Microscopic experimental study on the sweep and displacement efficiencies in heterogeneous heavy oil reservoirs

Abstract

Oil recovery is closely related to the sweep and displacement efficiencies in oil reservoirs. Therefore, several measures have been devised to enhance oil recovery based on these two factors. In heterogeneous heavy oil reservoirs, expansion of the swept volume of medium- and low-permeability layers is a suitable approach for enhancing oil recovery. However, the oil displacement efficiency is also an important factor. With regard to enhancing oil recovery, the selection of displacement agents will depend on whether the sweep efficiency or displacement efficiency is to be adjusted and on how these factors can be controlled to maximise the increase in oil recovery. In this study, the pixel method is used to distinguish between the sweep efficiency and displacement efficiency, and the joint effect of these factors is analysed. A heterogeneous microscopic model is used to conduct oil displacement experiments using polymer solutions with different viscosities and surfactant–polymer systems with similar viscosities and different interfacial tensions. The results indicate that under heterogeneous conditions, increasing the viscosity of the polymer solution leads to a slight expansion in the swept volume and an increase in the oil displacement efficiency in the low-permeability area; however, the overall sweep efficiency remains almost unchanged. There is a critical point for sweep efficiency: under the experimental conditions, when the oil viscosity is 305 mPa s, the critical viscosity of the polymer solution is 125 mPa s, and when the oil viscosity is 1050 mPa s, the critical value is 205 mPa s. When the viscosity of polymer solution reaches the critical value, for the surfactant–polymer system, a reduction in the oil–water interfacial tension has the same effect as a substantial increase in the polymer solution viscosity; moreover, the joint effect of the sweep efficiency and displacement efficiency can be enhanced. Therefore, for heterogeneous heavy oil reservoirs, the focus should be on improving oil displacement efficiency under certain sweep conditions rather than on expanding the swept volume. The findings of this study can help for better understanding of the selection of displacements agent in heterogeneous heavy oil reservoirs for enhanced oil recovery.