Welcome to the next PoreLab lecture!
Who: Dr. Johan Olav Helland, a senior researcher in the Computational Geosciences and Modelling group at NORCE – Norwegian Research Centre, Norway
Johan Olav Helland holds a PhD in petroleum technology from University of Stavanger (2005) and an MSc in applied mathematics from University of Bergen (2001). His research interests are related to modeling and simulation of transport and multiphase flow for recovery and storage in porous media. He has developed numerical methods and pore-scale models for three-phase flow that can simulate fluid configuration, capillary pressure, trapping, hysteresis, and complex interface phenomena, directly on segmented images of the pore structure.
When: Wednesday 23 October at 13:00 (Norway time)
Where: The lecture will be streamed in the Kelvin room (PoreLab Oslo) and the common room (PoreLab Trondheim). From anywhere else, you will be able to join via the following Zoom link:
https://uio.zoom.us/j/65837085049?pwd=WjZianUyN3FJa2liQkxBbzQrOCtGdz09
Title: Ostwald ripening of trapped gas in the presence of oil and water in subsurface storage – A level set pore-scale modelling approach
Abstract:
Subsurface storage technologies (e.g., natural gas, hydrogen, or carbon dioxide storage) require estimation of capillary trapped gas and its permanency in porous rock. Ostwald ripening is a mass transfer process that could redistribute and potentially change the amount of trapped gas over time. In this process, a gas concentration gradient will develop in the liquid and drive mass transfer by diffusion from bubbles with higher pressure to bubbles with lower pressure. Micro-CT images of multiphase fluid configurations in porous rock have revealed a large pressure range of trapped ganglia, which facilitate mass transfer by Ostwald ripening. Here, we will present a numerical model for Ostwald ripening that minimizes the Helmholtz free energy by calculating in alternate steps; (1) the mass transfer between the gas bubbles based on chemical potential differences, and (2) the corresponding stationary multiphase fluid configurations using a level set method for capillary-controlled displacement. The level set model accounts for local volume conservation, which allows for describing the evolution of the bubble pressures – this is a prerequisite for investigating ripening. The model is implemented within the SAMRAI framework (developed at Lawrence Livermore National Lab), enabling parallel simulations and adaptive mesh refinement using patch-based data structures.
We first investigate the capability of the level set model to simulate two- and three-phase capillary trapping on a segmented pore-space image of a sand pack, for which trapping measurements exist in the literature on the same core material. Then we perform two-phase simulations with the combined level-set mass-transfer model to demonstrate how Ostwald ripening depends on the pore geometry, the initial bubble configuration, the gas/liquid system, reservoir pressure, wettability, as well as whether the liquid phase surrounding the trapped gas bubbles is connected or disconnected. We proceed by presenting three-phase simulations of ripening of trapped gas in the presence of oil and water on 2D and 3D fluid configurations that we either constructed numerically or obtained by simulating the whole displacement history. For this purpose, we use a nitrogen/decane/water system and a CO2/decane/water system at typical immiscible and near-miscible reservoir conditions. The simulations show that the interfacial tensions and contact angles between gas and the liquids (oil and water), as well as the oil/water capillary pressure, have significant impact on the mass transfer behaviour during ripening and the resulting equilibrium three-phase configurations. Due to lower bubble capillary pressures, ripening of CO2 ganglia is slower for near-miscible conditions than for immiscible conditions, even though the coefficients in the mass transfer equation increase with reservoir pressure. Simulations of ripening on three-phase residual configurations in sandstone that contain trapped oil and CO2 leads to growth and ramification of large CO2 ganglia at the expense of dissolution of smaller bubbles, and redistribution of trapped oil. Overall, Ostwald ripening is a slow process on its route to equilibrium, but since the ripening rate typically diminishes over time, significant mass transfers can occur early in the process that potentially could mobilize trapped gas.