Evolution of pore-shape and its impact on pore conductivity during CO₂ injection in calcite: Single pore simulations and microfluidic experiments

Injection of CO₂ into carbonate rocks causes dissolution and alters rock transport properties. The extent of the permeability increases, due to the increased pore volume and connectivity, strongly depends on the regimes of transport and dissolution reactions. Identification of these regimes and their parametrization at the microscopic scale is required for an understanding of the injection processes, and, afterward, for calculating the effective macroscopic parameters for field-scale simulations. Currently, a commonly used approach for calculating the rock effective parameters is the Pore Network Method, PNM, but a better understanding of the validity of its basic assumptions and their areas of applicability is essential. Here, we performed a combined microscopic experimental and numerical study to explore pore-shape evolution over a wide range of transport and dissolution reaction regimes. Experiments were conducted by flowing an acidic solution through a microscopic capillary channel in a calcite crystal at two different flow rates. The experimental results were used to validate our pore-scale reactive transport model that could reproduce the measured effluent composition as well as pore shape changes. Two key stages in pore shape evolution were observed, a transient phase and a quasi-steady-state phase. During the first stage, the shape of the single pore evolved very fast, depending on the flow regime. Under advective-dominant flow, the pore shape remained nearly cylindrical, while under diffusive-dominant transport, the pore shape developed into a half-hyperboloid shape. During the quasi-steady-state stage, the pore volume continued to increase, however, without or with diminutive change of the pore shape. In this stage, only a long period of injection may result in a significant deviation of the pore shape from its original cylinder shape, which is a common assumption in PNMs. Furthermore, we quantitatively evaluated the impact of evolved pore shape spectrum on the conductance calculations and compared it to the formulations currently used for pore network modeling of reactive transport. Under low flow rates, neglecting the developed non-uniform pore shape during the non-steady stage may lead to an overestimation of pore conductance up to 80%.