Optimizing CO2 Injection for Hydrate-Based Solid Sequestration in Deepwater Shallow Sediments Based on a Coupled THMC Model

Deepwater shallow sediments can fall within the pressure and temperature stability field of CO2-hydrate. Under these conditions, injected CO2 can be immobilized as a solid phase, which is a potential option for long-term sequestration. However, CO2-hydrate formation can strongly change the coupled hydro-thermo-mechanical response of the near-well region. Hydrate bonding can increase stiffness and strength, while pore filling and connectivity reduction can decrease intrinsic permeability, lower relative permeability to CO2, and reduce injectivity. These competing effects imply that an effective injection strategy should balance maximizing the amount of CO2 stored in the solid hydrate form and maintaining sufficient permeability for injectivity. Excessive pressure buildup and geomechanical instability should also be avoided.

We develop a fully coupled thermo-hydro-mechanical-chemical model to simulate CO2 injection, hydrate kinetics, heat transfer, and sediment deformation. The formulation solves for pore pressure, temperature, displacement, and hydrate saturation. Additional governing equations representing reaction kinetics and mass transfer between gaseous and solid phases are also derived. Hydrate formation and dissociation are described using a kinetic model based on local thermodynamic disequilibrium, and the associated latent heat is included in the energy balance. The geomechanical field is represented by a coupled poromechanical model that accounts for effective stress, fluid pressure, and geomechanical properties. Sediment stiffness, strength, and permeability are modeled as functions of hydrate saturation and stress. Candidate injection schedules including rate, bottom-hole pressure, and temperature are considered in the investigation. The sequestration efficacy is quantified by solid-phase CO2 mass, injectivity, pressure evolution, and shear and tensile failure risks.

Numerical results indicate that hydrate formation localizes near the injection point during early time, leading to rapid permeability reduction and a progressive increase in the injection pressure to sustain the target rate. Injection schedules with step-wise and intermittent operations can delay near-well permeability damage and facilitate outward migration of the hydrate formation front, which helps the spatial distribution of solid CO2 while maintaining injectivity. Results also suggest that hydrate-induced strengthening can increase resistance to deformation, but also change stress concentrations and alter failure patterns depending on the degree of permeability damage and pressure buildup. Analyses indicate that stronger injection leads to greater solid CO2 storage, but can reduce injectivity and geomechanical safety margins. This critical threshold is controlled by sediment physical and geomechanical properties as well as bottom-hole boundary conditions.

This study provides a numerical model for designing CO2 injection strategies in hydrate-stable deepwater sediments. It can be used to provide quantitative predictions and references for injection optimization to achieve robust solid sequestration while avoiding excessive permeability damage and geomechanical instability.