Distributed Fiber Optic Strain Research Articles

Geomechanics Modeling of Strain-Based Pressure Estimates: Insights from Distributed Fiber Optic Strain Measurements

Summary Rayleigh frequency shift distributed strain sensing (RFS-DSS) has been demonstrated as a potential technology for characterizing fracture geometries and optimizing the completion design in unconventional reservoirs. The combination of RFS-DSS strain and pressure gauge measurements has been reported recently in field applications (Dhuldhoya and Friehauf 2022; Haustveit and Haffener 2023) to characterize conductive fracture dimensions and production profiles. Haustveit and Haffener (2023) demonstrated a novel technique that uses RFS-DSS strain change during production, measured from a far-field observation well, to monitor the drain profile of a horizontal multistage well. The strain change and pressure change were strongly related during production, and this relationship was applied to continuously estimate pressure drawdown along the full length of the well. However, the sensitivity or influencing factors of the relationship between strain change and pressure change along the fiber are not yet fully understood. Therefore, the main objective of this study is to investigate the relationship between strain change and pressure change under various fractured reservoir conditions and provide guidelines for better utilizing this novel strain-pressure relationship to estimate conductive fractures and pressure profiles. We use a coupled geomechanics and fluid flow model to simulate the strain and pressure changes in the near-well and far-field regions during stable production and shut-in periods. A linear relationship is used to fit the strain change and pressure change data obtained from both producing and monitoring wells. We analyze the impact of various fracture properties [size and permeability of the stimulated reservoir volume (SRV) around fractures, fracture conductivity, fracture area, and skin factor] on the linear relationship based on the R2 value and fitting error. The relationship between the strain change and pressure change during the stable production obtained by our numerical study is consistent with the field measurements. The slope of strain change vs. pressure change during one day of production is significantly influenced by fracture properties. Overall, the monitoring well shows a stronger linear relationship than the producing well during both stable production and shut-in periods. A better linear relationship between the strain change and pressure change is associated with a larger fracture conductivity or larger fracture spacing scenario on both producing and monitoring wells during the stable production period. The cluster spacing affects the linear relationship between strain and pressure change during rock deformation. When cluster spacing is reduced, the nonlocal effect of deformation becomes more pronounced, resulting in a weaker linear relationship. During the shut-in period, the monitoring well exhibits a stronger linear relationship with a high R2 value and small fitting error compared to the producing well. The linear relationship obtained during the shut-in period is less reliable for the producing well. In cases of homogeneous fractures, where the fracture properties are uniform, the strain change and pressure change are similar, except for two outer fractures. This study provides a better understanding of the relationship between strain change and pressure change measured in the near-well and far-field regions during stable production and shut-in periods. The results of our study also demonstrate the potential for utilizing the RFS-DSS strain measurement to characterize the effective fracture and drainage dimensions.

Real-time monitoring of rock fracture by true triaxial test using fiber-optic strain monitoring in adjacent wells

The real-time monitoring of fracture propagation during hydraulic fracturing is crucial for obtaining a deeper understanding of fracture morphology and optimizing hydraulic fracture designs. Accurate measurements of key fracture parameters, such as the fracture height and width, are particularly important to ensure efficient oilfield development and precise fracture diagnosis. This study utilized the optical frequency domain reflectometer (OFDR) technique in physical simulation experiments to monitor fractures during indoor true triaxial hydraulic fracturing experiments. The results indicate that the distributed fiber optic strain monitoring technology can efficiently capture the initiation and expansion of fractures. In horizontal well monitoring, the fiber strain waterfall plot can be used to interpret the fracture width, initiation location, and expansion speed. The fiber response can be divided into three stages: strain contraction convergence, strain band formation, and postshutdown strain rate reversal. When the fracture does not contact the fiber, a dual peak strain phenomenon occurs in the fiber and gradually converges as the fracture approaches. During vertical well monitoring in adjacent wells, within the effective monitoring range of the fiber, the axial strain produced by the fiber can represent the fracture height with an accuracy of 95.6% relative to the actual fracture height. This study provides a new perspective on real-time fracture monitoring. The response patterns of fiber-induced strain due to fractures can help us better understand and assess the dynamic fracture behavior, offering significant value for the optimization of oilfield development and fracture diagnostic techniques.

The Impact of Liquids and Saturated Salt Solutions on Polymer-Coated Fiber Optic Sensors for Distributed Strain and Temperature Measurement.

The application of distributed fiber optic strain and temperature measurement can be utilized to address a multitude of measurement tasks across a diverse range of fields, particularly in the context of structural health monitoring in the domains of building construction, civil engineering, and special foundation engineering. However, a comprehensive understanding of the influences on the measurement method and the sensors is essential to prevent misinterpretations or measurement deviations. In this context, this study investigated the effects of moisture exposure, including various salt solutions and a high pH value, on a distributed strain measurement using Rayleigh backscattering. Three fiber optic sensors with different coating materials and one uncoated fiber were exposed to five different solutions for 24 h. The study revealed significant discrepancies (∼38%) in deformation between the three coating types depending on the surrounding solution. Furthermore, in contrast to the prevailing literature, which predominantly describes swelling effects, a negative deformation (∼-47 με) was observed in a magnesium chloride solution. The findings of this study indicate that corresponding effects can impact the precision of measurement, potentially leading to misinterpretations. Conversely, these effects could be used to conduct large-scale monitoring of chemical components using distributed fiber optic sensing.

Large-scale laboratory investigation of pillar-support interaction

Underground mine pillars provide natural stability to the mine area, allowing safe operations for workers and machinery. Extensive prior research has been conducted to understand pillar failure mechanics and design safe pillar layouts. However, limited studies (mostly based on empirical field observation and small-scale laboratory tests) have considered pillar-support interactions under monotonic loading conditions for the design of pillar-support systems. This study used a series of large-scale laboratory compression tests on porous limestone blocks to analyze rock and support behavior at a sufficiently large scale (specimens with edge length of 0.5 m) for incorporation of actual support elements, with consideration of different w/h ratios. Both unsupported and supported (grouted rebar rockbolt and wire mesh) tests were conducted, and the surface deformations of the specimens were monitored using three-dimensional (3D) digital image correlation (DIC). Rockbolts instrumented with distributed fiber optic strain sensors were used to study rockbolt strain distribution, load mobilization, and localized deformation at different w/h ratios. Both axial and bending strains were observed in the rockbolts, which became more prominent in the post-peak region of the stress-strain curve.

Load Test Performance of Bored Pile with Distributed Fibre Optic Strain Sensing

Load Test Performance of Bored Pile with Distributed Fibre Optic Strain Sensing

Monitoring fluid migration in a CO2 storage reservoir by distributed fiber optic strain sensing: A laboratory study

To ensure the safety of carbon dioxide (CO2) geological storage, it is important to effectively monitor the pore pressure, reservoir deformation and fluid plume migration, which poses new challenges to monitoring techniques and tools. In this paper, an innovative monitoring method is presented based on distributed fiber optic strain sensing (DFOSS) to detect the strain response distribution induced by fluid injection in real time. The original strain field distribution in the reservoir rock and the effect of the effective stress on the strain response were obtained for different confining and pore pressure combinations. Then, the rock strain during displacement was measured to dynamically monitor the movement front of the wetting phase. The results showed that the core strain response curves under the influence of the confining and pore pressures exhibit significant edge effects, with lower strain values in the vicinity of the artificial confinement facility than in the reservoir. In addition, the overall strain response exhibited an increase corresponding to the increase in the confining pressure. A higher confining pressure was observed to mitigate the pore pressure expansion-related strain in the rock itself. This proves that the selection of a large-depth subsurface reservoir could help to increase the CO2 injection pressure and rate, and the formation pressure threshold value identified in this study varied between 6 to8 MPa. Furthermore, the full-field strain history showed that fiber optic technology can be employed to accurately monitor the detailed characteristics of front movement and core deformation during wetting fluid displacement, including the history of the strain distribution due to two-phase motion and the change in the two-phase moving speed. The analysis results collectively validated the applicability of this monitoring technique for core-scale testing. The data and conclusions obtained could contribute to a deeper comprehension of the DFOSS reservoir monitoring capability. Furthermore, these findings hold relevance for subsequent experimental analyses, numerical simulations, and field applications at carbon capture and storage sites.

Integrated wellbore-reservoir-geomechanics modeling for enhanced interpretation of distributed fiber-optic strain sensing data in hydraulic-fracture analysis

Fiber-optic distributed strain sensing (FO-DSS) has been successful in monitoring strain changes along horizontal wellbores in hydraulically fractured reservoirs. However, the mechanism driving the various FO-DSS responses associated with near-wellbore hydraulic fracture properties is still unclear. To address this knowledge gap, we use coupled wellbore-reservoir-geomechanics simulations to study measured strain-change behavior and infer hydraulic fracture characteristics. The crossflow among fractures is captured through explicit modeling of the transient wellbore flow. In addition, local grid refinement is applied to accurately capture strain changes along the fiber. A Base Case model was designed with four fractures of varying properties, simulating strain change signals when the production well is shut-in for 10 d after 240 d of production and reopened for 2 d. Strain-pressure plots for different fracture clusters were used to gain insights into inferring fracture properties using DSS data. When comparing the model with and without the wellbore, distinct strain change signals were observed, emphasizing the importance of incorporating the wellbore in FO-DSS modeling. The effects of fracture spacing and matrix permeability on strain change signals were thoroughly investigated. The results of our numerical study can improve the understanding of the relation between DSS signals and fracture hydraulic properties, thus maximizing the value of the dataset for fracture diagnostics and characterization.

The Challenges and Advantages of Distributed Fiber Optic Strain Monitoring in and on the Cementitious Matrix of Concrete Beams

Distributed fiber optic strain measurement techniques have become increasingly important in recent years, especially in the field of structural health monitoring of reinforced concrete structures. Numerous publications show the various monitoring possibilities from bridges to special heavy structures. The present study is intended to demonstrate the possibilities, but also the challenges, of distributed fiber optic strain measurement in reinforced concrete structures. For this purpose, concrete beams for 3-point bending tests were equipped with optical fibers on the reinforcement and concrete surface as well as in the concrete matrix in order to record the strains in the compression and tension zone. In parallel, an analytical approach based on the maximum strains in the uncracked and cracked states was performed using the Eurocode 2 interpolation coefficient. In principle, the structural design correlates with the measured values, but the strains are underestimated, especially in the cracked zone. During load increase, structural distortions in the compression zone affected the strain signal, making reliable evaluation in this zone difficult. The information content of distributed fiber optic strain measurement in reinforced concrete structures can offer tremendous opportunities. Future research should consider all aspects of the bond, sensor selection and positioning. In addition, there is a lack of information on the long-term stability of the joint and the fiber coating, as well as the effects of dynamic loading.

High-frequency dynamic distributed fiber optic strain sensing for civil infrastructure applications

High-frequency dynamic distributed fiber optic strain sensing for civil infrastructure applications

Hierarchical Bayesian calibration of deck deflection models using distributed fiber optic strain data

A naval hovercraft was outfitted with a distributed fiber optic sensing system (FOSS) on its cargo deck in which two fibers were placed along orthogonal axes. A series of trucks were loaded onto the cargo deck to simulate operational loading conditions and the resulting strain profiles were measured by the distributed FOSS. The aim of this study is to quantify the deck deflections that correspond to the measured strains under different vehicular loads. This is accomplished by idealizing the cargo deck as a rectangular plate and using the data to estimate the associated plate model parameters. In order to account for measurement noise and model prediction error, as well as the inherent variability of inferences made with different data sets, a hierarchical Bayesian scheme that considers both the model parameter uncertainty and the prediction error variance is utilized to derive the posterior distributions of the model hyper-parameters, i.e., their mean and variance. Given the volume of FOSS strain data available, different approaches were taken for incorporating the data into the hierarchical approach in order to determine whether the inclusion of all available strain data meaningfully reduced the uncertainty compared to using only time-averaged strain. Comparisons between the different approaches are explored in the context of 95% confidence intervals for strain profiles, deck deflection envelopes, and reliability indices based on allowable deflections.

Application of Distributed Fiber Optic Strain Sensors to LMQXFA Cold Mass Welding

The future High Luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN will include the low-beta inner triplets (Q1, Q2a/b, Q3) for two LHC insertion regions. The Q1, Q3 components consist of eight 10 m-long LMQXFA cryo-assemblies fabricated by the HL-LHC Accelerator Upgrade Project. Each LMQXFA Cold mass contains two Nb3Sn magnets connected in series. A stainless-steel shell is welded around the two magnets before the insertion into the cryostat. There is a limit on how much coil preload increase induced by the shell welding is allowed. Distributed Rayleigh backscattering fiber optics sensors were used for the first time to obtain a strain map over a wide area of a Nb3Sn magnet cold mass shell. Data were collected during welding of the first LMQXFA cold mass and the results confirm that the increase of the coil pole azimuthal pre-stress after welding do not exceed requirements.

Fatigue weld crack detection using distributed fiber optic strain sensing

We demonstrate the application of distributed fiber optic strain sensing based on optical frequency-domain reflectometry for early detection and location of fatigue cracks in welds in steel tubular test specimens. To do so, we have subjected welded tubular specimens instrumented with surface-mounted optical fiber sensors to resonant bending load, and we have measured the strain distributions in the test samples continuously and without any interruption of the test throughout its whole duration, with a 2.6 mm spatial resolution and a 1 με strain resolution. We show that the fatigue cracks, which initiate from the inner surface of the wall of the specimens, can be successfully detected and located in real time using our measurement method. We conclude that the detection of the crack initiation may provide relevant information serving the estimation of the remaining lifetime of the component.

Space tether system state estimation using distributed fibre optical strain measurement, inverse finite element method and Kalman filter

This paper proposes a robust finite element Kalman filter for the state estimation of flexible space tether systems, which are critical for space exploration and satellite missions. Due to the limited access to placing sensors along the flexible tether to measure them directly, the measurement of the state of these systems is challenging. To address this, a fully expandable, scalable, and robust approach is developed that fuses position/velocity sensors at tether ends, distributed fiber optic strain measurement along the tether, and a position/velocity estimation by a finite element-based Kalman filter to estimate the state of the tether system. The proposed estimator is mathematically proven to be observable, stable, and robust. The effectiveness of the estimator is demonstrated through numerical simulations of a space tether system in orbit, and the method can be extended to any flexible body system. The results of the simulations show that the proposed approach can accurately estimate the state of the tether system, even in the presence of disturbances and noise.

Distributed fiber optic strain sensing in axially loaded glued-in steel rods in glued-laminated timber

The rise in popularity of mass timber structures globally has increased the demand for high-performance timber connections. Glued-in rod connections which are advantageous compared to other dowel-type connections because of their high strength and stiffness, have been the subject of extensive research in the literature; However, the lack of an in-depth understanding of their behavior and a uniform set of guidelines for their design continues to limit the use of glued-in rods in practice. To develop an improved understanding of the behavior of glued-in rod connections this paper examines, for the first time, the use of distributed fiber optic sensors to measure the complete strain distribution along the length of a glued-in rod connection. Axial pullout tests were conducted on twenty-five glued-in rod connections in glued-laminated timber with varying embedment lengths and glue-line thicknesses. Results of the study demonstrate the ability of fiber optic sensors to capture the full strain distribution along the length of the steel rod and along the epoxy-adhesive interface, enabling the study of localized behavior that would be difficult to measure using conventional strain gauges. In addition, results demonstrate the ability of distributed fiber optic strain sensors to infer axial stress and force distributions along the length of a glued-in rod connection, which are compared with theoretical distributions from the Volkersen model, as well as measure bond-slip behavior.

Unified inverse isogeometric analysis and distributed fiber optic strain sensing for monitoring structure deformation and stress

Inverse analysis is an efficient and reliable approach to the estimation of structural state such as deformation and stress distribution from measured real-world data and numerical algorithms. In this work, a new method of inverse analysis is proposed which integrates Isogeometric Analysis (IGA) and distributed fiber optic sensing (DFOS) using a framework based on non-uniform rational B-spline (NURBS) functions. This formulation expands on the existing inverse Finite Element Methods (iFEM) by developing a new approach to use the exact geometry of strain-sensing optical fibers. The constraint equations of theoretical and measured strains are derived using a discrete least-squares functional and then coupled with isogeometric finite element equations to provide regularization for ill-posed inverse problems. The explicit strain fiber model not only separates the structure mesh and strain sensing locations but also can adapt to the sensing parameters such as spatial resolution of DFOS instrument, which is not yet possible with the current models. The proposed method is validated with four numerical examples using plate, hemisphere and cylindrical geometries under prescribed force or displacement boundary conditions. Furthermore, the sensitivity of the inverse solution to the sampling interval and measurement noise is studied. The unified inverse IGA and DFOS is tested under simulated measurement noise up to 30 micro-strains with strain sampling intervals ranging from 10 mm to 1 m, and validated with the comparisons of reference and inverse solutions.

Evaluation of the Structural Health Monitoring Results of the Applied Fiber Optics in the Pile-Raft Foundations of a High-Rise Building

Fiber optics are increasingly being used in structural health monitoring applications. In addition to the well-known advantages, they most reliably transmit the stress–strain condition of the structure in the long term. This article presents the results of structural health monitoring on the embedding of fiber optics in the upper layer of the raft over the entire area of a high-rise building in Astana city (Kazakhstan), including the 75-storey 320 m block R. Temperature and strain data were collected constantly via a distributed fiber optic strain sensing system (DFOSS). The analysis of strains in fiber optic chainage indicated that the compression strains were observed over the entire area of the building, with values from −20 to −40 µε and with progression of the compression loaded up to −80 µε along the core wall closer to the eastern facade of the building. Tension strains were concentrated along the extreme axes of the building, with a predominance in the southern part of the R block, as well as in the immediate vicinity of the columns, in a range of −20 to −40 µε, with separate spots from −40 to −60 µε. Individual patches of tension strains near the columns have been explained by the increased deflection of the raft under the application of a concentrated load (columns). Fiber optic monitoring results at the time of testing did not exceed the permitted values for high-rise building operation; however, they characterized the general picture of the strain in the raft plane and made it possible to determine the initiation of cracks in concrete at an early stage.

Distributed viscosity and flow velocity measurements using a fiber-optic shear stress sensor

The understanding and precise prediction of fluid and solid displacement is of great interest in many technical applications. Density and viscosity are two key parameters that govern process mechanisms. The possibility to measure transient processes over longer distances is desirable. We present a novel distributed shear stress sensor that allows to derive fluid rheological parameters such as the viscosity along a fiber-optic cable being exposed to a moving medium. This works because flow velocity and fluid viscosity directly translate to a shear stress and consequently to a tensile strain on the fiber optic cable. Using the technology of fiber-optic distributed strain sensing, strain changes (and temperatures) are detected in real-time at any location along the fiber. Given the cable mechanical properties and geometry of the flow path, the strain translates to a shear stress which can be correlated to either the flow velocity or fluid viscosity. We derive a theoretical characterization of the sensor based on the principles of fluid mechanics. Also, we perform laboratory experiments with the sensor and demonstrate that we can distinguish differences of 1 mPa s dynamic viscosities in a range as low as 1 – 7 mPa s. In the next phase, we are implementing this sensor into a real working environment in a wellbore application to investigate the applicability of this novel sensor technology.

Multi-physical field joint monitoring of buried gas pipeline leakage based on BOFDA

Determining the leakage location in an underground gas pipeline is a serious technical problem. Distributed fiber optic testing technology based on Brillouin optical frequency domain analysis (BOFDA) can be used for monitoring the change in the strain and temperature of the surrounding media caused by the gas leakage. In this study, a point source leakage model was built. The distributed fiber optic strain and actively heated fiber optic were installed around the pipeline to monitor the gas pipeline leakage. Variations in the strain and temperature characteristics of the pipeline under different gas pressure conditions were found to be in the range of −392–402 μϵ and 3.5 °C during the 1.0 MPa gas leakage, respectively. When the soil model was loaded with a gas leakage pressure of 0.3–1.0 MPa, a significant variation in strain and temperature was noticed. The copper mesh actively heated fiber optic can effectively identify the location of the leak. Meanwhile, the heating rate of the actively heated fiber optic cables in different media environments were obtained and ranked from fast to slow: air, followed by general soil, followed by leakage soil. This led to the generation of new technology to detect the leakage location of buried gas pipelines through actively heated optical cables.

Experimental investigation on pipe-soil interaction due to ground subsidence via high-resolution fiber optic sensing

Understanding the pipe-soil interaction is critical for the design and safety assessment of buried pipelines subjected to large differential ground motion. Laboratory model tests were conducted to study the mechanical response of polyvinyl chloride pipes to ground subsidence. The optical frequency domain reflectometry technique was employed to perform distributed strain sensing of the pipe-soil system. An analytical method was proposed to estimate greenfield settlements in terms of fiber optic strain measurements. The pipe-soil interaction mechanism due to ground subsidence was investigated concerning four aspects, i.e., displacement profile of the pipe-soil system, pipe bending, soil arching, and interfacial shearing. Accounting for three-dimensional soil arching effects, a limit equilibrium solution for the earth pressure imposed on the pipe crown was given. This solution was developed from the two-dimensional Terzaghi soil arching theory and was validated based on the measured earth pressures. The feasibility of distributed fiber optic strain sensing technique in monitoring pipe-soil deformation was discussed through the comparison with the data of displacement transducers. In addition, the change in relative pipe-soil stiffness with increasing ground movements was studied based on the bending behavior of the pipe. A simplified method was given for quantifying the axial pipe-soil interaction based on longitudinal strains. This paper presents new inspiration on the intricacies of pipe-soil interaction using high-resolution distributed fiber optic sensing, which can be applied to structural health monitoring and optimum design of buried pipelines.

Monitoring Flexure Behavior of Compacted Clay Beam Using High-Resolution Distributed Fiber Optic Strain Sensors

Abstract Understanding the mechanical characteristics of clayey soil is critical to estimate the stability of clay-related geotechnical engineering works. However, the distributed information of the internal deformation of clay is hard to capture through traditional methods. In this paper, a set of four-point bending tests were performed on a compacted clay beam instrumented with fiber optic strain sensors to investigate the flexure and cracking behaviors. The optical frequency domain reflectometry technique with high spatial resolution and precision was employed to establish a distributed strain sensing (DSS) system in this study. The results show that the DSS system exhibits a high accuracy in the collection of the clay deformation in terms of compression, tension, and cracking. The comparison of the internal strain profiles with the observation of particle image velocimetry analysis system verifies the feasibility of fiber optic sensing technology in the quantitative assessment of clay deformation. The performance of fiber optic sensors in the rebound deformation and creep deformation monitoring was preliminarily issued in the study. In addition, the progressive failure on the fiber–soil interface was analytically studied to evaluate the deformation compatibility between the fiber and surrounding clayed soil. The conclusions drawn in this paper can help to guide the state interpretation of clayey soil instrumented with fiber optic sensors.