The time evolving strain field contains a wealth of information that can be used to interpret subsurface behavior. For example, injecting or removing fluids from reservoirs or aquifers causes deformation that can be used as a diagnostic signal in some cases, while it can interfere with geodetic interpretations in other cases. We've recently completed a field study that demonstrated the feasibility of measuring the strain tensor at a depth of 30 m caused by injection into a reservoir at 530 m depth. The observed strain signals were interpreted using four independent analytic and numerical methods that resulted in estimates of the poroelastic properties and geometry of the reservoir that was consistent with data from well logs. However, studies like these are only possible if these deformations can be reliably measured. Advances in optical fiber sensing systems have led to their introduction in a number of areas including quasi-static and dynamic subsuface deformation monitoring. Optical fiber-based interferometers are capable of measuring very small displacements while being completely passive in their operation. The low attenuation and significantly reduced bending loss in rare-earth doped, high numerical aperture glass optical fibers allows for the embedding of long lengths of fiber into compact, durable and exceptionally sensitive downhole sensing packages. We have expanded on years of lab and field work developing and deploying long baseline and embedded single-component borehole strainmeters to the design of three novel horizontal tensor strainmeters. Each design represents a unique embedding approach for measuring directional strain across the diameter of a borehole with differing advantages in terms of ease of fabrication and assembly, as well as directional resolution. We will present the design details along with laboratory calibration results and preliminary field data comparing their relative performances across tidal and seismic frequencies.

Strains occur at shallow depths in response to pressure changes during well tests in an underlying aquifer, and recent developments in instrumentation have made it feasible to measure essentially the full strain tensor. Simulations using poroelastic analyses indicate that shallow normal strains are approximately proportional to the logarithm of time when a well is injecting into or pumping from a uniform aquifer or reservoir. The drawdown is also a function of log time, as shown by the classic Cooper-Jacob type-curve analysis. The time when the semi-log straight line intercepts the zero-strain axis is similar to the time determined from the Cooper-Jacob pressure analysis, and it can be used to estimate hydraulic diffusivity, suggesting that horizontal strain data can be used directly to estimate aquifer properties. This approach is applied to data measured with shallow (30 m) borehole strainmeters during an injection test at a 530-m-deep sandstone aquifer/reservoir in Oklahoma. The results show intercept times for the shallow normal strain data are essentially the same as for deep pressure data from an equivalent radial distance. The slopes of the semi-log plots of the pressure and the strain increase at the same time, suggesting that they both respond to a lateral aquifer boundary. These results confirm the type-curve approach for interpreting strain data. Significantly, though, strain was measured at shallow depths while the pressure data was measured at 530 m depth. This suggests that strain data from shallow depths could be an effective way to improve the characterization of an underlying aquifer.

Storing and recovering water, carbon and heat from geologic reservoirs is central to managing resources in a changing climate. We tested the hypothesis that the strain tensor caused by injecting or producing fluids can be measured at shallow depths and interpreted to advance understanding of underlying aquifers or reservoirs. Geodetic-grade strainmeters were deployed at 30m depth overlying the Bartlesville Formation, a 500-m-deep sandstone near Tulsa, OK. The strainmeters are 220m east of injection well 9A completed in a permeable lens at the base of the Bartlesville Formation. Water was injected into well 9A at approximately 1.0 L/s during four tests that ranged in duration from a few hours to a few weeks. The horizontal strain increased (tension) and the circumferential strain was a few times larger than the radial strain. The vertical strain decreased (compression) during injection. Strain rates were approximately 100 n/day during the first few hours, but the rates decreased and were approximately 10 n/day during most of the tests. Four independent methods of poroelastic simulation and inversion predict reservoir properties and geometries that are similar to each other and consistent with independent information about the reservoir. All strain interpretations predict that a boundary to the permeable lens occurs beneath the vicinity of the AVN strainmeters, which is consistent with core data from the site. The boundary of the permeable lens is located by matching the radial and circumferential strains, which demonstrates the value of measuring the strain tensor.