Article outline

Displacement of oil by less viscous media leads to instability of the front and the formation of “viscous fingers”, the structure of which depends on various physical parameters. The most informative dynamics of the change in the interface between the injected fluid and the displaced medium with a change in physical parameters can be studied using the Hele-Show cell. The use of a cell with a small gap allows one to track the effect of changes in surface tension on the formation of “viscous fingers”. Carrying out experiments at different temperatures makes it possible to effectively change the ratio of the dynamic viscosities of the media under study. By adjusting the injection of fluids with a constant volumetric flow rate, it is possible to study the dynamics of the front development under various displacement modes.

At a cylindrical wriggling interface between liquids, the capillary pressure is determined by two main curvatures of the interface. One corresponds to the curvatures of the bending front line; the other, with full wetting, is half the size of the gap (10 μm), it is several orders of magnitude higher than the curvature of the front. The growth of the main “viscous finger” predominates in the direction of the greatest pressure gradient. The capillary forces of the convex part of the front exert a braking effect, the concave one - accelerating, but their stabilizing effect is much less than the hydrodynamic forces leading to the development of “fingers”. The capillary forces caused by the interaction with the surface with a huge curvature are very significant; they play the main role in the formation of residual pillars of the displaced liquid.

In the process of unstable displacement of oil by water, 2 stages can be distinguished - the flow before the breakthrough and after the breakthrough. The resulting structures of “viscous fingers” before the breakthrough form fractal structures, which are initial for the structures formed in the process of subsequent displacement. For practical use, in particular for evaluating the displacement efficiency, the processes influencing the expansion of the displacement zone that occur after the breakthrough are important.

Changing the temperature and concentration of surfactant solutions makes it possible to change the viscosity ratio between the displaced and displaced liquid and the surface tension at the interface. The Hele-Shaw cell is a flat channel between two optical plates, the volume of the cell corresponds to the volume of the “pore”. The liquid flow rate was set with a syringe pump and was additionally controlled by the gravimetric method, the pressure was measured with a manometer and a pressure transducer (MPX 5100). To prevent blurring of the picture, the shooting was carried out with the minimum exposure of the video camera. The computer was used to capture video frames of the flow image, and also synchronized and recorded measurements of the pressure sensor and precision balance. The resulting video recordings were processed using the ImageJ program. The high quality of the images made it possible to obtain high-quality binary pictures and to calculate the residual saturation of the displaced phase. Since the sequential structures of “viscous fingers“ are self-similar, their fractal analysis was carried out. The fractal dimension of the displacing phase was calculated in the HarFa 5.5L program using the box-counting method.

The paper presents the results of an experimental study of the development of “viscous fingers” during unstable displacement of oil by water and an aqueous solution of surfactants from the Hele-Show cell. Physical parameters were changed: temperature, dynamic viscosity, surface tension and flow regimes influencing the formation of “viscous fingers”. It was found that an increase in the displacement fluid flow rate leads to an increase in the area of coverage and a decrease in the width of the “iscous fingers”. After the breakthrough, a drop in the share of oil displacement is observed.

Fractal analysis of the dimensions of “viscous fingers” made it possible to establish a correlation between fractal dimension and displacement efficiency; an increase in fractal dimension corresponds to an increase in oil recovery. Thus, the change in physical parameters during unstable displacement makes it possible to find the determining factors for increasing oil recovery at various stages of displacement.

With unstable oil displacement in the Hele-Shaw cell, a change in physical parameters leads to significant changes in the front structure, a large difference in the nature of displacement before the breakthrough and after - during additional washing. The efficiency of oil recovery at all stages of displacement varies ambiguously from the physical parameters.

By the time a breakthrough occurs, the strongest changes in displacement efficiency lead to a decrease in the viscosity ratio at a low volumetric flow rate of displacing water.

In the stage of additional washing, the most effective is the use of a high flow rate along with a decrease in the ratio of viscosities and surface tension.

Thus, the change in physical parameters during unstable displacement makes it possible to find the determining factors for increasing oil recovery at various stages of displacement.