FIBRE OPTIC PRESSURE AND TEMPERATURE SENSOR FOR GEOTHERMAL WELLS / PRESENTED BY; Jose dominic EI-S8 Roll.no:26 Guide: Muhzina MH

CONTENTS  INTRODUCTION  SENSOR TYPES  FEATURES OF FIBRE OPTIC SENSOR  APPLICATIONS OF OPTICAL FIBER SENSOR  PRINCIPLE OF OPERATION  EXPERIMENTAL SET UP  EXPERIMENTS  CONCLUSION  REFERENCES

INTRODUCTION  Optical fibres are made of transparent dielectric whose function to guide light over long distances.  In industries, fibre optic sensor used to monitor quantities such as displacement,pressure,temperature, flow rate etc.  The use of geothermal energy is an important issue of future energy supply within strategies for the mitigation of climate changes.  A fibre optic sensor is developed and tested to measure pressure and temperature under simulated wellbore conditions.

 The sensor consists of: a.miniature all-silicafibre optic Extrinsic Fabry-Perot Interferometer (EFPI) pressure sensor b.an encapsulated Fibre Bragg Grating (FBG) for temperature sensing.  The fibre optic sensor head is formed from silica glass components only by splicing a Single Mode (SM) FBG,a silica glass capillary and a 200μm silica glass fibre together.  Therefore the fibre optic sensor provides a simple, miniature and robust sensor configuration to measure pressure and temperature in geothermal wells

FEATURES OF FIBRE OPTIC SENSORS Highly reliable & secure due to immunity of the sensed signal to electromagnetic interference. Safe in explosive & nuclear environments,free from risk of fire & sparks. Most suitable for remote sensing & telematry. Corrosion resistant. Small size & weight. High accuracy & sensitivity. Robust construction

* military and law enforcement * partial discharge detection * medical fields for diagostics and surgical application * aircraft jet engines. * computer application APPLICATIONS OF OPTIC FIBER SENSORS

PRINCIPLE OF OPERATION A schematic of the fibre optic pressure and temperature sensor is illustrated in Fig. 1.

 The fibre optic sensor fabricated by splicing the 200μm silica glass fibre.  SM FBG to the glass capillary to obtain a robust sensor structure.  The 200μm fibre cleaved & polished using raw polishing paper several 100 micrometers from the glass capillary/200μm fibre splice, to avoid light reflections at the outer surface of 200μm fibre.  Incident light Io propagating to the sensor head is reflected at the FBG for a wavelength equal to the Bragg wavelength λB λB = 2neff Λ, (1) where neff - refractive index of the core material Λ - period of the grating.  All other wavelengths propagate through the fibre & reflected at the glass/air interface of the SM fibre and at the air/glass interface of the 200μm fibre.

 Both reflections transmit back into the SM fibre and generate light interference.  Due to the low reflections coefficients of the glass/air and air/glass interface,the function of the light interference can be calculated as IR = Io ⋅ 2R(1+ cos ϕ C ).(2) R - reflection coefficient of the glass/air and air/glass- interface φC - phase shift between both reflected light waves.  φc is defined as : (3) n - refractive index of the EFPI cavity, λ - free space optical wavelength L - EFPI cavity length.

 When pressure is applied to the fibre optic sensor, the glass capillary deforms and hence changes the EFPI cavity length.  The cavity length change ΔLp due to applied pressure (4) μ - Poisson’s ratio of the glass capillary E - Young’s modulus, Ls - effective length of the pressure sensor, ro and ri are the inner and outer radius of the glass capillary.

 Due to the thermal expansion of all glass components, the EFPI cavity is also sensitive to temperature.  The change of the EFPI cavity length as a result of temperature can be calculated as: (5) αC and αF are the Coefficient of Thermal Expansion (CTE) of the glass capillary and the SM fibre. P and T are the pressure and temperature during sealing the EFPI cavity.

 The FBG is entirely encapsulated in the glass capillary, which keeps Bragg wavelength changes less, due to pressure induced.  The temperature sensitivity of the FBG is due to effect on induced refractive index change and on the thermal expansion coefficient of the SM fibre.  The shift of the Bragg wavelength due to temperature can be expressed as: (6) dneff/dT - thermo optic coefficient

 Using pressure and temperature coefficients from equation (4) and (6),the following equation can be constucted: a11 represents the pressure sensitivity of the FBG and was negligible for the developed FOPS due to the encapsulated FBG within the glass capillary. In order to obtain pressure and temperature readings from the fibre optic sensor, the matrix in Equation 7 has to be inverted.

EXPERIMENTAL SET UP. The fibre optic sensor was interrogated using the interrogation system shown below.

* * The interrogation system consists of a Broad-Band Source (BBS) (INO FBS-C), anoptical circulator and an Optical Spectrum Analyser (OSA)(ANDO AQ6330). *Light from the BBS is guided through the optical circulator to the sensor and is reflected at the sensor head back to the optical circulator again. *From the optical circulator the reflected spectrum of the fibre optic sensor is transferred to the OSA. *The OSA captures and normalises the reflected fibre optic sensor spectrum. A computer is used to acquire and analyse the spectrum.

In Fig. below shows an example of the reflected spectrum of the fibre optic sensor is depicted.

Down-hole temperature and pressure conditions were simulated using an oil-filled pressure chamber. The pressure applied to the pressure chamber with hydraulic pressure hand pump Reference pressure was measured using an electrical pressure reference sensor. Pressure chamber was inserted in a temperature stabilized water bath to keep the temperature constant during pressure experiments. The reference temperature was measured using PT25 temperature sensor

EXPERIMENTAL SET UP

The pressure input was connected to the left port of the pressure chamber. The fibre optic sensor was mounted to the right port.

EXPERIMENTS The pressure and temperature response of the fibre optic sensor, evaluated by measuring pressure at different temperatures. Pressure experiments started at ambient pressure ( 0MPa) and increased to 30MPa for four different temperatures (25°C, 40°C, 55°C and 70°C). The temperature kept constant during each pressure experiment.

The change of the EFPI cavity length due to applied pressure and temperature are shown in Fig.

The EFPI cavity shows a good linear correlation to applied pressure. The temperature sensitivity is much smaller compared to the pressure sensitivity. For a relatively small temperature range, the cross-sensitivity of the EFPI cavity to temperature can be neglected.

The temperature response of the FBG sensor is illustrated.

 Experimental results illustrate that the developed fibre optic sensor can measure pressure and temperature at the point of measurement.  A fibre optic pressure and temperature sensor for down- hole applications has been successfully tested by this experiment.

Looking at the industry trends in the past 2 decades and the exponential curve it seems to me that there is going to be a lot of research and improvements to the existing sensors. optical sensors are here to stay !!!! CONCLUSION

REFERENCES o o o E.HUENGES “Geothermal Energy System- Exploration,Development & Utilization” o F. T. S. Yu & S. Yin “Fibre Optic Sensors” o M. J. Economidies and K. G. Nolte “ Reservoir stimulations ”, 3 rd edition