Carbon Storage Simulation Near the Saturation Pressure and Temperature, and Critical Point of CO₂

Depleted oil and gas reservoirs are increasingly considered viable candidates for carbon capture and storage (CCS), due to their well-characterized geology, established infrastructure, and proven containment. However, simulating CO₂ injection in these systems presents significant challenges, particularly under low-pressure and low-temperature conditions near the saturation curve and critical point of CO₂.

This study presents a numerical formulation for accurately simulating pure and near-pure CO₂ behavior in compositional reservoir simulators. The approach addresses the instability and nonlinearity associated with phase behavior near the saturation curve and the Widom line, where small changes in pressure and temperature can lead to abrupt phase transitions. 

The methodology introduces isenthalpic flash calculations to properly model two-phase behavior for pure CO₂ systems, overcoming limitations of traditional isothermal flash approaches. In addition, an interpolation technique is applied near the saturation curve and critical region to smooth phase properties and improve numerical stability without significantly compromising accuracy.

The approach is validated across a range of scenarios, including CO₂ vaporization, condensation, near-critical conditions, and transition from multicomponent systems to pure CO₂. 

A field-scale case demonstrates the applicability of the method in a heterogeneous reservoir model. Results show stable simulation performance during continuous CO₂ injection, including conditions where temperature drops below saturation and liquid CO₂ forms near the wellbore. The method enables smooth transitions between phases and avoids convergence failures typically encountered in standard compositional simulations. 

Why This Matters

• Improves stability of CO₂ storage simulations
  The method addresses numerical challenges near saturation and critical conditions, where conventional approaches often fail. 
• Accurately models pure CO₂ phase behavior
  Isenthalpic flash calculations enable correct representation of liquid–vapor transitions in single-component systems. 
• Handles transition from multicomponent to pure CO₂ systems
  The workflow ensures smooth switching as injected CO₂ displaces in-situ hydrocarbons. 
• Supports realistic CCS modelling in depleted reservoirs
  Captures key physical processes such as condensation, vaporization, and near-critical behavior. 
• Enables field-scale application
  Demonstrated robustness in heterogeneous models with sustained CO₂ injection and complex phase behavior.