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.

In the petroleum industry, advances in technology are required for cost-effective production in key reservoir exploration areas and in oil-well production management, in order to increase yield and profit. This manuscript reviews our researches on advanced fiber Bragg gratings (FBGs) and their reservoir exploration applications in local geology. Several cladding FBGs are introduced for seismic exploration, which are interrogated by a simple power detection system. They show high sensitivities to vibration/acceleration, together with strong orientation dependence. Orthogonal cladding FBG inscriptions can perceive two-dimensional (2D) vibration information, i.e., vector vibration sensing. Power perturbations are referenced with respect to the core resonance that is insensitive to vibration, thus stabilizing the sensing system. Seismic physical modeling (SPM) is another method for gathering seismic data and bridging computation modeling and field exploration. In previous reports, aiming to SPM imaging, several wide-frequency FBG sensors were developed to detect ultrasound wave (UW) in the 100 kHz to 10 MHz range. The interrogation technology uses a low-cost spectral side-band filtering technology. The key point in prior research relates to narrowing the spectral band in order to increase the UW sensitivity of the sensor. Compared with piezoelectric transducers (PZTs), FBG sensors have the high sensitivity, compact structure, large detection angle, and high spatial resolution, stability, and immunity to electromagnetic interference. Combining the two technologies can help humans learn more specific physical properties (geometric, kinematic and dynamic) of hydrocarbon reservoirs.

We have designed, fabricated, and characterized a fiber-optic three-dimensional vector accelerometer for vibration measurement. The accelerometer is based on fiber Bragg gratings (FBGs) inscribed with three cores of seven-core fiber by femtosecond laser with a phase mask. Through monitoring the wavelength shifts of two of the seven cores, we obtain the vibration azimuthal angle and the acceleration in the XY plane by angle reconstruction and algorithmic calculation. Similarly, by monitoring the wavelength shift of the core fixed along the Z-axis, we obtain the vibration acceleration in the Z-axis direction. The full 3D vector information about the vibration signal is obtained by vector synthesis. The accelerometer has achieved a working frequency bandwidth from 10 to 220 Hz with the maximum sensitivity of 355 pm/g, and the best azimuthal angle accuracy of 0.269 ^° in the linear range of 0 ^° -100 ^° and 170 ^° -280 ^° . Moreover, it is possible to increase the overall bandwidth, sensitivity and accuracy of the azimuth of sensors in the XY plane by adjusting the length of the optical fiber and adjusting mechanical parameters to increase the bandwidth and sensitivity of the sensor in Z-axis. Such improvements will further enhance application to small-scale seismic surveys.

We have designed and developed a compact fiber optic accelerometer for vibration detection. The mechanical vibrating element is a hexagonal structure which is fabricated using 3D-printing technology. Such a structure can efficiently convert vertical vibrations into periodic deformations of a longitudinally positioned fiber structure into which a fiber Bragg grating (FBG) is inscribed. By demodulating the wavelength shift of the reflection spectrum, we established the relationship between vibrational acceleration and wavelength shift. The measured sensitivity is 421.4pm/g. A second FBG inscribed in the fiber enables temperature compensation. The proposed design has achieved a working bandwidth from 10 to 210Hz with a resonant frequency of 124.9 Hz. Both laboratory and in-field measurements using our sensor have been compared with those of a traditional electromagnetic accelerometer to validate the effectiveness of the derived theoretical models and the simulation results. The comparison has effectively supported the parameter determination of the sensor design.

Fiber Bragg grating (FBG) accelerometers measure vibrations by demodulating wavelength shifts induced by dynamic strain. Their high-temperature resistance and multiplexing capabilities provide them with opportunities for quasi-distributed applications in harsh environments. The basic principle of an FBG accelerometer is based on vibration-strain coupling. In the design of a sensor's mechanical structure, improving the coupling efficiency can effectively enhance the sensitivity of the accelerometer. Moreover, an FBG accelerometer can be developed as a multidimensional sensor, which is a unique feature compared to distributed-fiber vibration-sensing technology. In this paper, we review the recent advances in FBG accelerometer developments and by focusing on two main trends: sensitivity enhancement and orientation identification. We emphatically introduce new techniques for FBG accelerometers including hybrid strain loading, thin-cladding fiber FBGs, multicore FBGs and cladding FBGs.

We demonstrate a fiber-optic strain sensor based on a seven-core fiber (7-CF) for accurate extrusion bending and tensile strain sensing. The sensor structure we designed is the 7-CF spliced to a piece of multimode fiber on each side with a lateral core-offset and then conventionally fused to single mode optical fiber as the input and output ports. The experimental results indicate that the optimal curvature sensitivity of the sensor is achieved 25.96 nm/m-1, and the sensitivity of the tensile strain is 0.094 nm/με, which allows us to monitor bending and stretching strain respectively with high precision by the simple, low-cost, and effective sensor.