Changes in porosity, permeability, water retention curve and reactive surface area during carbonate rock dissolution

Abstract

Chemical reactions between fluids and the solid phase change porous media by dissolution and precipitation processes that modify the pore space geometry and associated hydraulic and chemical properties (saturated and relative permeability, porosity, capillary pressure curve, pore size distribution, reactive surface area). Knowledge of these properties and how they evolve with time is important for understanding and modelling multi-phase flow or reactive transport. Most published studies for reservoir rocks concentrate on the changes of porosity and saturated liquid permeability, but few data are available on the other parameters, and even less on their evolution during interaction with acidic solutions.We investigate the joint evolution of transport parameters of sedimentary carbonate rocks upon reaction with dilute acid (HCl). We first characterize the initial permeability, porosity and pore size distribution. We then attack the rock samples by permeation with HCl (pH3.5 or 4.1) and characterize them again after partial dissolution. Several dissolution-characterization cycles were performed on each sample in order to study the evolution and interaction of the different parameters.In two coarse calcite cores C1 (dissolved at pH3.5) and C2 (pH4.1) with high initial permeability, significant dissolution occurred only in pores with diameters greater than 0.022mm. Permeability increased very little even though porosities increased by 2.6–5.8%. In fine-grained dolomite cores, dissolution affected a broader range of pore sizes, down to 0.001mm in D1 (pH3.5), and down to 0.0066mm in D2 (pH4.1). A wormhole broke through in D1 after percolation of only 5000 pore volumes of acid. Effective reactive surface areas decreased by a factor of 240–250 in the cores dissolved at pH3.5, but only by a factor of 2–25 in the cores dissolved at pH4.1. A simple 1-D model was unable to simulate the evolution of the reactive surface during wormhole formation. Effective reactive surface area correlated negatively with permeability, and was consistent with the geometric area of the pores that carried most flux, which was calculated from the retention curves.