Capillary Dynamics of Nanoconfined Water and CO2 Induced by Time-Dependent Wettability Alteration

Abstract Formation wettability alteration (WA) is ubiquitous in nature and relevant to many applications spanning from geological CO2 storage to enhanced oil/gas recovery. WA and the induced capillary dynamics are of great significance in caprocks and shale reservoirs. However, the impact of nanoconfinement effect has rarely been studied. In this work, we developed a new dynamic capillary pressure model for nanoconfined water and CO2 considering time-dependent WA. A sorption-based WA model was adopted to describe the variation of wettability over time. Dynamic capillary pressure model was then established by coupling the time-dependent WA model and numerical modelling of H2O-CO2 drainage and imbibition in nanoscale bundle-of-tubes. To adapt to the nanoconfined environment, slip and entrance/exit loss modified Lucas-Washburn flow model was developed by molecular dynamics (MD) simulations and interfacial tension (IFT) was predicted using nanoscale-extended Cubic-Plus-Association equation of state and the density gradient theory. Cycles of H2O-CO2 drainage and imbibition were implemented in the nanoscale bundle-of-tubes, until the wettability of all the tubes reached its final state. Based on the results, a quantitative dynamic capillary pressure model was established with a term characterizing the speed and extent of change. The impact of nanoconfinement was investigated, where slip and entrance/exit loss effect accelerated the capillary pressure alteration, while IFT shift due to fluid-wall interaction slowed it down. The influence of pore structure was also examined. The capillary pressure of pores with triangular cross-sections changed fastest, followed by those with square cross- sections, and circular cross-sections; tortuosity reduced the speed of capillary pressure change. This new nanoscale-extended dynamic capillary pressure model accounts for the rock-fluid interaction and long-term WA induced by WA agent adsorption, which are usually ignored by conventional capillary pressure models.