Deformation-Based Monitoring of Water Migration in Rocks Using Distributed Fiber Optic Strain Sensing: A Laboratory Study

Rock deformation induced by pore-fluid pressure carries useful information about fluid flow owing to hydromechanical coupling. Thus, obtaining spatiotemporal changes in rock deformation could provide improved understanding of the fluid and pressure migration in aquifers or the role of fluid in the evolution of rainfall-induced landslide. Here we deployed high-resolution Rayleigh-scattering-type distributed fiber optic strain sensing (DFOSS) to measure rock deformation while injecting water into low-permeability dry sandstone. X-ray computed tomography imaging was simultaneously used to visualize the water migration. DFOSS measurements showed the rock developed a dilation deformation that grew during water saturating process. The movement of water wetting front can be revealed by the changes in the measured distributed strain. Strain changes were shaped by poroelastic changes due to the fluid pressure buildup and swelling by the water-clay reaction (i.e., adsorption). The latter mechanism caused increase in the strain when water first entered the dry pore spaces and the change in pore pressure was slight. The mechanism continued contributed to the overall deformation to peak magnitude of similar to 600 mu epsilon together with poroelastic mechanism. However, after the rock was fully saturated, further deformation during the flow test can be explained by the poroelastic mechanism alone. Our study suggests that the two factors can be employed as signatures for effectively monitoring fluid behavior in natural sediments using DFOSS. Moreover, we obtained the spatial hydromechanical properties, permeability and stiffness, from the distributed strain measurement and pressure responses. Using DFOSS in the field may substantially improve our ability to monitor and model fluid activity related to rock deformations in reservoirs and rainfall-induced landslides that would help in warning people of risks and preventing disasters. Plain Language Summary Deployment of distributed fiber optic strain sensing (DFOSS) in underground geoengineering may be a new and better way to solve some of the problems with conventional deformation monitoring methods. For example, interferometric-synthetic-aperture-radar-based earth surface deformation monitoring lacks constraints along the vertical direction across strata formations. Other discrete strain measurement tools (e.g., extensometers and strainmeters) can be difficult to install at a particular depth and they have low spatial resolution. In order to demonstrate the strain monitoring ability of DFOSS, in particular for monitoring deformations induced by underground fluid activity as a consequence of hydromechanical coupling, in this study, we experimentally measured the distributed strain in a sandstone core while injecting water into it as a proof of concept. The results showed that water migration in the rock can be successfully sensed by the spatially distributed strain changes using high-resolution DFOSS. Moreover, by tracking the water wetting front using strain data, the spatial (one-dimensional) permeability of the rock can be estimated. Our study suggests that distributed strain sensing can provide not only information about geomechanical deformations (which provides important constraints in geomechanical risk assessment), but also unique insights into the hydromechanical link between fluid flow and rock deformation in subsurface fluid injection/extraction operations or rainfall-induced landslides.