Deformation, Failure and Permeability Evolution of Sealed Fractures/Foliations in EGS Collab Poorman Schist

It is widely recognized that reactivation of pre-existing fractures by injection can play a crucial role in permeability creation during EGS stimulation. A numbers of previous studies have focused on the permeability enhancement by shear slip and propagation of open fractures in response to injection. The contributions of sealed fractures (e.g. filled fractures, foliations, and veins) to permeability creation during hydraulic injection have not scrutinized. These weak planes are supposedly reactivated by injection to provide high conductive flow paths and potentially form a network by coalescence with hydraulic fractures. In this work, a Poorman Schist sample containing a calcite-filled vein and foliations from the EGS Collab site (Sanford Underground Research Facility) was used to conduct laboratory injection tests. The test included three steps: (1) characterization of the sample permeability before reactivation/stimulation; (2) reactivation of a relatively thick vein (mineralized sealed fracture) in the sample by pressurized fluid; and (3) evaluation of permeability enhancement by the reactivation of the fracture. The results demonstrated that the sealed fracture or vein is more permeable than the rock matrix. In addition, the foliation planes are relatively weaker and can be reactivated by fluid injection before the vein/sealed fractures, resulting in flow rate/permeability enhancement. The failure behavior and the temporal-spatial evolution of AE hypocenters during the test indicate that natural discontinuities (foliation, and veins) increase the complexity of hydraulic stimulation (fracturing and shear stimulation) in Collab stimulation at Poorman Schist formation.