Multiphase Flow in Heterogeneous Porous Media

Abstract

Immiscible fluid displacement in porous media plays a central role in diverse energy, environmental, and industrial processes. The two types of immiscible fluid displacement are drainage, the process in which a non-wetting fluid displaces a wetting fluid, and imbibition, the process in which a wetting fluid displaces a non-wetting fluid. While these processes are well studied in homogeneous porous media, in many cases, the medium is structurally heterogeneous. In this thesis, I investigate how two forms of structural heterogeneity -- pore size gradients and stratification -- affect drainage and imbibition.

In the first part of this thesis, we investigate the influence of pore size gradients on the fluid pathways that are taken during drainage. Using microfluidic experiments and computational pore-network models, we show that the non-wetting fluid displacement behavior depends on the competition between a pore size gradient and pore-scale disorder. By analyzing capillary forces at the pore scale, we identify a non-dimensional parameter that describes the physics underlying these diverse flow behaviors.

In the second part of the thesis, we investigate the influence of stratification on the physics of imbibition. We directly visualize forced imbibition in three-dimensional (3D) porous media with two parallel strata. We find that imbibition is spatially heterogeneous: for small capillary number Ca, the wetting fluid preferentially invades the fine stratum, while for Ca above a threshold value, the fluid instead preferentially invades the coarse stratum. We also develop an analytical model combined with a pore network model that explicitly describes fluid crossflow between the strata. By numerically solving these models, we examine the fluid dynamics and residual fluid saturation left after breakthrough. We find that the residual saturation of nonwetting fluid is minimized when the imposed capillary number Ca is tuned to a threshold value that depends on both the medium structure and the viscosity ratio between the two fluids. Lastly, we explore how stratification impacts the process by which trapped ganglia of nonwetting fluid are mobilized from the porous medium.

Taken together, the results presented in this thesis expand the current understanding of flow in complex porous media and provide quantitative guidelines for predicting and controlling flow.