Wettability of Shales at Geological Carbon Dioxide Storage Conditions: A Comparative Study of Results Obtained from Sessile and Captive Bubble methods

Summary Global warming and climate change have become pressing concerns, primarily due to the excessive release of CO2 into the atmosphere. One proposed mitigation strategy involves injecting and storing CO2 in subsurface reservoirs. However, caprock integrity remains a significant concern, as leakage paths even through tiny pores may compromise tightness and lead to escaping gas to the environment. Capillary pressure, which is governed by the wetting situation within the caprock, can either contribute positively or negatively to the pore flow and consequently to storage safety. Poor water-wetting can reduce capillary pressure, and in cases where wettability shifts from water-wet to gas-wet conditions, the capillary pressure adds up to the gas pressure, thereby increasing the risk of leakage. To address this concern, it is essential to evaluate the system's wettability under various conditions, which can be achieved through contact angle measurements. This study aimed to investigate the influence of key factors on contact angle using the captive bubble and sessile drop methods, which represent two different scenarios; gas displacing water in water-saturated rocks and water displacing gas in locations that are either originally dry or susceptible to drying out by CO2, respectively. Six primary factors were examined: Bond number, pressure, rock mineralogy, total organic carbon (TOC), water salinity and surface roughness. Results indicated minimal dependence of contact angle on all investigated factors when the captive bubble method is applied, with rocks consistently exhibiting apparent water-wetness under all conditions. This observation can only be explained by a thin water film separating the CO2 phase and the rock surface. The thickness and stability of this film depends on the interactions between CO2 and the rock minerals as well as on the surface roughness. In contrast, the sessile drop method representing the real three phase contact, demonstrated a strong pressure- dependent behavior, increasing from 76° at 50 bar to 119° at 150 bar for the Sultani shale. Similarly, the Longmaxi formation exhibited an increase from 63° to 88° at similar conditions. These results indicate a reduction in water wettability and a progressive shift toward CO2-wet conditions. Furthermore, the role of exposure to carbonated water in altering the shale surface is investigated. Chemical reactions, particularly those involving the dissolution of carbonates could be demonstrated by surface imaging techniques, in addition to water analysis.