Micro-CT imaging of multiphase flow at steady state
Creators: name: Gao, Ying
Publisher: name: Imperial College London
Year: 2019

description: Pore-scale imaging provides an effective way to understand multiphase flow in porous media at the pore scale and provides benchmark data for larger-scale modelling. In this thesis, an X-ray tomography-based experimental and image analysis method was devised for investigating the relative permeability and associated pore occupancy during steady-state two-phase flow in a sandstone and a complex carbonate, and to identify dynamic flow regimes from low to high capillary numbers. First, a two-phase flow experiment at different fractional flows at steady state was conducted on a Bentheimer sandstone sample at mm-scale using a laboratory micro-CT scanner at both low and high capillary numbers. An intermittent flow, defined as the regions occupied by oil and water alternately, was observed at high capillary number during the hour-long scan time. Differential imaging method was used to quantify the fraction of the intermittency in pore space. Imbibition relative permeabilities and fractional flow curves were obtained with precise differential pressure measurements which compared well with literature measurements on larger cm-scale cores. This method was then expanded and applied into a micro-porous Estaillades carbonate using X-ray micro-tomography. Differential imaging method was applied to (i) distinguish micro-pores and quantify micro-porosity; (ii) determine fluid pore occupancy in both microporous regions; and (iii) identify the intermittency in macro-pore space. The brine saturation and relative permeabilities were obtained, which were impacted by the presence of water-wet micro-porosity which provides additional connectivity to the phases. Pore and throat occupancy of oil, brine and intermittency were obtained from images at different fractional flows using the generalized pore network extracted from the image of macro-pores. The intermittent flow was observed at high capillary number and was predominantly located in the small and intermediate size pores and throats. As laboratory-based instruments take around 1 hour to acquire an image, meaning that intermittency can only be observed indirectly, fast synchrotron X-ray micro-CT was used to capture changes in occupancy over time scales of approximate 1 minute. Thus, fluid behaviours as a function of flow rate (capillary number) at a fixed fractional flow of 0.5 were studied during steady state multiphase flow through a Bentheimer sandstone using synchrotron X-ray micro-tomography. Combining the fluid rearrangements observed from the continual scans taken every 60s with the pressure differential provided by the sensitive differential measurements, three flow regimes were classified: capillary-dominated flow, onset of dynamics, and intermittent flow. The pressure differential has a linear relationship with flow rate during the first two flow regimes. However, there is a power-law scaling of flow rate with pressure drop across the system during the intermittent flow regime. We observed that when there is sufficient energy to allow the oil to short circuit some pores and throats at high capillary number, the overall flow resistance decreases, meaning that oil moves along pathways that intermittently disconnect and reconnect.