From connected pathway flow to ganglion dynamics : understanding the effect of pore-scale properties on dynamic fluid connectivity and average flow functions

Abstract

Since the turn of the industrial revolution in the early 1900s, the global economy has relied on fossil fuels for energy, transport, and other day to day industrial, commercial, and domestic activities. The combustion of fossil fuels (coal, petroleum (oil) and natural gas) is the primary cause of atmospheric carbon dioxide (CO2) emissions which result in climate change and global warming. Until we fully transition to cleaner alternative energy sources, the global economy will continue to rely on fossil fuels. The injection and storage of CO2 in subsurface geological formations such as saline aquifers and depleted oil and gas reservoirs, has been identified as a promising solution for mitigating climate change and global warming. Changes in reservoir rock and/or fluid properties at the pore-scale (scale of several microns) have been known to have an impact on flow and transport properties at the Darcy-scale (scale of several centimetres to metres). As such, successful implementation of CO2 storage technology at the large scale, relies heavily on our ability to understand and predict changes that occur in the subsurface at the pore scale and their subsequent effect on average flow functions. One of the major, unresolved challenges in upscaling multiphase flow from the pore scale to the Darcy scale lies in addressing the effects of connected and disconnected fluid fractions. Direct numerical simulations (DNS) were coupled with flow through experiments in miniature replicas of porous rocks fabricated on glass substrates (micromodels) to investigate the effects of pore-scale flow and transport properties on dynamic fluid connectivity and average flow functions such as displacement efficiency and the saturation function. Flow and transport properties investigated include surface roughness, wettability, as well as fluid velocity. Three pore-scale flow regimes were identified from the investigations conducted: two disconnected pore scale flow regimes namely, the ganglion dynamics (GD) regime and the droplet traffic flow (DTF) regime and a regime in which fluid displacement occurred by connected flow paths (the connected pathway flow (CPF) regime). It was established that there is a relationship between the dominant pore-scale mechanism and the kinetics of fluid displacement processes. Disconnected flow regimes were found to accelerate the fluid displacement process. The impact of disconnected and connected flow regimes was studied and it was determined that the GD regime can have a negative impact on the efficiency of subsurface fluid displacement processes and would adversely impact CO2 storage operations. In contrast, the DTF regime was found to enhance fluid displacement efficiency. Transitions between connected and disconnected flow regimes were also investigated and it was found that the shape of the saturation function is strongly influenced by transitions between pore-scale flow regimes. This work shows that the impact of pore-scale dynamic fluid connectivity on flow transport kinetics and the saturation function is highly significant and should not be ignored. Pore-scale property induced changes in the rate of change of saturation and the shape of the saturation function and could potentially have a knock-on effect on saturation dependent Darcy-scale functions such as relative permeability-saturation curves. Further work should be done to ascertain the relationship between dynamic fluid connectivity and relative permeability-saturation curves.