oil production has gradually shifted from conventional to production in complex environments such as high water-bearing oil and gas and shale gas.Due to the increasing difficulty of extraction,the traditional flow detection methods are gradually unable to meet the exploration needs of people due to their low sensitivity and weak resistance to high temperature and pressure.Fiber Bragg Grating(FBG)sensing technology has gradually gained the attention of researchers due to its high sensitivity,high temperature and high pressure resistance,antielectromagnetic interference and small size,and has become an important direction of research.The fiber grating differential pressure flow sensor proposed in this thesis is suitable for realtime monitoring of small and medium flows in oil and gas wells because of its low start-up and high sensitivity.In this paper,based on the pressure differential sensing mechanism,combined with the basic theory of fiber grating sensing,a series of flow sensing research has been conducted,mainly including sensor structure design,simulation optimization and experimental measurement and other related work.The specific contents include.(1)To analyze the common ways of current oil and gas downhole flow measurement,summarize the composition structure,working principle,advantages and disadvantages of each electric type of flow sensor,compare and analyze the advantages of FBG flow sensors,and discuss the characteristics of various types of FBG flow sensors.The work of this paper is presented according to the actual problems faced by oil and gas downhole flow measurement.(2)The sensing mechanism of FBG differential pressure flow measurement is described at the theoretical level,including coupled mode theory,Bragg condition,thermo-optical effect,elasto-optical effect and thermal expansion effect.The relationship between flow and strain in the fluid domain is analytically calculated and the relevant physical model is established。(3)A low-start fiber grating flow sensor based on differential pressure sensing mechanism was designed and developed.The FBG and differential pressure sensing mechanism were analyzed,and the relationship between the amount of change of its central wavelength and the flow rate was derived.The cross-sensitivity of temperature strain can be eliminated by temperature test,then the correctness of the theoretical part is verified by static pressure experiment,and finally the sensitivity of the sensor is obtained by dynamic pressure experiment.The experimental results show that the sensitivity of the hydrostatic response of the encapsulated sensor is 86.6925 nm/MPa and 68.6286 nm/MPa,respectively;the temperature sensitivity of the two FBGs is almost the same,0.0298 nm/°C and 0.0288 nm/°C,respectively.Therefore,the effect of temperature on the flow measurement can be eliminated;the sensitivity of this flow sensor is 0.00029 L/s.(4)An FBG flow measurement method using a flat solid support beam for flow conversion unit is proposed.With the design idea of improving the sensitivity and reducing the sensor size,a flow sensor model is established,and the parameters of the sensor are optimized by combining the actual requirements of the sensor in oil and gas wells.The cross-sensitivity of temperature strain can be eliminated by temperature test is determined,and then the sensitivity of the sensor is obtained by dynamic pressure experiment.The experimental results show that the temperature sensitivities of the two FBGs are almost the same,0.0279 nm/°C and 0.0287 nm/°C,respectively.Therefore,the influence of temperature on the flow measurement can be eliminated;the sensitivity of this flow sensor is 0.00017 L/s.