Hydromechanical Storage Measured by Fiber Optic Distributed Acoustic Sensing

Subsurface storage and recovery of gas, carbon, heat, and water, key to the geoenergy transition, are governed by hydromechanical processes in the geologic medium. Despite the importance of hydromechanical storage, the physics of storage has received less attention than the physics of fluid flow. Storage is typically assumed to be a linear, homogeneous function of pore pressure. This approximation may hold for permeable unconsolidated formations under small hydraulic forcings but can break down in stiff or low-permeability media subjected to high-pressure injection and withdrawal.

One reason the physics of hydromechanical behavior in geologic reservoirs remains incomplete is the scarcity of local in-situ measurements of deformation during flow. Commonly available measurements, such as surface deformation (e.g., InSAR) or borehole displacement (e.g., extensiometers), vertically integrate formation strain. The advent of distributed fiber-optic sensing provides a new opportunity to resolve depth-dependent deformation in response to pressure variability.

I describe three experiments in which distributed strain was observed during fluid injection and withdrawal: (1) a sparsely fractured crystalline rock, (2) a semi-consolidated porous sandstone, and (3) a permeable aquifer. In all cases, fiber-optic distributed acoustic sensing (DAS) was used to measure along-wellbore deformation. The nanostrain sensitivity of DAS provided new insight into the physics of hydromechanical deformation associated with subsurface fluid movement. Displacement was vertically heterogeneous in all experiments, and storage parameters derived from displacement measurements were not directly comparable to those inferred from hydraulic responses. These results illustrate the need for coupled hydromechanical frameworks and in-situ measurements that jointly resolve fluid flow and deformation, particularly in the context of subsurface technologies central to the geoenergy transition.