The presence of naturally developed collapse breccia pipes poses a risk for subsurface development of natural resources in underground excavations (wellbores, shafts and mines). These features introduce heterogeneities affecting the fluid flow and mechanical behavior of the integrated host rock and breccia pipes system. The interplay of collapse shapes and stresses, geomechanical properties and pore pressure fluctuations dictates the mechanical integrity of such systems. Injection could, through breccia pipes, breach and communicate with upper layers. This research work contemplates geomechanical simulations of an ore deposit layer-cake formation in western Canada, where breccia pipes extend from the ground level to depths beneath the injecting reservoir. The goal is to understand injection conditions that prevent water inrush of underground infrastructures and contamination of aquifers due to fracturing and loss of confinement. Pre-stack time-migrated 3D seismic data helped to characterize shapes and sizes of the collapse breccia-pipes. Results indicate that it is not strictly necessary to include lateral variations of breccia-pipes permeability as long as a calibrated average permeability for the entire system is applicable. Conversely, the Young's modulus (E) contrast (i.e., E ratio of breccia-pipe to host rock) introduces the highest uncertainty in the estimation of in-situ stress magnitudes. Low values of E ratio induces a stress barrier surrounding the collapse breccia pipes. The analysis shows that under the assumption of a closed boundary scenario, tensile fracturing occurs inside the injection layers, but none of the intersected collapse breccia pipes becomes fractured. Pore pressure transmission vertically inside a collapse breccia pipe leads to their incipient tensile fracturing at shallower depths but with relative low propagation. The workflow helps to better understand the conditions for potential risks associated with collapse breccia pipes and water inrush for underground infrastructures.
The presence of naturally developed collapse breccia pipes, also known as inclusions, chimneys, collapse structures, karsts or paleokarst features, are widely distributed around the world. They are of significant interest as they pose some risks for subsurface development of natural resources in underground excavations (wellbores, shafts and mines). These features introduce heterogeneities that affect the fluid flow and mechanical behavior of the integrated system, that is, host rock (reservoir and surroundings burdens) and inclusions (breccia pipes). The interplay of collapse shapes, geomechanical properties, stresses around collapses and pore pressure fluctuations dictates the mechanical integrity of such systems. Brine injection could, through breccia pipes, breach and communicate with upper layers affecting underground infrastructures and petroleum reservoirs. The subject of coupled interaction between geomechanics and fluid flow of collapse breccia pipes structures under injection has not been widely explored in the literature.
