Numerical Simulation Of Multiphase Hydromechanical Processes Induced By CO₂ Injection Into Deep Saline Aquifers

Greenhouse effect caused by excessive emissions of CO₂ has become one of the most severe environmental problems in the world,it is urgent to reduce the concentration of CO₂ in the atmosphere.CO₂ geological sequestration is an effective approach to solve this problem.And CO₂ storage in deep saline aquifers is one of the most feasible methods for CO₂ storage with great storage potential.During this process,there is a strong coupling mechanism between fluid flow and solid deformation in the reservoir,CO₂ injection into deep saline aquifers changes the pore pressure and formation stress field,which leads to the deformation of the solid skeleton of the reservoir.Besides,the medium deformation will affect the migration of CO₂ in the reservoir,even lead to formation fracture,and affect the security of storage.It is of great significance to study the hydromechanical process of CO₂ sequestration in deep saline aquifers,to better understand the migration law of CO₂ in reservoir and provide theoretical support for the evaluation of reservoir stability and safety.Based on the modified Darcy's law,the concept of Terzaghi effective stress and the Biot consolidation theory,a coupling model of CO₂,brine two-phase seepage and solid mechanics for CO₂ injection into deep saline aquifers was established,and the main factors of hydromechanical process induced by CO₂ injection into deep saline aquifers were analyzed,and the stability of reservoir was evaluated based on Mohr-Coulomb failure criterion.Numerical simulation results indicate:1)With the increase of the injection velocity of CO₂,the CO₂ saturation of reservoir in steady state increased continuously and the storage capacity of CO₂ increased exponentially;2)The capillary entry pressure(P_entry)has a significant effect on the saturation distribution of CO₂,when P_entry is higher,carbon dioxide is much easier to be trapped in the saline aquifers,and the saturation distribution of CO₂ is more uniform;while pore distribution coefficient does not have a significant effect on the CO₂ saturation distribution;3)When the injection velocity is 1.0×10^-6 m/s or Biot coefficient is equal to 1,the new stress condition is closer to the failure envelope.Based on the measured data from the In Salah site project in Algeria,numerical simulation of CO₂ injection into the deep saline aquifer was carried out.Numerical simulation results indicate:1)After three years of injection,the surface was raised about 18.6 mm,it is consistent with the measured values;2)After three years of injection,the pore pressure the reservoir increased and the increase in horizontal direction is greater than that in vertical direction,the maximum pore pressure increased by about 8 MPa;3)There are complicated coupling mechanisms in the reservoir,coupling of fluid and mechanics will affect the migration of CO₂ in the reservoir,the propagation of CO₂ is slightly enhanced by deformation.which is more significant near gas injection wells.