Hydrogen can be stored in underground wells to mitigate the imbalance between hydrogen supply and demand in the future hydrogen economy. The concentration of stored hydrogen can vary due to microbial reactions and leakage through caprocks in the subsurface storage facilities such as salt caverns, saline aquifers, and depleted hydrocarbon reservoirs. Thus, monitoring hydrogen concentration in subsurface storage is essential to ensure integrity of the storage infrastructure and to detect early signs of gas leakage. Optical fiber-based hydrogen monitoring has advantages of stability in harsh environments, real-time and remote sensing, and improved safety compared to electrical-based sensors in flammable gases. An optical fiber sensor with a palladium nanoparticles-incorporated SiO2 film (Pd/SiO2) was previously demonstrated for hydrogen sensing over a wide range of hundreds ppm to 100% in dry conditions. However, the Pd-based hydrogen sensitive materials are susceptible to water vapor interference, which leads to a significant reduction in hydrogen sensitivity under humid conditions. To address this challenge, this study focused on the enhancement of hydrogen sensing response under humid conditions by applying a hydrophobic filter film over the Pd/SiO2 sensing layer. The optical fiber sensor covered by the filter layer showed significant improvement on the baseline drift issue and reduction in hydrogen sensitivity caused by high humidity (99% RH). In addition, the developed optical fiber sensor demonstrated negligible impact by hydrocarbon contaminants such as CO2 and CH4 which are present in the subsurface hydrogen storage reservoirs. The Pd/SiO2-coated optical fiber sensor coupled with the filter layer has high potential to be deployed in the subsurface hydrogen storage areas to monitor hydrogen concentration without cross-sensitivity of hydrocarbons and humidity.
In the oil and gas, CO2 sequestration, H2 subsurface storage, and geothermal energy sectors, subsurface pH measurements are critical for monitoring the geochemical conditions and structural integrity of wellbore systems. Real-time pH measurements in these conditions are vital for detecting and predicting corrosion deterioration of wellbore components that may jeopardize the safety and continued operation of wellbore systems. The viability of TiO2-coated optical fibers has previously been demonstrated as an effective sensor design for continuous distributed pH monitoring at elevated temperatures and ambient pressures. However, real wellbore conditions contain high pressures and the effects of high pressures on sensor results and the sensing layer have not been well studied. As TiO2 has been established in the literature as being stable at temperatures and pressures substantially higher than those expected in typical wellbore conditions, it makes for a promising sensing material for applications requiring high-pressure, high-temperature (HPHT) conditions. In this study, a sol-gel deposition method is used to coat the optical fiber sensors with TiO2 sensing layer. The sensor performance was measured using optical transmission measurements at various pH and using optical backscatter reflectometry for distributed pH sensing demonstration in wellbore-relevant pressures (up to 1000 psi) and temperatures (~80 °C). The TiO2 sensing layer was characterized using scanning electron microscopy (SEM) and full spectrum UV-Vis-NIR transmission data for a planar substrate. The TiO2-coated optical fiber sensor is tested for any pressure-derived effects and the viability of this sensor design for real-time in-situ wellbore pH monitoring is discussed.
pH is a critical parameter for wellbore integrity and geochemical monitoring in wells for oil and gas production, CO2 storage, H2 subsurface storage, and geothermal systems. In situ real-time pH monitoring in subsurface wells is of significant value for wellbore integrity monitoring and predictive analysis of well component deterioration such as casing steel corrosion and cement carbonation. However, harsh environments in subsurface wells have limited many commonly used pH sensors. We have previously demonstrated optical fiber pH sensors coated with metal oxide-based sensing materials such as TiO2, which offer stability at high pressures and temperatures. In this study, we demonstrated TiO2 coated optical fibers for real-time distributed pH monitoring based on backscattered light interrogation. TiO2 coated optical fibers were tested under ambient conditions and wellbore relevant conditions at elevated temperatures. TiO2 coating was deposited on the optical fibers through a facile sol-gel method. TiO2 coated optical fibers have shown promising pH sensing results under elevated temperatures and high pH conditions, making them suitable for wellbore cement monitoring.
It is important to monitor and locate internal corrosion along natural gas pipelines to prevent methane leaks and catastrophic failures. Corrosion proxy materials enable optical fiber sensors to provide insight into corrosive environments where the pipeline materials tend to corrode. A distributed optical fiber corrosion sensor was demonstrated using a metallic film-coated, single-mode optical fiber interrogated with an optical backscatter reflectometer (OBR), based on strain changes during metallic film dissolution. Electroless plating leads to inherent internal stress in the metallic coating and therefore induces strains on optical fibers. As the metallic film gets dissolved or corroded, the internal stress will be released and cause opposite changes in strain, which can be used for corrosion monitoring. The microstrains induced solely by electroless plating or metal dissolution were measured in situ and in real time using the OBR, and interferences from temperature changes and water-induced swelling were compensated through comparison between the treated section (sensitized and activated) and an untreated control section. High-pH Ni plating had a faster deposition rate with branching on the film and induced negative microstrains, whereas low-pH Ni plating had a slower deposition rate with smooth coating and induced negligible (near-zero) strains. Eletroless Fe plating with high pH didn’t cause significant microstrains. When exposed to corrosive HCl solutions, dissolution of high-pH Ni plated films induced positive microstrains, opposite to the Ni plating. The OBR allows for distributed monitoring of strain changes due to Ni dissolution, demonstrating the capability of identifying corrosion locations along the optical fiber.
The presence of water can provide aqueous electrolytes for corrosion to occur inside the pipelines. The capability of monitoring water vapor condensation enables in-situ monitoring of internal corrosion in natural gas transmission pipelines. Previously, a fully distributed optical fiber sensor for water and humidity monitoring has been demonstrated, consisting of an unmodified off-the-shelf single mode (SM) optical fiber connected to an optical backscatter reflectometer. The intrinsic polymer jacket of the SM fiber is hygroscopic and can serve as a water sensing layer due to expansion/swelling from water absorption. In this work, strain changes were measured and calibrated in jacketed and unjacketed sections at different relative humidity levels (RH, 0% to 100%) and different temperatures (T, 21 to 50°C). In the jacketed section, the sensitivity to humidity decreased from 1.2 to 0.6 με/%RH and then diminished as T increased from 21 to 50 °C, which could be due to the intrinsic absorption property of polymer at higher T or the wet gas flow at room temperature being absorbed in the polymer jacket. The unjacketed section demonstrated a minimal sensitivity to humidity (<0.2 με/%RH) at 21-50 °C and a relatively consistent sensitivity to temperature.
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