Effect of Flow Rate on Vertical Immiscible Displacement in Geologic Heterogeneous Porous Media

Abstract:

CO2 emplacement in the subsurface results in immiscible buoyant flow in layered porous media. This can create different patterns of invasion and trapping depending on the nature of the heterogeneity, and the fluid release. In this work we focus on the final state of the system with regards to non-wetting phase distribution and saturation at the intermediate scale, as a function of flow rates/capillary numbers.

We conduct gravity unstable, immiscible displacement experiments at the intermediate scale (60 cm x 60 cm x 2cm) in heterogeneous bead-packs with cross-stratified geometry, generated using an automated technique reported previously. The technique enables generation of bead-packs that mimic natural outcrops and display heterogeneity at a hierarchy of scales (mm to 10s of cm). The experiments are conducted at ambient conditions using a surrogate fluid pair that mimics density and viscosity contrasts, and interfacial tension of in-situ reservoir brine and supercritical CO2. Light transmission enables visualization of flow behavior and measurement of saturations in real time. Keeping constant the fluid pair, grain size contrast and fabric heterogeneity, we vary the flow rate over three orders of magnitude.

We find that the competing effects of capillary heterogeneities and gravity, and viscous forces lead to a non-linear relationship between the flow rate and invasion and trapping behavior. With increasing flow rates, the sweep efficiency and non-wetting phase saturations increase as expected. At the same time, enhanced vertical connectivity and viscous effects lead to more non-wetting phase escaping during redistribution. But ultimately, higher flow rates end up leading to larger trapped saturations. The main takeaway from these experiments is how the underlying rock heterogeneity continues to influence displacement behavior irrespective of three orders of magnitude variation in the flow rates. The findings of these experiments show that the influence of capillary heterogeneities on non-wetting phase trapping is not limited to post injection, far field conditions and that local capillary trapping can persist at viscous dominated flows close to the injection wells.