Multiphase flow behaviour during CO2 geo-sequestration: Emphasis on the effect of cyclic CO2–brine flooding

Geo-sequestration is considered to have a significant global potential for reducing the amount of anthropogenic greenhouse gas emissions, in particular carbon dioxide. In simple terms CO2 geo-sequestration involves capturing the gas at its emitting source and then compressing and injecting it into a deep underground geological reservoir with a known seal where it will exist as a dense supercritical fluid. In this way the unwanted gas will be sequestered for perpetuity.It is known that during many long term CO2 geo-sequestration processes three will be parts of the underground reservoir subjected to cyclic CO2–brine flooding. The multiphase flow characteristics of the fluids–rock system can change after each flooding cycle or in other words each flooding stage would be different from the other from the multiphase flow behaviour point of view. This continual change is due to various factors including: the capillary hysteresis effect, chemical reactions between the in-site created solute and the host formation rock, the effect of the stress applied to the reservoir and any formation damage or enhancements caused by the CO2 or alternating CO2–brine injection process. To the authors' knowledge there has not been an extended experimental study conducted to date on the effects of such cyclic flooding pattern on the multiphase flow characteristics of the system for more than two flooding cycles. This paper presents in details the equipment, procedure, protocol and the results of an experimental work carried out to understand the change in supercritical CO2–brine–rock interactions subjected to this cyclic flooding pattern under in-situ reservoir conditions of pressure and temperature, with emphasis on change in multiphase flow characteristics of the system. The experiments were run on a number of sandstone samples with different permeability and porosity values to allow an evaluation of the change in experimental results with change in the sample properties. With this knowledge, we can better understand the significance of change in fluid flow characteristics during cyclic flooding which occurs in many CO2 geo-sequestration processes in one form or the other.

► Brine displacement by CO2 is strongly non-uniform.
► IFT strongly influences the displacement of brine by CO2.
► Effect of cyclic CO2–brine flooding on saturation profiles is minimal.
► Cyclic CO2–brine flooding strongly influences differential pressures across the porous medium.
► Capillary hysteresis and alteration of rock physical properties are the most influencing factors.