Distributed Strain Sensor Research Articles

Toward Retrieving Distributed Aquifer Hydraulic Parameters From Distributed Strain Sensing

AbstractSubtle elastic rock deformation during aquifer testing may bear hydraulic parameter (permeability and compressibility) information owing to the poroelastic hydromechanical coupling effect. Here we report that such in situ rock deformations (∼50 µε) during an aquifer pumping test are successfully measured along a vertical well by a high‐resolution fiber optic distributed strain sensing (DSS) tool with an accuracy of 0.5 µε. We investigate the feasibility of hydraulic parameter estimation at meter scale using DSS data through a coupled hydromechanical model. Both synthetic and field cases are tested with sensitivity analysis. The results indicate that the simultaneous estimation of permeability and compressibility using DSS data is possible at low noise levels. However, only non‐global near‐optimal solutions can be obtained using the applied gradient‐based inversion algorithm, because of parameter crosstalk and sensitivity problems when the data contain large noise. In particular, estimation is difficult for zones with relatively low permeability due to the low sensitivity to the strain changes. The estimated permeability/compressibility structures for the field test are largely consistent with other geological information from well logs. Our study suggests that DSS data can be quite useful in aquifer characterization and fluid flow profiling in addition to geomechanical monitoring. The obtained hydraulic information is beneficial for the optimized reservoir management of water and oil/gas storage.

In situ hydromechanical responses during well drilling recorded by fiber-optic distributed strain sensing

Abstract. Drilling fluid infiltration during well drilling may induce pore pressure and strain perturbations in neighbored reservoir formations. In this study, we report that such small strain changes (∼20 µε) have been in situ monitored using fiber-optic distributed strain sensing (DSS) in two observation wells with different distances (approximately 3 and 9 m) from the new drilled wellbore in a shallow water aquifer. The results show the layered pattern of the drilling-induced hydromechanical deformation. The pattern could be indicative of (1) fluid pressure diffusion through each zone with distinct permeabilities or (2) the heterogeneous formation damage caused by the mud filter cakes during the drilling. A coupled hydromechanical model is used to interpret the two possibilities. The DSS method could be deployed in similar applications such as geophysical well testing with fluid injection (or extraction) and in studying reservoir fluid flow behavior with hydromechanical responses. The DSS method would be useful for understanding reservoir pressure communication, determining the zones for fluid productions or injection (e.g., for CO2 storage), and optimizing reservoir management and utilization.

One Stage Forward or Two Stages Back: What Are We Treating? Identification of Internal Casing Erosion during Hydraulic Fracturing—A Montney Case Study Using Ultrasonic and Fiber-Optic Diagnostics

SummaryPlug-and-perforation (plug-and-perf) multistage hydraulic fracturing completions in unconventional reservoirs rely on complete hydraulic isolation from the previous stage to ensure effective treatment of the active stage. Failure to isolate stages can be a result of partially set plugs, plugs set in wellbore debris or deformed casing, unqualified pressure/temperature rating of plugs, and so on. This paper presents a case study with field examples in which unexpected casing erosion occurred at the setting depths of the dissolvable fracturing (frac) plugs during hydraulic fracturing and subsequently resulted in loss of interstage isolation.A 12-well, four-layer, cube pilot was designed with permanent fiber-optic cable to collect distributed acoustic sensing (DAS), distributed temperature sensing (DTS), and distributed strain sensing (DSS) data as well as downhole pressure gauges for development insight and future completion optimization. The cable was mapped, and oriented perforation techniques placed entry holes opposite the fiber along the wellbore, and no loss of communication was observed during perforating operations. However, fiber-optic signal was lost during hydraulic fracturing operations on one or more stages in all four instrumented horizontal wells. Real-time DAS/DTS analysis indicated the fiber breaks were consistently occurring below the lowermost perforation cluster in the stage, at or very near the frac plug setting depth. Step-down tests were also performed and showed significantly enlarged effective treating area. Based on this observation, post-frac downhole imaging tools were deployed to investigate potential casing and perforation erosion.Downhole imaging data clearly showed the casing was severely eroded at several locations. Additional interrogation of the damage with respect to plug design components indicated that damage always occurred near the plug sealing element. By integrating the analysis of DAS/DTS, step-down tests, and ultrasonic imaging, it was determined that the frac plug bypass was creating a loss of casing integrity at the plug set location. Casing integrity loss resulted in multiple fiber-optic cable breaks and lowered the ability to evenly distribute slurry into treatment clusters. Fiber-optic data analysis showed that 50% of the larger outer diameter (OD) dissolvable frac plugs had bypass compared to 100% bypass for the smaller OD high-expansion, dissolvable plugs.To establish key patterns and identify critical variables that influence stimulation effectiveness, it is important to obtain several different diagnostic data sets and perform an integrated evaluation using all available information. This study also reinforces the need for operators and manufacturers to work together to design and qualify frac plugs against realistic downhole conditions, particularly in areas with potential casing deformation issues. Industry innovation is required to enable fracturing operations to continue through deformed casing. This includes advancing equipment, tools, and techniques for plug-and-perf and other multistage completion methods.

The Sensitivity Improvement Characterization of Distributed Strain Sensors Due to Weak Fiber Bragg Gratings.

Weak fiber Bragg gratings (WFBGs) in a phase-sensitive optical time-domain reflectometer (phi-OTDR) sensor offer opportunities to significantly improve the signal-to-noise ratio (SNR) and sensitivity of the device. Here, we demonstrate the process of the signal and noise components’ formation in the device reflectograms for a Rayleigh scattering phi-OTDR and a WFBG-based OTDR. We theoretically calculated the increase in SNR when using the same optical and electrical components under the same external impacts for both setups. The obtained values are confirmed on experimental installations, demonstrating an improvement in the SNR by about 19 dB at frequencies of 20, 100, and 400 Hz. In this way, the minimum recorded impact (at the threshold SNR = 10) can be reduced from 100 nm per 20 m of fiber to less than 5 nm per 20 m of fiber sensor.

Distributed Strain Sensing to Study a Composite Liner for Cast Iron Water Pipe Rehabilitation

Abstract The use of cured-in-place pipe lining systems for the rehabilitation of cast iron water pressure pipe has grown significantly in the past 20 years. With limited information on the performance of these lining systems in service, strain measurements are needed to evaluate theoretical findings about three-dimensional strength limits. The application of distributed optical fiber sensing to study the properties of a polymer composite liner was investigated in this research. Polyimide optical fibers were installed into flat liner coupons fabricated in the laboratory, and the specimens were subsequently tested in uniaxial tension and four-point bending. Using optical frequency domain reflectometry, Rayleigh backscatter was monitored from preinstalled optical fiber sensors and strains of up to 4,000 με were obtained. Comparisons between experimental optical fiber measurements and results obtained from conventional strain measurement techniques (i.e., extensometer and crosshead displacement) showed a strong correlation with an average difference of 3 % in four-point bending and 6 % in tension. However, the orientation and diameter of the composite fiber reinforcements were observed to affect distributed sensing performance.

Modeling of fiber-optic strain responses to hydraulic fracturing

We modeled cross-well strain/strain rate responses of fiber optic sensing, including distributed strain sensing (DSS) and low-frequency distributed acoustic sensing (DAS), to hydraulic stimulation. DSS and low-frequency DAS have been used to measure strain or the strain rate to characterize hydraulic fractures. However, the current application of DSS/DAS is limited to acquisition, processing, and qualitative interpretations. The lack of geomechanical models hinders the development of the technology toward quantitative interpretation and inversion. We have developed a strategy to use the displacement discontinuity method to model the strain field around kinematically propagating fractures. For a horizontal monitoring well, modeling results were able to explain the heart-shaped extending pattern before a fracture hit, the polarity flip due to fracture interaction during stimulation, and the V-shaped pattern when a fracture does not intersect with the monitoring well. For a vertical monitoring well, modeling shows the different characters of strain rate responses when a fracture is near and far away from a vertical monitoring well. We also investigated the effects of fractures with various geometries such as elliptic and layered fractures. We compared and verified the modeling with field data from the Hydraulic Fracturing Test Site 2, a research experiment performed in the Permian Basin. Our modeling work can be used to identify patterns in field observations. The results also help to improve acquisition design and lay the groundwork for quantitative interpretation and inversion.

Damage Detection of CFRP Stiffened Panels by Using Cross-Correlated Spatially Shifted Distributed Strain Sensors

This paper presents a cross-correlation function-based method applied to a spatially shifted differential strain readout vectors using distributed sensors under backscattering random noise and impact excitations. Structural damage is generated by low/medium energy impact on two aeronautical 24-ply CFRP (carbon fiber reinforced plastic) stiffened panels. Two different drop impact locations, two different sensor layouts and two different post-impact solicitations are provided for a skin-stringer debonding detection and length estimation. The differential signal with respect to an arbitrarily selected grounding is used. Then the effects of noise filtering are evaluated post-processing the differential signal by cross-correlating two strain vectors having one sensor gauge position lag. A Rayleigh backscattering sensing technology, with 5 mm of spatial resolution, is used to log the strain map. The results show a good coherence with respect to the NDI (nondestructive inspection) performed by ultrasonic C-scan (an ultrasonic imaging system) flaw detector.

A machine learning method for inclinometer lateral deflection calculation based on distributed strain sensing technology

Due to its unique advantages, the distributed fiber optical sensing (DFOS) technology has been used to study the performance of inclinometer so as to monitor landslide deformation. Strain distribution of inclinometer can be obtained by distributed strain sensing (DSS) cables, and the strain-deflection relationship can be established by using the widely accepted methods (e.g., the quadratic integral method and classical conjugate beam method). However, the application of quadratic integral method and classical conjugate beam method are based on many assumptions, and there will be remarkable deviation between calculated deflection and actual displacement with the increase of integral length. Given this, a new deflection calculation method based on machine learning is proposed. Through learning on the monitoring data, an implicit function model between depth, strain, and measured displacement is established by using the BP (back propagation) neural network algorithm. The efficiency of the proposed model has been verified against measured displacement, which demonstrates the capability of this method for landslide deformation prediction. Compared with the traditional integral method, the lateral deflection curve of inclinometer calculated by the proposed method is closer to the actual measured displacement both in trend and values. The proposed model shows great potential in the application of deflection calculation in engineering.

Single-Shot COTDR Using Sub-Chirped-Pulse Extraction Algorithm for Distributed Strain Sensing

Distributed optical fiber sensing, which uses telecom fiber as sensing medium, can realize high-precision temperature/strain monitoring along the fiber, making it an indispensable tool for structural health monitoring, security monitoring, seismic wave detection, etc. In this article, for the first time, a novel sub-chirped-pulse extraction algorithm (SPEA) is proposed to replace the time-consuming sweeping process in conventional COTDR, and a single-shot coherent optical time domain reflectometry (SS-COTDR) for dynamic strain sensing is demonstrated with very high performance. In order to quantitatively evaluate the maximum measurand change that can be measured per unit time, the measurement slew rate (SR) as an important parameter of COTDR has been analyzed in phase-demodultion scheme and in our scheme. As the proof-of-concept experiments, ultra-high strain sensitivities $\text{2.3 } p\varepsilon /\sqrt{\text{Hz}}$ is demonstrated in $\text{134} \text{ m}$ fiber with $\text{2} \text{ m}$ spatial resolution. Without distributed amplification, high-performance distributed strain measurements are demonstrated on $\text{75} \text{ km}$ fiber with strain sensitivity better than $\text{100} \,p\varepsilon /\sqrt{\text{Hz}}$ . In addition, this scheme is inherently immune to interference fading, which is disadvantageous for phase demodulation scheme.

Strain-based stability analysis of locally loaded slopes under variable conditions

With the rapid development of the distributed strain sensing (DSS) technology, the strain becomes an alternative monitoring parameter to analyze slope stability conditions. Previous studies reveal that the horizontal strain measurements can be used to evaluate the deformation pattern and failure mechanism of soil slopes, but they fail to consider various influential factors. Regarding the horizontal strain as a key parameter, this study aims to investigate the stability condition of a locally loaded slope by adopting the variable-controlling method and conducting a strength reduction finite element analysis. The strain distributions and factors of safety in different conditions, such as slope ratio, soil strength parameters and loading locations, are investigated. The results demonstrate that the soil strain distribution is closely related to the slope stability condition. As the slope ratio increases, more tensile strains accumulate in the slope mass under surcharge loading. The cohesion and the friction angle of soil have exponential relationships with the strain parameters. They also display close relationships with the factors of safety. With an increasing distance from the slope edge to the loading position, the transition from slope instability to ultimate bearing capacity failure can be illustrated from the strain perspective.

Graphene sensing meshes for densely distributed strain field monitoring

The objective of this study is to design and validate distributed strain field monitoring using a patterned nanocomposite “sensing mesh” that is coupled with an electrical impedance tomography (EIT) measurement strategy and algorithm. Although EIT has been used in other studies and in conjunction with a piezoresistive thin film for spatial damage detection, different strain components cannot be directly extracted from reconstructed EIT conductivity maps. Therefore, this study seeks to address this issue by patterning piezoresistive graphene-based thin films to form a mesh-like pattern. The high aspect ratio of each nanocomposite grid interconnect acts as a linear distributed strain sensor, capable of resolving strains along the entire length and direction of the element. This study first began with the design, fabrication, and characterization of the strain sensing response of a graphene-based thin film of high strain sensitivity. Second, the strain-sensitive film was spray-coated onto patterned polymer substrates to form the sensing meshes, which were then subjected to load tests. Upon validating distributed strain field monitoring through EIT, its applicability for field implementation and damage characterization was also demonstrated by instrumenting sensing meshes in the column of a seven-story reinforced-concrete building subjected to shaking table earthquake excitations. The large-scale shaking table test results successfully validated distributed damage detection.

Distributed fiber-optic strain sensing with millimeter spatial resolution for the structural health monitoring of multiaxial loaded GFRP tube specimens

Due to their high strength-to-weight ratio and excellent fatigue resistance, glass fiber reinforced polymers (GFRP) are used as a construction material in a variety of applications including composite high-pressure gas storage vessels. Thus, an early damage detection of the composite material is of great importance. Material degradation can be determined via measuring the distributed strain profile of the GFRP structures. In this article, swept wavelength interferometry based distributed strain sensing (DSS) was applied for structural health monitoring of internal pressure loaded GFRP tube specimens. Measured strain profiles were compared to theoretical calculation considering Classical Lamination Theory. Reliable strain measurements with millimeter resolution were executed even at elongations of up to 3% in the radial direction caused by high internal pressure load. Material fatigue was localized by damaged-induced strain changes during operation, and detected already at 40% of burst pressure.

On the Fluid Structure Interactions of a Wire-Wrapped Pin Bundle

Presently there exist no experimental methods readily available to characterize the comprehensive motion of wire-wrapped pins for the purpose of measuring fluid structure interactions. Specifically, the dearth of capabilities lies in the need to capture pin-to-pin interactions within the bundle that do not have visual access. This study leverages recent previous efforts that have demonstrated the successful use of a distributed strain sensor to characterize the motion of a single wire-wrapped pin under fluid flow and expands through the use of multiple instrumented pins to characterize the simultaneous motion of pin-to-pin interaction. The outcome of this study demonstrates the direct measurement of pin-to-pin contact, rubbing, and interaction over a range of relevant flow rates on a 19-pin wire-wrapped bundle.

Distributed UV Light Sensor Probe Based on Photo-Responsive Polymer Coated Optical Fiber

A novel distributed ultraviolet (UV) light sensor based on a functional polymer-coated optical fiber is proposed. An azobenzene polymer was prepared by mixing an azobenzene compound and UV curing agent and applied as coating on a distributed strain sensor probe as optical fiber using the UV curing process. This coating was reversibly stretched by the UV light, and in the coated region, it induced tensile strains by the UV light along the fiber. This strain, measured by a distributed fiber optic sensor (DFOS) interrogator, was up to 124.5 με under a UV power of 10 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Using this method, distributed UV monitoring can be realized in conventional strain/temperature DFOS interrogators.

Embedded optical fibres for monitoring pressurization and impact of filament wound cylinders

Filament wound composite pressure vessels have found use throughout many industries for storing high-pressure gases. The ability to investigate the vessels for possible damage from accidental impact loads is important for applications in the transport sector. Standard non-destructive methods are complicated. This paper describes a method to permanently monitor pressure vessels for their structural integrity.The method is based on standard telecommunications grade optical fibres embedded in the composite overwrap as a network of distributed strain sensors. Challenges regarding the practical implementation of optical fibres, and post-processing of signals, are discussed. During pressurization, the optical fibres were used to measure strains in several thicknesses of the composite layup. The cylinders were impacted and the backscattered light from optical fibres was then analysed to visualize the position and the severity of damage. The interrogation of embedded optical fibre networks is successfully demonstrated as a promising method for structural health monitoring of composite pressure vessels.

Crack detection and localization in RC beams through smart MWCNT/epoxy strip-like strain sensors

In recent years, self-sensing structural materials have drawn enormous attention of scientific community due to their potential to enable continuous monitoring of the integrity of structures. The new paradigm of smart condition-based maintenance advocates the use of next-generation structures completely or partly constituted by self-sensing materials, that is, the structure or part of it also behaves as a sensor. In this context, the remarkable mechanical and electrical properties of multi walled carbon nanotubes (MWCNTs) have fostered an increasing number of applications as fillers for composites with multifunctional properties. Among a wide spectrum of potential applications, the development of skin-type piezoresistive distributed strain sensors shows great promise. Such sensors can be deployed onto large-scale structures, enabling a continuous monitoring of the strain state in the global area of the structure. In this paper, a theoretical study on the potential application of smart MWCNT/epoxy strip-like strain sensors for damage detection/localization/quantification in reinforced concrete (RC) beams is presented. A micromechanics-based finite element model is proposed for the electromechanical analysis of MWCNT/epoxy strips. Furthermore, a damage detection algorithm through model updating approach is introduced. To do so, an Euler–Bernoulli model for beams equipped with a smart MWCNT/epoxy strip is developed. Finally, two numerical case studies are presented including: a 2D concrete beam with multiple prescribed crack-like damages, and a 3D RC beam under four-point flexural conditions with non-prescribed cracking. Results show that the proposed smart strips are capable of exploiting the damage-induced variations in the electrical output to locate and quantify damages for real-time distributed structural health monitoring of RC beam structures.

Strain resolution enhancement in Rayleigh-OTDR based DSS system using LWT-MPSO scheme

This paper presents a scheme for performance improvement of a Rayleigh-optical time domain reflectometry (OTDR) based distributed strain sensing (DSS) system using the combined effect of lifting wavelet transform (LWT) and modified particle swarm optimization (MPSO) method. In our proposed scheme, MPSO evolutionary algorithm produces an optimum threshold and helps in minimizing the noisy lifting wavelet coefficients effectively. This allows an order-of-magnitude increase in strain resolution of the proposed system. The proposed sensing system is capable of measurement up to 50 km using a 10 mW of input laser source power. We have applied 30 με strain at 30 km position of the fiber under test. The proposed signal processing scheme offers better performance over a conventional system. The system achieved a strain resolution of ∼ 1.8 με. Relevant numerical simulations are presented using MATLAB 15.0.

Intra-Arterial Image Guidance With Optical Frequency Domain Reflectometry Shape Sensing.

Intra-arterial liver cancer therapies, such as trans-arterial chemoembolization, are the preferred therapeutic approaches for advanced hepatocellular carcinoma. However, these palliative techniques are challenging for delivering therapeutic agents selectively in the tumor without real-time 3-D visualization of the catheter within the hepatic arteries. The objective of this paper is to develop and evaluate in pre-clinical tests an advanced interventional guidance platform using a distributed strain sensing device based on optical frequency-domain reflectometry (OFDR) to track the tip and shape of a catheter. The scattering properties of a fiber triplet are enhanced by focusing an ultraviolet beam on these fibers, producing a fully distributed strain sensor, which avoids interpolation errors observed with traditional shape tracking systems. A 3-D roadmap of the hepatic arteries is obtained from a combined fully convolutional and residual networks trained on MR angiography and combined with a 4-D flow dynamic sequence enabling to map blood flow velocities. An anisotropic curvature matching method is proposed to map the sensed data onto pre-operative MR and using 3-D ultrasound to correct for non-rigid deformations. Experiments were conducted in a controlled environment setting as well as in both synthetic phantoms and in five porcine models to assess the performance for device navigation, yielding satisfactory tracking accuracy with 3-D mean errors of 2.8 ± 0.9 mm. We present the first pilot study of MR-compatible UV-exposed OFDR optical fibers for non-ionizing device guidance in intra-arterial procedures, with the potential of avoiding multiple hospitalizations required to perform invasive selective chemoembolizations.

A numerical study on the feasibility of evaluating CO2injection wellbore integrity through casing deformation monitoring

AbstractDistributed sensing technologies have been increasingly used for downhole monitoring in the oil field in recent years. A casing deformation monitoring system based on distributed strain sensors has been developed and proposed for wellbore integrity monitoring. However, due to the lack of field data on casing deformation induced by various exploration and development activities, the relationship between casing deformation and wellbore integrity remains unclear. To study the feasibility of evaluating short‐term CO2injection wellbore integrity through casing deformation monitoring, a staged finite element analysis is conducted for an existing well in the Weyburn field to study the relationship between casing deformation and wellbore integrity during CO2injection. The model is generated with HypermeshTMand the analysis is conducted with AbaqusTM. The staged finite element model applies loads and materials in multiple steps to replicate the actual drilling process. The wellbore stress, casing strain, and interface gap distance change at the surface casing shoe and production casing shoe are calculated. A parametric study is also conducted to study the effect of various parameters on CO2injection wellbore integrity in the Weyburn field. The result shows that the casing deformation can be correlated to the wellbore integrity by cement expansion/shrinkage through its effect on the interface gap distance change and the stress difference between wellbore components induced by CO2injection. © 2017 Society of Chemical Industry and John Wiley &amp; Sons, Ltd.

Temperature-dependent strain and temperature sensitivities of fused silica single mode fiber sensors with pulse pre-pump Brillouin optical time domain analysis

This paper reports a distributed temperature and strain sensor based on pulse pre-pump Brillouin optical time domain analysis. An uncoated, telecom-grade fused silica single-mode fiber as a distributed sensor was calibrated for its sensitivity coefficients under various strains and temperatures up to 800 °C. The Brillouin frequency of fiber samples changed nonlinearly with temperature and linearly with strain. The temperature sensitivity decreased from 1.113 to 0.830 MHz /°C in the range of 22–800 °C. The strain sensitivity was reduced from 0.054 to 0.042 MHz /με as the temperature increased from 22 to 700 °C and became unstable at higher temperatures due to creep effect. The strain measurement range was reduced from 19 100 to 6000 με in the temperature range of 22–800 °C due to fused silica’s degradation. The calibrated fiber optic sensor demonstrated adequate accuracy and precision for strain and temperature measurements and stable performance in heating–cooling cycles. It was validated in an application setting.