New Strainmeters used to Monitor Deformation During Injection and Withdrawal

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. This has motivated us to develop instrumentation and methods to characterize the strain field resulting from injection and pumping. Three new instruments have been deployed at our field stations near Clemson University and at the Avant Field north of Tulsa, OK. Two use non-contact eddy current transducers configured to measure four components of strain and two tilts to 1 part-per-billion. One system is designed for permanent installation, the other is removable for short term deployments. Another system is a low cost volumetric strainmeter consisting of an embedded optical fiber that is interrogated using laser interferometry. This strainmeter is designed to be a permanently installed and has a resolution of several parts-per-trillion. The field sites are designed to characterize strains during pumping or injection over different scales and in different geologic settings. The Clemson field station is underlain by biotite gneiss, a low permeability crystalline rock overlain by moderate permeability, soft saprolite above 30m depth. The water table is at approximately 9m depth. The strainmeters are in the crystalline rock at approximately 40m depth, and pumping occurs in the overlying saprolite. In contrast, wells at the Avant Field site are much deeper. They are approximately 500m deep and completed in a 25-m-thick oil-bearing sandstone. Strainmeters at the Avant Field are at 30m depth. These two sites provide contrasting approaches to characterizing strain at 30-40m depth. Water is pumped from an overlying formation at the Clemson site, whereas it is pumped from a much deeper underlying formation at the Avant Field. Preliminary results are available from a brief injection test, and from a longer shut-in test at the Avant Field. Injection is characterized by an increase in tensile strains in both the radial and circumferential directions approximately 220m from the well. The shut-in was characterized by radial tension and circumferential compression in response to a well approximately 1km from the strainmeter. These are the expected signals caused by injection and shut-in according to poroelastic simulations.