Distributed Acoustic Sensing of Formation Poroelastic Deformation for Water Resources and Geothermal Energy

Geomechanical strain resulting from fluid pressure changes is generally called poroelasticity and its theoretical underpinnings are well known.  However, the application of theory to real-world problems is often limited by the measurement of subsurface poroelastic heterogeneity.  The nanostrain resolution of DAS presents an opportunity to study poroelastic and hydraulic behavior in the subsurface in unprecedented detail.  Ultra-low frequency response in DAS data can be used to monitor the poroelastic behavior during dynamic hydraulic tests either in single or cross-hole testing mode.  We will present results from multiple field sites in which we have used DAS as a distributed dynamic strain meter to elucidate poroelastic behavior in consolidated and unconsolidated formations, for water resource and geothermal energy applications.

Single well hydraulic tests in alluvium were monitored with DAS fiber strapped to the screened well casing and mechanically coupled to the aquifer formation via a gravel pack.   In a short-screen interval well, DAS demonstrated the non-radial behavior of poroelastic response to extraction of water.  In a test conducted in an aquifer with a long (235 m) well screen, DAS-measured formation expansion and contraction in response to fluid injection were several hundred nanostrain.  Strain, and the implied storage distribution, was highly localized in specific strata and demonstrated complex hydromechanical behavior.  In a crystalline bedrock, DAS observed nanometer fracture displacements in response to injection at a companion well, 30 meters away.  The strain response was consistent with expected flow in sparse fracture networks.  Geomechanical response indicated some shear along the fractures.  An installation of fiber in a 3000 m deep geothermal borehole is characterizing hydraulic connectivity during flow recirculation.  These geothermal experiments are ongoing, but early DAS data show strain in response to recirculation tests between laterals separated by 100 m.