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Distributed fiber optic sensing (DFOS) as a reliable well and reservoir surveillance has become more prevalent in recent years, due to the continuous advancement of the technology and the reduction of the cost. DFOS found widespread application not only in the oil and gas industry but also in other deep subsurface injection/extraction operations such as geologic CO2 storage (GCS) and geothermal energy. Under the umbrella of DFOS, distributed acoustic sensing (DAS) and distributed temperature sensing (DTS) have had remarkable increases of applications in subsurface monitoring, and distributed strain sensing has opened new possibilities in applications such as high-resolution production logging and near-wellbore stress measurements. The applications of DAS include, but are not limited to, vertical seismic profiling (VSP), seismic, microseimic, frac-hit detection based on far-field low-frequency DAS, fluid allocation during hydraulic fracturing, and production logging. DTS has shown promising results in well integrity leak diagnosis, production logging in gas production, gas-lift monitoring, hydraulic fracturing characterization, and monitoring of GCS and geothermal projects. Furthermore, the integration of all kinds of DFOS data starts to deliver a complete depiction of what is happening downhole.

Even with the boom of DFOS applications, challenges remain with how to tie the physics underneath the measurements and how to quantitatively interpret the data. This special section explores various successful applications of DFOS and in-depth studies in DFOS data interpretation. The three papers in this section focus on crosswell strain signals during hydraulic fracture characterization, time-lapse monitoring of subsurface CO2 sequestration, and estimation of hydraulic fracture extents in different shale formations based on strain response. These three papers showcase the importance and potential of DFOS technology in the field of well and reservoir surveillance.

Interpreting strain signals is important for effectively characterizing hydraulic fracturing operations in unconventional reservoirs. Accurate interpretation and visualization of these signals can provide valuable insights into the fracture process and guide optimization efforts, ultimately improving production outcomes. Ning and Jin investigate crosswell strain signals recorded by offset wireline and disposable fibers during the zipper fracturing treatment of seven wells targeted in four layers. The authors demonstrate methods of visualizing and interpreting complicated low-frequency DAS signals, providing guidance for hydraulic fracture characterization during complex stimulation operations in unconventional reservoirs.

Leggett shows how the fiber optic strain response informs the geometry of height-bounded fractures. A new interpretation method is applied to estimate final extents of hydraulic fractures in the Austin Chalk and Montney Shale.

CO2 sequestration is a popular method used to reduce greenhouse gas emissions. Ma et al. use field VSP data for monitoring CO2 sequestration to analyze the imaging differences between DAS and geophone data. The authors convert DAS to geophone data via a deep-learning approach to mitigate the differences, aiming to potentially facilitate time-lapse monitoring subsurface CO2 sequestration.

As DFOS technology continues to evolve and improve, it will undoubtedly play a critical role in advancing our understanding of subsurface operations and optimizing production outcomes. Our aspiration is that this special section will contribute to our ongoing endeavors to advance the development and utilization of DFOS technologies.