Research On Zonal Fluid Fiber Bragg Grating Flow Sensor In Water Injection Wells

With the rapid development of deep oil exploration,low permeability reservoirs are steadily increasing,and they are characteristic of complex pore throats.During exploration and development,the pressure of the reservoirs is commonly maintained through injection of water,so that the development efficiency can be well improved.Accurate measurement of the flow rate of each layer in the water injection well is conducive to the study of important parameters such as reservoir physical permeability,downhole pressure of water injection wells,and oil production,and the accurate allocation of water injection volume in each layer,providing data support for adjusting production efficiency.Under the environment of high temperature,high pressure and strong electromagnetic interference,the original flow detection methods cannot meet the requirements of production work.Fiber Bragg grating(FBG)sensors have the advantages of high sensitivity,high temperature and pressure resistance,no electromagnetic interference,small size and easy to form quasi-distributed,which is very suitable for the requirements of downhole flow measurement of water injection and has great research value.Starting from the traditional target FBG flow sensor with cantilever beam as transducer,this paper improves the target FBG flow sensor step by step.Target FBG flow sensor based on lever-hinge structure and target FBG flow sensor based on differential structure are presented.The pressure of fluid is converted to FBG strain by lever-hinge structure,which significantly reduces the size of target FBG flow sensor.The differential structure constructed by two FBGs further enhances the flow measurement resolution and obtains good low-start characteristics,and can be used in water injection wells with a flow rate of 100 m3/d and low configuration water injection wells with a flow rate below 25m3/d.The specific contents include:1.The basic principle of the proposed target flow detection technology is explained theoretically and the relationship between FBG center wavelength and flow rate is given.Through finite element calculation,the structural parameters of the sensor are analyzed,and the optimal solution of the parameters is obtained,which provides a basis for manufacturing the physical structure.2.A sensor model made of carbon fiber material was developed based on the target type FBG flow sensor structure and its flow and temperature sensing characteristics were analyzed through experiments.The results show that the flow sensor made of carbon fibers can greatly reduce the temperature interference to FBG,and has the characteristics of corrosion resistance,light weight and high sensitivity.3.An FBG flow sensor based on lever-hinge structure is presented.Compared with the above sensor structure,the size is reduced by 80%,and the resolution of the sensor is improved.The flow rate ranges within 4300 L/h and the resolution of the sensor is 0.054 L/s.However,when measuring small flow,the resolution decreases to 0.062 L/s.The sensor features temperature and flow measurement with a temperature sensitivity of 21 pm/℃.The experiment has proven that the sensor has good stability and repeatability.In addition,two flow sensors with different ranges were combined to form a quasi-distributed FBG sensing array,verifying the feasibility of using the sensor for measuring stratified fluid flow in water injection wells.4.An FBG flow sensor with low startup characteristics is presented.Based on the lever-hinge structure,the differential structure is constructed by double FBG,which improves the flow resolution of the sensor again and achieves low start-up characteristics.After the improvement,the flow resolution reaches 0.039 L/s,and remains unchanged when measuring small flow.The sensitivity of the sensor to temperature is 17.95 pm/℃.Compared with the lever hinge structure,this structure has an 18% increase in flow resolution at high flow rates and 37% increase in resolution at low flow rates.Compared with other target FBG flow sensors,this structure has an increase in resolution by 17% to 48.7%.After gradual improvement,the flow sensor proposed in this paper has the advantages of small size,good resolution,large range and low start-up.The multi-stage quasi-distributed layout can be realized by connecting several fiber grating sensors in series to measure the flow rate of stratified fluid downhole for water injection and provide accurate flow data for down-hole stratified injection,stable production and enhanced recovery.