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Assessment of Geomechanical Impacts on Breccia Pipes in Underground Potash Mining With Brine Injection Operations

Abstract

The presence of natural developed breccia pipes, aka collapse structures or karsts, poses a potential risk for subsurface development of natural resources such as potash mines and petroleum reservoirs. They introduce heterogeneities affecting the mechanical and fluid flow behaviour of the interacting systems i.e.; host rock (reservoir and surroundings burdens) vs inclusions (breccia pipes). Pressure fluctuations, geomechanical properties, shapes and stresses around collapses, dictate conditions whether the two components are broken or not. Brine injection into saline aquifer-reservoirs could breach or communicate with upper layers trough breccia pipes impacting underground excavations (shafts and mines).Modeling of a Potash ore layer-cake formations breached by collapse structures are presented. No-flow boundary conditions were simulated under both constrains of bottom-hole pressure and reservoir rate control. Fluid-flow is coupled to calibrated geomechanical FEM’s, shapes and sizes of collapses were extracted from pre-stack time-migrated 3D seismic data. Results indicate that the “most reasonable” scenario is for a bottom hole injection pressure threshold of 22 MPa for vertical wells. Injected brine flows through the collapses dissipating substantially any pressure build-up, then flows further into the reservoir, corroborating the observed pressure maintenance in the field. Results from streamlines suggest that it is possible to have less interaction with breccia pipes. Neither the reservoir nor the breccia pipes were re-pressurized when uncoupled fluid-flow simulations were used. However, when considering changes of stresses other impacts become evident around the collapse structures.Regarding fluid-flow modeling, a reliable average permeability can be estimated for the entire system. Conversely, Young’s modulus (E) of breccia pipes, introduces high uncertainty for the in-situ stress magnitudes. For instance, some sort of stress barrier (i.e. stress increase) is observed surrounding the breccia features when low values of E are used. Inclusion of thermo-poroelastoplastic stresses show that the onset of tensile fracturing, within the injection layers, starts after 5 years of injection, but none of the intersected breccia pipes became fractured at the injection depths. Nonetheless, a vertical pressurization is transmitted inside the breccia pipes at shallower depths of up to ~1100 m TVD initiating incipient tensile fracturing in some segments. This poses a risk of water inrush in case any excavation reaches that depth at those specific segments. The results of this study provides practical considerations and observations that contribute to a better understanding of potential risks associated to water inrush and impact of breccia pipes on seal integrity for underground potash mining in southern Saskatchewan, Canada.