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.

1. INTRODUCTION

The EGS Collab project conducts meso-scale hydraulic stimulation and interwell flow experiments at Sanford Underground Research Facility (SURF) to characterize rock failure behavior and probe the fluid-flow and heat transfer process for geothermal reservoir development (Kneafsey et al., 2018, 2019). EGS Collab Experiment 1 boreholes are entirely within the Poorman Formation, a metasedimentary rock consisting of sericite-carbonate-quartz phyllite (the dominant rock type), biotite-quartz-carbonate phyllite, and graphitic quartz-sericite phyllite (Caddey et al., 1991). Carbonate minerals present consist of calcite, dolomite, and ankerite. The rock is highly deformed and contains carbonate, quartz veins/boudinage, pyrrhotite, and minor pyrite. Other mineral phases (in addition to those listed above) include graphite and chlorite. The Poorman schists contains different type of discontinuities such as open and sealed natural fractures, foliations planes, and different mineralization bands distributed at different scales (Uzunlar, 2019). The mechanical deformation, failure behavior, and transport properties of subsurface rock mass are highly influenced by these natural discontinuities and fabric features. In our previous work (Ye, Ghassemi, & Kneafsey, 2019), we have carried out laboratory injection tests on Poorman Schist sample with an open fracture to characterize its seismo-hydro-mechanical response during fracture shear slip. The results demonstrated that the open fractures in Poorman Schist can be reactivated and propped by pressurized fluid injection for permeability enhancement. Creating permeability by the shear reactivation of pre-existing fractures has been known for a long time. However, most previous experiments have focused on the permeability enhancement by shear slip and propagation of open fractures in response to injection (Esaki et al., 1999; Ishibashi et al., 2016; Park et al., 2013; Vogler et al., 2016; Ye & Ghassemi, 2018, 2019). The contributions of sealed fractures (e.g. filled fractures, foliations, and veins) to permeability creation during hydraulic stimulation have not been examined. Specifically, in Collab stimulation experiments it is observed that the hydraulic fractures tend to have complexity, involving multiple failure planes. In this paper, from the perspective of reservoir geomechanics, we used the term of sealed fractures to represent the natural discontinuities that are closed or partially/completely sealed by minerals. In general, the sealed fractures are likely more conductive and weaker than the rock matrix, and can supposedly be reactivated by injection to provide high conductive flow paths and potentially form a network by coalescence with hydraulic fractures.

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