Water-CO2-mineral systems: Interfacial tension, contact angle, and diffusion-Implications to CO2 geological storage

The interfacial interaction between mineral surfaces and immiscible fluids determines the efficiency of enhanced oil or gas recovery operations as well as our ability to inject and store CO2 in geological formations. Previous studies have shown that the interfacial tension and contact angle in CO2-water-mineral systems change noticeably with fluid pressure. We compile previous results and extend the scope of available data to include saline water, different substrates (quartz, calcite, oil-wet quartz, and polytetrafluoroethylene (PTFE)), and a wide pressure range (up to 20 MPa at 298K). Data analysis provides interfacial tension and contact angle as a function of fluid pressure; in addition, we recover the diffusion coefficient of water in liquid CO2 from long-term observations. Results show that CO2-water interfacial tension decreases significantly as pressure increases in agreement with previous studies. Contact angle varies with CO2 pressure in all experiments in response to changes in CO2-water interfacial tension: it increases on nonwetting surfaces such as PTFE and oil-wet quartz and slightly decreases in water-wet quartz and calcite surfaces. Water solubility and its high diffusivity (D = 2 x 10(-8) to 2 x 10(-7) m(2)/s) in liquid CO2 govern the evolution of interparticle pendular water. CO2-derived ionic species interaction with the substrate leads to surface modification if reactions are favorable, e. g., calcite dissolution by carbonic acid and precipitation as water diffuses and migrates into the bulk CO2. Pressure-dependent interfacial tension and contact angle affect injection patterns and breakthrough mechanisms, in other words, the performance of geological formations that act as either reservoirs or seals.