Abstract

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.