Pore network modelling of fingering phenomena during unsteady-state waterflooding of heavy oils
Abstract
Although thermal methods have been popular and successfully applied in heavy oil
recovery, they are often found to be uneconomic or impractical. Therefore, alternative
production protocols are being actively pursued and interesting options are water and
polymer flooding.
Such techniques have been successfully tested in recent laboratory investigations, where
oil recovery was found to be much higher than expected. Moreover, in some of the core
scale waterflood experiments reported using 2D slabs of Bentheimer sandstone, X-ray
scans performed during the flooding sequence provided evidence of an interesting new
phenomenon – post breakthrough, highly dendritic water fingers were seen to thicken
and coalesce, forming braided water channels and improving sweep efficiency.
However, despite encouraging results, these experimental studies show that the
mechanisms governing water displacing extra heavy oil are still poorly understood. This
means that the optimization of this process for eventual field applications is still
somewhat problematic. Ideally, a combination of two-phase flow experiments and
simulations should be put in place to help inform our understanding of the process.
To this end, a new fully dynamic network model is described. It has been developed to
investigate unsteady state drainage floods and has been applied here in the context of
waterfloods of heavy oil in oil-wet media. It has subsequently been used to investigate
finger thickening during water flooding of extra-heavy oils. The displacement physics
has been implemented at the pore scale and, following a successful benchmarking
exercise against numerous micromodel experiments, a range of slab-scale (30cm x
30cm) simulations has been carried out and compared with the corresponding
experimental observations. They reveal that the model is able to replicate finger
architectures similar to those observed in the experiments. Subsequently, for the first
time to our knowledge, finger thickening following water breakthrough is reproduced
and interpreted. The simulator is then used to investigate the effects of different system
parameters on finger swelling behaviour. Finally, a sensitivity study is performed to
better understand the effects of different system variables upon the sweep efficiency, the
displacement front stability and unsteady-state relative permeability.