Effect of pH on Spontaneous Imbibition in Calcareous Rocks

Reactive transport in porous media exhibits multifaceted interactions that are dependent on the matrix and fluid properties, and which ultimately alter these properties. A set of calcareous rock samples with unique mineralogy and varying petrophysical properties are selected for this study. A capillary rise experiment is performed in each sample, first with deionized water and then with a dilute, pH 2, HCl solution. Pre- and post-acid petrophysical properties such as porosity, permeability, pore size distribution, and contact angle are measured for each sample along with the capillary rise profile. The latter is tracked by applying image analysis on video recording. The rock mineralogy significantly affects the acidic fluid intrusion into the rock samples. Calcite dissolution is the main reaction that results in the opening of the pore space. This is more prominent in all the carbonate samples where a higher proportion of calcite minerals is present. A higher capillary rise is consistently observed compared to the neutral fluid along with an increase in porosity and the mean pore size. The contact angle also undergoes changes making the carbonate matrix from oil-wet to neutral-wet. Coupling capillary interactions with fluid reactivity is often neglected in fluid transport phenomena. This study offers new insights into the relative importance of reactivity at the timescale of spontaneous imbibition. This is important in understanding dissolution and precipitation processes during capillary flow. In the field of geochemistry, the coupled processes of reactivity and capillary interaction are influenced by the properties of rocks and fluids. However, during capillary rise with reactive fluid, the rocks can initiate a cycle wherein reactivity continuously changes the properties of the rocks thereby affecting the fluid height which results in a different in comparison to unreactive fluids. To comprehend the underlying mechanisms of this phenomenon, a series of capillary rise experiments were conducted in three different carbonate rocks: first with DI water and then with a dilute acidic solution. Rock and fluid properties were measured before and after the reactive capillary rise. A higher fluid height was observed in acidic fluids, indicating that these fluids altered the pore structure and surface chemistry of the rocks. The reactivity promoted both pore enlargement and reduction due to calcite dissolution and precipitation. These processes also altered the surface chemistry of calcites, making them more water wet. Despite the small length scale and short duration of this study, the results show the importance of reactivity during capillary phenomenon. This understanding is crucial for grasping how reactive contaminants can move through the vadose zone. Acidic imbibition (pH 2) in calcareous rocks results in higher capillary rise compared to neutral fluid Pore enlargement and reduction were observed due to calcite dissolution and precipitation during acid-based capillary rise Calcite dissolution is the main reaction observed during the acidic imbibition and result in a more water-wet matrix