Fiber-Optic Sensor Detects Rock Strain Before CO2 Plume Arrives

Deep beneath the surface of any carbon capture and storage site, two things happen when carbon dioxide is pumped into porous rock: the gas itself creeps slowly through the pore network, and pressure waves race ahead of it through the connected brine. The first process is relatively easy to image. The second has long been a blind spot, because pore pressure inside a formation is notoriously difficult to measure at high spatial resolution. Now a laboratory study published in Communications Engineering shows that a single helically bonded optical fiber can sense the mechanical fingerprint of that hidden pressure field, revealing rock deformation that begins well before the carbon dioxide front becomes visible in X-ray tomography.

The research team, led by Jinrong Cao of the Geological Carbon Dioxide Storage Technology Research Association and the Research Institute of Innovative Technology for the Earth in Japan, combined three observational streams in a single experiment: distributed fiber-optic strain sensing, time-lapse X-ray computed tomography, and coupled numerical modeling of two-phase flow and poroelastic deformation. Their specimen was a cylindrical tuffaceous sandstone core, 34.83 millimeters in diameter and 179.50 millimeters long, drilled from the Sarukawa oil field in Oga, Akita Prefecture, an area selected for a Japanese advanced carbon storage program targeted to begin operations by 2030. The rock had a porosity of 25.06 percent and an intrinsic permeability of 5.41 millidarcys, with a mineral framework dominated by the zeolite clinoptilolite at roughly 63 percent, alongside quartz, albite, anorthite, and heulandite.

The sensing element was a single-mode silica fiber bonded to the core surface with epoxy at an optimized wrap angle of 80 degrees relative to the core axis. This helical geometry converts one continuous fiber into a dense array of quasi-independent strain sensors, resolving circumferential, or hoop, strain at one-centimeter spatial resolution along the full specimen length. The interrogator, a Neubrex 7020 unit, works by measuring Rayleigh frequency shifts in backscattered laser light using a tunable wavelength coherent optical time-domain reflectometer. Because strain and temperature both shift the Rayleigh spectrum, the team held the experiment at a stable 40 degrees Celsius and applied a calibrated strain coefficient of minus 0.153 gigahertz per microstrain, yielding a strain resolution of about half a microstrain for comparable measurements, far finer than the tens to roughly one hundred microstrains observed during injection.

Before introducing carbon dioxide, the researchers ran a single-phase water flow test to pin down permeability and validate their poroelastic model. When upstream pressure reached 10.5 megapascals, circumferential strain near the inlet jumped to approximately 75 microstrains within a single sampling interval of 36 seconds or less, while the downstream end of the core registered only a few microstrains. Numerical simulations showed that the sub-second initial response could not be resolved experimentally, but by roughly half a second to one second the simulated strain gradient matched the first measured profile, and a steady inlet-to-outlet strain gradient established itself within about one second. This close agreement between measurement and model confirmed that, in fully water-saturated rock, deformation is governed by pore water pressure gradients acting through Biot-style poroelastic coupling.

The main event was the supercritical carbon dioxide flood. Time-lapse CT scans, taken at 60 seconds per volume using an iodide-rich contrast solution to sharpen fluid-rock contrast, tracked the drainage process in detail. During the first 1.6 minutes, corresponding to 0.036 injected pore volumes, carbon dioxide accumulated mainly near the inlet. By 2.67 minutes the saturation field had become spatially nonuniform, reflecting variability in pore accessibility and capillary entry conditions across the zeolite-rich framework. At 4.27 minutes the front reached the specimen midpoint, and breakthrough occurred at 8.0 minutes, when average carbon dioxide saturation hit 19.4 percent. Differential pressure during drainage held steady between 0.45 and 0.48 megapascals at a flow rate near one milliliter per minute.

The striking result was how differently the strain field evolved from the saturation field. At the onset of injection, a pronounced strain gradient appeared immediately: strain near the inlet climbed to roughly 80 microstrains, while even the outlet showed a small but detectable response. The strain front was far more spatially diffuse than the comparatively sharp saturation front imaged by CT. By breakthrough, strain had reached about 100 microstrains near the inlet and 20 microstrains near the outlet. Most tellingly, at a position 0.09 meters from the inlet, a distinct strain increase appeared near the very start of injection, while local carbon dioxide saturation remained at zero for the first four minutes. The delay between strain onset and saturation onset grew with distance from the inlet, exactly the pattern expected if pressure disturbances propagate through the connected aqueous phase faster than the capillary-controlled gas front advances.

The coupled numerical model, a two-dimensional axisymmetric finite-element representation of the full specimen, provided the physics-based bridge between the observable strain and the unmeasured internal pressure. The team calibrated two-phase flow parameters exclusively against the CT-derived saturation profiles, then compared the predicted strain against the fiber measurements as an independent test. Simulated pore gas and pore water pressures evolved concurrently but with different spatial patterns, related through capillary pressure, and the modeled strain reproduced both the magnitude and the timing of the measured response. Because the sandstone’s apparent drained bulk modulus of roughly one to two gigapascals makes it relatively compliant, a given pressure perturbation produces a large, measurable strain. The authors are careful to stress that the fiber does not measure pressure directly; the strain signal is a model-constrained poroelastic proxy for the evolving pressure footprint, not a standalone pressure gauge.

The engineering implications reach well beyond the laboratory. Operational surveillance of storage sites typically relies on wellhead and bottomhole pressure, flow rates, sparse downhole gauges, and episodic plume imaging, none of which uniquely constrains the internal pressure field. Distributed strain sensing adds a spatially continuous constraint that can be folded into coupled flow and geomechanical models, helping to characterize uncertain formation properties and to evaluate deformation relevant to reservoir, caprock, and wellbore integrity. Field deployments already hint at feasibility: distributed fiber sensing has been demonstrated under field-scale injection conditions at the CO2CRC Otway site in Australia, a pumping test at Mobara in Japan captured pressure-related reservoir deformation, and behind-casing water-injection measurements show that injection-induced strain is detectable through a casing-cement-formation system.

The authors are equally candid about the limitations that separate their benchtop result from routine field practice. The adhesive layer bonding the fiber to the core was not independently calibrated, so perfect strain transfer was assumed. Field fibers are usually installed axially behind casing, where recorded amplitude depends on cable orientation, completion design, cement properties, gauge length, and strain-transfer efficiency, and axial strain is not generally equivalent to the hoop strain measured here. Stiffer formations or rocks with a lower Biot-Willis coefficient would produce smaller strains that may fall below detection thresholds, meaning an absent strain signal cannot be read as absent pressure communication. The homogeneous axisymmetric model also cannot capture localized effects of bedding, fractures, or anisotropic permeability, and longer-term chemical interactions between carbon dioxide, brine, and reactive minerals may gradually alter rock stiffness and fracture behavior.

Even with those caveats, the study delivers a compelling proof of concept: the mechanical consequences of injection arrive before the plume does, and a helically wound strand of glass can watch them happen. Future priorities include experiments on rocks with contrasting stiffness and capillary behavior, direct internal pressure measurements, three-dimensional models that explicitly represent the helical fiber geometry, and field studies that integrate distributed strain with pressure gauges, time-lapse geophysics, distributed acoustic and temperature sensing, and surface deformation observations. As global net-zero pathways call for roughly ten gigatons of carbon dioxide storage per year by 2050, tools that convert invisible pressure transients into measurable, interpretable strain signals could become an essential part of the monitoring toolkit, giving operators and regulators earlier and more defensible warnings about where the subsurface is being mechanically stressed.

Subject of Research: Distributed fiber-optic sensing of poroelastic strain precursors during supercritical CO2 core flooding in sandstone

Article Title: Poroelastic strain precursors during laboratory carbon dioxide core flooding revealed by helically bonded fiber sensing

Article References: Cao, J., Park, H., Xue, Z., Nakajima, T., Amer, R., & Ouchi, W. (2026). Poroelastic strain precursors during laboratory carbon dioxide core flooding revealed by helically bonded fiber sensing. Communications Engineering, 5(1), Article 164. https://doi.org/10.1038/s44172-026-00783-5

Image Credits: AI Generated

DOI: 10.1038/s44172-026-00783-5

Keywords: carbon capture and storage, distributed fiber-optic sensing, poroelasticity, CO2 core flooding, X-ray computed tomography, pore pressure, geomechanics, sandstone, capillary pressure, geological storage, strain sensing, two-phase flow