10-05-2012, 12:58 PM
1Fiber Optic Pressure and Temperature Sensors for Oil Down Hole
Application
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Introduction
Even in US, the low production efficiency during recovery of fossil oil is still a severe problem. With current
technologies, one third of the total crude oil still remains under the earth after exploitation. Oil well down-hole
conditions, such as pressure and temperature are of great values for oil exploration and reservoir management [1].
Under normal conditions, current electrical sensors show good performances such as high resolution, low drift, etc.
However, few sensors can work under the harsh conditions in the oil down-hole environment (pressure as high as
10000psi, temperature as high as 200 C°). In the oil recovery industry some sensors may need to be deployed in the
down hole several kilometers. How to power the sensors and how to transfer the signals back to the control system are
also critical problems for electrical sensors. Other requirements for down-hole sensors include: long lifetime, small size
and immune to EMI.
Fiber optic sensors, with advantages such as high resolution, immune to EMI, small size, and long distance access, are
the best candidates for the oil down-hole applications. Impelled by the great potential values, numerous researchers
performed research to exploit the possibility to apply fiber optic sensor for oil down hole applications [2][3].
Under the support of Department of Energy and Chevron Company, a three and half years project is underway in the
Center for Photonics Technology (CPT) of Virginia Poly-tech. The purpose of this project is to develop fiber optic
1 Contact Author: Bing QI. Phone: 540-231-7070 FAX : 540-231-2158 E-Mail: bqi[at]vt.edu
Fiber Optic Sensor Technology and Applications 2001, Michael A. Marcus, Brian Culshaw, Editors,
182 Proceedings of SPIE Vol. 4578 (2002) © 2002 SPIE · 0277-786X/02/$15.00
sensor systems, which can satisfy the requirements for oil down hole applications. In this paper, we report on the
experimental results of the fiber optic pressure and temperature sensors.
The inteferometric based fiber optic sensors are used both in pressure measurement and temperature measurement. The
sensor head consists of two pieces of polished fiber bonded inside a glass tube by CO2 laser. By choosing different
sensor parameters and materials, the sensor head can works as a pressure sensor with very small temperature sensitivity
or as a temperature sensor with almost zero pressure dependence. A special sensor packaging technique based on soft
metal foil was developed to improve the mechanical stability of the sensor head and protect it from water penetration.
The size of the sensor head with protecting package is about 25mmx 1 mm. The deployability of the fiber optic sensor
had been proven by the experiments in the field site. Adapting to different applications, two types of signaldemodulated
structures were developed. The first one is the Self-Calibrated Interferometric/Intensity-Based (SCIIB)
system, whose advantages include: high speed, medium resolution and low cost. The second type is the white light
interferometer system, which satisfies requirements of high resolution and long-term stability. Both of them have been
realized based on commercialized components and test in a laboratory environment. One set of multimode white light
pressure system has been tested in the oil site of Chevron Company; the preliminary results will be presented in this
paper.
The structure and fabrication of the sensor head
After more than 3 decades from its birth, the fiber optic sensors have been developed from the simple intensity -based
conFigureurations to diverse elaborate structures [4]. Among various kinds of fiber optic sensors, the interferometricbased
fiber optic sensors attracted the attention of many researchers. The advantages of interferometric based fiber optic
sensors include: high sensitivity, intrinsic immunity to the fluctuation of intensity induced by the instability of light
source and drift of fiber loss. To date, four types of interferometer, (Mach Zehnder, Michelson, Sagnac and Fabry-
Perot), had been constructed with optic fiber components. These interferometric-based fiber optic sensors have been
used in the measurement of various physical parameters, such as displacement, pressure, temperature and electrical
field.
The geometrical structure of the sensor head is shown in
Figure.1. The sensor head consisted of two optical fibers
joined in a silica capillary tube by a CO2 laser. The leadin
fiber is connected with the light source and signaldemodulation
system through a transmitting fiber. The
cleaved end-faces of the two fibers compose a low fitness
Fabry-Perot interferometer. The glass tube has several
functions: it serves as a package to isolate inside
environment from outside environment, assures the
alignment between the two fibers and transfers the
physical parameter to be measured to the air-gap of F-P
cavity.
In general, if the air-gap of the F-P cavity has a monotonically increasing or decreasing relationship with a physical
parameter, then this physical parameter can be measured. Yet, to improve the sensitivity of the sensor head to the
physical parameter to be measured and to minimize cross talk from other factors, the geometrical parameters of the
sensor head and the materials for both the fiber and the glass tube need to be selected very carefully.
For pressure measurement, when a hydrostatic pressure is applied on the sensor head, the glass tube will deform. As a
consequent, the air gap of F-P cavity will change. Assuming the outer radius and inner radius of the glass tube are ro and
ri, then the relation between applied pressure p and air-gap change G can be expressed as:
Where L0 is the effective sensor gauge length defined by the distance between the two bonding points, p0 is the pressure
inside tube, which is approximately a constant, E is the Young's Modulus of glass tube and μ is the Poisson's ratio of the
glass tube.
Equation (1) describes how the air-gap of the pressure sensor changes with applied pressure. Obviously, the sens itivity
of the sensor head can be adjusted by changing the geometrical parameters of sensor head, such as the gauge length L0
and the outer radius and inner radius of the glass tube. On the other hand, the selections of these geometrical parameters
are limited by other factors such as the requirements for the size and stability of sensor head.