With the oil and gas industry growing rapidly, increasing the yield and profit require advances in technology for cost-effective production in key areas of reservoir exploration and in oil-well production-management. In this paper we review our group’s research into fiber Bragg gratings (FBGs) and their applications in the oil industry, especially in the well-logging field. FBG sensors used for seismic exploration in the oil and gas industry need to be capable of measuring multiple physical parameters such as temperature, pressure, and acoustic waves in a hostile environment. This application requires that the FBG sensors display high sensitivity over the broad vibration frequency range of 5 Hz to 2.5 kHz, which contains the important geological information. We report the incorporation of mechanical transducers in the FBG sensors to enable enhance the sensors’ amplitude and frequency response. Whenever the FBG sensors are working within a well, they must withstand high temperatures and high pressures, up to 175 °C and 40 Mpa or more. We use femtosecond laser side-illumination to ensure that the FBGs themselves have the high temperature resistance up to 1100 °C. Using FBG sensors combined with suitable metal transducers, we have experimentally realized high- temperature and pressure measurements up to 400 °C and 100 Mpa. We introduce a novel technology of ultrasonic imaging of seismic physical models using FBG sensors, which is superior to conventional seismic exploration methods. Compared with piezoelectric transducers, FBG ultrasonic sensors demonstrate superior sensitivity, more compact structure, improved spatial resolution, high stability and immunity to electromagnetic interference (EMI). In the last section, we present a case study of a well-logging field to demonstrate the utility of FBG sensors in the oil and gas industry.

We demonstrate a fiber-optic 3D vector displacement sensor based on the monitoring of Bragg reflection from an eccentric grating inscribed in a depressed-cladding fiber using the femtosecond laser side-illumination and phase-mask technique. The compact sensing probe consists of a short section of depressed cladding fiber (DCF) containing eccentrically positioned fiber Bragg gratings. The eccentric grating breaks the cylindrical symmetry of the fiber cross-section and further has bending orientation-dependence. The generated fundamental resonance is strongly sensitive to bending of the fiber, and the direction of the bending plane can be determined from its responses. When integrated with axis strain monitoring, the sensor achieves a 3D vector displacement measurement via simple geometric analysis.

A highly sensitive fiber-optic accelerometer based on detecting the power output of resonances from the core dip is demonstrated. The sensing probe comprises a compact structure, hereby a short section of specific core (with a significant core dip) fiber stub containing a straight fiber Bragg grating is spliced to another single-mode fiber via a core self-alignment process. The femtosecond laser side-illumination technique was utilized to ensure that the grating inscription region is precisely positioned and compact in size. Two well-defined core resonances were achieved in reflection: one originates from the core dip and the other originates from fiber core. The key point is that only one of these two reflective resonances exhibits a high sensitivity to fiber bend (and vibration), whereas the other is immune to it. For low frequency (<10 Hz) and weak vibration excitation (<0.3 m/s2) measurement, the proposed sensor shows a much higher resolution (1.7 × 10−3 m/s2) by simply monitoring the total power output of the high-order core mode reflection. Moreover, the sensor simultaneously provides an inherent power reference to eliminate unwanted power fluctuations from the light source and transmission lines, thus providing a means of evaluating weak seismic wave at low frequency.

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The paper presents selective fiber Bragg grating (FBG) inscription in four-core fiber based on a phase mask scanning method. The inscription factors are systematically investigated, which involves fiber core position and focused laser beam size in fiber, etc. Several specific inscriptions (including individual, dual and all inscriptions) are demonstrated. Two orthogonally positioned cores are selectively inscribed and applied to two-dimension vector bending measurement. The measured bending sensitivities of two FBGs range from −54.3 pm/m −1 to 52.2 pm/m −1 and −53.7 pm/m −1 to 52.8 pm/m −1 , respectively. More importantly, it has been revealed that their sensitivities versus bending direction follow regular cosinoidal and sinusoidal distribution. The direction and amplitude of the vector bending can be recovered using measured central wavelength shifts of those two FBGs.

Femtosecond laser exposure is used to fabricate a fiber Bragg grating (FBG) over a fiber core cladding on a short section of photon-sensitive thin-core fiber (TCF) using a sideillumination technique. The cladding modes are excited in the fusion region due to the core mismatch between the leading-in single-mode fiber and the TCF, and one resonant cladding mode and core mode are then transmitted downstream as separate wavelengths by the FBG. Furthermore, the transmission intensities for the two resonant dips could be controlled by changing the focal-line position of laser beam. Because the cladding modes have unique field shapes, they react to perturbations both inside and outside the fiber, which makes them good candidates for measuring fiber bending and ambient refractive index variation.Index Terms-Femtosecond laser, FBG inscription over cladding.

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