Pore Structure Changes Occur During CO₂ Injection into Carbonate Reservoirs

Area of Science:

• Geochemistry and Petroleum Engineering
• Carbon Capture and Storage (CCS)
• Reservoir Characterization

Background:

• CO2 injection in carbonate reservoirs forms acidic brine, altering rock properties.
• Impacts on pore structure and flow properties are poorly understood, especially concerning pressure gradients.
• Mineral precipitation due to ionic equilibrium shifts requires further investigation.

Purpose of the Study:

• To investigate fluid-rock interactions and their impact on pore structure and hydraulic properties in carbonate reservoirs during CO2 injection.
• To evaluate the influence of pressure drop on ionic equilibrium and mineral precipitation.
• To introduce a novel experimental procedure for simulating reservoir conditions.

Main Methods:

• Injected CO2-saturated brine into a composite carbonate core with a pressure gradient (approx. 250 psi).
• Utilized medical X-ray CT, nuclear magnetic resonance, and scanning electron microscopy for pore-scale analysis.
• Analyzed changes in porosity, permeability, pore structure, and mineralogy.

Main Results:

• Significant calcite dissolution near the injection point led to increased porosity, permeability, and wormhole formation.
• Heterogeneous dissolution created intra-granular micro-pores and moldic porosity as ooid interiors became accessible.
• Distant regions showed minimal changes; minor calcite precipitation occurred but did not impact permeability.

Conclusions:

• CO2 injection primarily alters near-wellbore carbonate rock, enhancing porosity and permeability.
• Pressure gradients induce localized dissolution and minor precipitation, with limited impact on overall reservoir flow.
• The experimental procedure effectively simulates geochemical consequences of pressure gradients in CO2 storage.