A New Multiscale Computational Model For Coupled Flow And Geomechanics In Karst-Carbonates Containing Solution-Collapse

Abstract

Summary We developed a new three-scale (micro/meso/macro) computational model based on a reiterated homogenization procedure to describe coupled flow and geomechanics in carbonate rocks containing complex geological structures at multiple length scales, such as fractures and mainly solution-collapse breccias. Within this framework we construct a Hierarchical Karst-Fracture Model wherein larger geological objects are incorporated in the mesoscopic model explicitly, whereas the high density smaller structures, which prevail microcopically, are homogenized and replaced by equivalent continua with hydromechanical properties computed using a self-consistent homogenization procedure. Using the terminology adopted in the cave-collapse literature, in the upscaling method we subdivide the different clastic-arrangements in the breccia into crackle, mosaic, totally open and chaotic substructures, where equivalent properties, such as permeability and elastic constants, are assigned to each layer of the breccia. After establishing the mesoscopic coefficients we upscale the problem to the macroscale, characterized by a length of order of hundreds of meters, associated with a representative cell of a coarse grid of a reservoir simulator. In this latter procedure we apply the Discrete Fracture Model (DFM) combined with robust computationally schemes with ability to handle strong heterogeneity induced by the collapse-breccia. In this setting equivalent properties are computed through a straightforward averaging process. Numerical results, hinging on outcrop-based flow simulations, are presented with the input small-scale features extracted from drone images. In particular our simulations aim at analyzing the influence of different scenarios on the magnitude of the macroscopic properties which are subsequently explored as pre-processors in reservoir simulators.