Effect of Cumulative Surface on Pore Development in Chalk

Abstract

Pore development in natural porous media, as a result of mineral dissolution in flowing fluid, generates complex microstructures. Although the underlying dynamics of fluid flow and the kinetics of the dissolution reactions have been carefully analyzed in many scenarios, it remains interesting to ask if the preferentially developed flow paths share certain general petrophysical properties. Here we combine in situ X-ray imaging with network modeling to study pore development in chalk driven by acidic fluid flow under ambient condition. We show that the trajectory of a growing pore correlates with the flow path that minimizes cumulative surface—the overall surface area available to fluid within the residence time—calculated along streamlines. This correlation is not a coincidence because cumulative surface determines conversion of reactant and thus defines the position of dissolution front. Model simulations show that, as fluid channelizes, the growth of the leading pore in the flow direction is guided by migration of the most far-reaching dissolution front, even in an ever-changing flow field. In addition, we present a complete tomographic time series of microstructure erosion and show a good accord between the in situ observation and the model simulation. Our results suggest that the microscopic pore development is a deterministic process while being sensitive to stochastic perturbations to the migrating dissolution front.