Pore-Scale and Conventional Wettability Measurement Considerations for Improving Certainty of Geological CO2 Sequestration

Abstract

Parallel to the approach of developing zero-carbon-emission energies, other solutions have been recently proposed to decrease the amount of CO2 emissions into the atmosphere. Geological CO2 sequestration (GCS) has provided economic benefits and slight adverse environmental effects. GCS involves capturing CO2 from large producers, then injecting it into deep layers of the earth’s subsurface to be stored for hundreds to thousands of years. A safe and economic GCS requires a profound knowledge of immiscible CO2-water/brine fluid flow in CO2 storage sites including capillary pressure which has a barrier effect against leakage. The main uncertainty in measuring capillary pressure is due to the wettability, which is quantified by contact angle of water/brine interface on rock surface. The objective of this study is to explore the reasons of uncertainty observed in conventional contact angle measurement and introduce a more realistic pore level contact angle measurement. The contact angle of water/brine on select minerals found in common rocks (silica and mica) was measured using a high-pressure, high-temperature chamber developed for a captive bubble test method. As an innovative method, pore-scale static and dynamic contact angles were also measured inside a high-pressure micromodel using a microscope. The results showed that the heterogeneity on minerals surface plays an important role in controlling contact angle variation with time. With unsaturated fluid (water/brine-CO2) condition, which is more realistic in the short-term after CO2 injection, the contact angle can increase due to a pinned triple line (the line on which the three phases of the liquid, gaseous, and solid surface meet) as a result of heterogeneity. An increased contact angle causes the capillary pressure to decrease resulting in a higher leakage risk. The micro-scale dynamic contact angle results showed that rocks were not as water-wet as assumed in literature when conventional measurement methods on flat surfaces of minerals were used. An increase of pressure and salinity changed the glass (silica) behavior from water-wet to intermediate-wet. Pore-scale contact angle measurement provides more realistic wettability behavior of geo-materials and increases the certainty the simulations used for assessing safety and efficiency of storage sites.