Flow regime analysis for geologic CO₂ sequestration and other subsurface fluid injections

Carbon dioxide (CO₂) injection into a confined saline aquifer may be modeled as an axisymmetric two-phase flow problem. Assuming the CO₂ and brine segregate quickly in the vertical direction due to strong buoyancy, and neglecting capillary pressure and miscibility, the lubrication approximation leads to a one-dimensional nonlinear advection-diffusion equation that describes the evolution of the sharp CO₂-brine interface. The interface evolution is driven by two forces: the force from fluid injection (the lateral pressure gradient due to injection), and buoyancy. Analytical solutions can be derived when one of the two forces is dominant. The solutions depend on the viscosity ratio (M) between the displaced and injected fluids, and a buoyancy parameter (Γ) that measures the relative importance of buoyancy compared to the driving force from injection. Different combinations of these two parameters give different forms of the solutions. But for all the solutions, the radius of the lateral spreading of the injected fluid follows,r∝t^1/2 with the proportionality coefficient differing for the different solutions. In this paper, we identify the kinds of solutions appropriate for practical CO₂ injection projects as well as other subsurface fluid injection applications. We use data from eight CO₂ injection projects, twenty-four acid gas injection projects, two liquid waste disposal projects, and one CO₂-WAG enhanced oil recovery project. The solutions provide guidance for the expected behavior of fluid spreading under the different injection operations while providing general insights into overall fluid flow behavior.