Research And Application Of Downhole Fiber Bragg Grating Temperature Sensing Technology

Geological movement and energy conversion are closely related to both our social development and security of life.With the expanding field of human exploration,unknown environment is becoming more complicated,in terms of oil and gas wells,the bad factors like high temperature,high pressure,high corrosion etc will occur with the Increasing depth;Harsh conditions from spontaneous combustion danger zones of mine goaf make it is difficult to achieve the long-term effective temperature monitoring.Which put forward more high requirements for the temperature sensor technology.The traditional electrical temperature sensor can not meet the real-time and high precision monitoring of downhole temperature due to the limitation of the size,structure and test range.There is no doubt the optical fiber sensing technology apply most promising application platform for underground sensing in the special environment.Compared to the electronic sensor,fiber grating sensor with the characteristic of anti electromagnetic interference,high sensitivity,small size,light weight,wide dynamic range,good temperature resistance,strong multiplexing and transmission distance(can reach some kilometers to the end of the demodulation),corrosion resistance,easy deformation and passive devices etc.In this paper,we design a fiber grating temperature sensor with high temperature resistance and the protection structure applied to the complicated underground environment in mine and oil well,and the sensor with high response speed and resolution has been applied to make a temperature monitoring experiment as well,the specific as follows:(1)The structure and sensing principle of optical fiber are described,and the relationship between the wavelength shift of fiber Bragg grating and temperature is discussed to offer the knowledge base for fabrication of fiber temperature sensor.According to the requirements of sensitivity and response of underground temperature monitoring,We fabricate the usual fiber grating temperature sensor with the base material of high strength,high elasticity and low thermal expansion coefficient,it can reach the linear degree of 0.998 and the sensitivity of 10.0pm/? when at 0-80?;In addition,in order to meet the higher application requirements,the structure and packaging method of the sensor are optimized: The sensitivity of the sensor is increased by using the coefficient of thermal expansion of copper and the paste method;Reducing the metal shell size,and adding sensitization device into the middle of the heat conduction medium to further enhance the response speed of the sensor,the final temperature sensitivity can reach 32.3pm/?,and the response speed of up to 3 times.(2)Based on the environment of high temperature deep well,We compared several kinds of high temperature resistant fiber,and chose polyimide coated optical fiber as the central sensing part,however,removing the polyimide coated layer used the traditional methods will produce damage and cannot be used in the field of the premise,we put forward a new method of removing.Using optical fiber fusion splicer,setting the electrode discharge intensity and discharge time and other parameters for the removing,and comparing the microscopic characteristics and spectral characteristics in different parameters,finally,get a method of removing method without damage and can be used in the engineering field as well.(3)Aiming at the high temperature environment and on the basis of non-damaged fiber,a kind of high temperature resistant fiber Bragg grating temperature sensor was fabricated through the design of the cone can be easily penetrated into the soil,high stability of the ceramic temperature sensor,to ensure the sealing of the stainless steel structure,such as the card sleeve,Combining with UV glue which can work stably at high temperature of 300?,The sensor with the sensitivity of 9.8pm/ ? at 300?,and can maintain for a long time stable work.(4)There are random dangers of inflammable,explosive and falling ores in mine,so we make the stress and deformation simulation of protective material by utilizing ANSYS,besides,we analysis By analyzing the force and deformation,force and Curves of force.and combined with the practical engineering experience,designing the reasonable and applicable fiber protection structure and laying scheme.Finally,it can reach the protection capacity of more than 50 meters through the practical application in the mine,which provides the technical support for the temperature monitoring of the mined goaf area in the future.This work design and fabricate an ordinary fiber Bragg grating temperature sensor as well as a high sensitivity fiber Bragg grating temperature sensor base on the principle of fiber Bragg grating temperature sensing.In addition,a method for removing the high temperature resistant optical fiber coating layer is proposed,further fabricate a kind of high temperature resistant fiber Bragg grating temperature sensor which can be applied in underground monitoring.Finally,a kind of protection structure is proposed by utilizing ANSYS that is suitable for fiber Bragg grating temperature sensor and transmission optical cable.Better results are obtained in practical mine application.This paper show the excellent commercial value and broad application prospects of optical fiber sensor entirety.