Carbon storage efficiency and permeability decay during microbubble and conventional carbon dioxide injection in high-permeability sandstone

Technical Summary: Carbon Storage Efficiency and Permeability Decay During Microbubble and Conventional CO₂ Injection in High-Permeability Sandstone

Problem Statement
Geological carbon storage (GCS) in saline aquifers faces challenges regarding sweep efficiency and injectivity, particularly in high-permeability formations where CO₂ tends to channel through dominant pathways, bypassing significant pore volumes. While microbubble (MB) CO₂ injection has been proposed to enhance sweep efficiency and dissolution trapping by temporarily blocking large pores (the "Jamin effect") and increasing surface area for dissolution, its implications for injectivity and near-wellbore integrity in high-permeability sandstones remain poorly constrained. Previous studies have largely focused on low-permeability reservoirs or single-flow-rate experiments, leaving a gap in understanding how MB injection performs under the variable flow rates typical of field operations and how it affects long-term rock permeability through cumulative exposure.

Methodology
The study employed repeated volume-balance core-flooding experiments on high-permeability Berea sandstone cores (~1000 mD) under deep saline aquifer conditions (40 °C, 10.7 MPa). The experimental design compared conventional supercritical CO₂ injection against microbubble CO₂ injection across a range of flow rates (0.05, 0.1, and 0.5 cc/min).

• Apparatus & Setup: A double syringe pump system was used for injection, with back-pressure and confining pressure maintained by ISCO pumps. Microbubbles were generated mechanically using a ceramic filter (0.5 cm, micron-scale pores) placed at the sample inlet, where brine flowed through the filter followed by CO₂.
• Experimental Protocol:
  • Replication: Sample BO5 underwent multiple floods at a fixed low rate (0.05 cc/min) to replicate protocols from previous literature (Zhai et al., 2020).
  • Sequential Testing: Sample BO3 was subjected to sequential floods with increasing flow rates (0.05 → 0.1 → 0.5 cc/min) for both injection types to evaluate flow-rate sensitivity.
  • Permeability Monitoring: Brine permeability was measured before and after sequential floods to track decay. A control sample (BO7) was flooded with nitrogen gas to isolate CO₂-specific effects.
• Analysis: CO₂ saturation was calculated via volume balance. Permeability decay was modeled using a single-exponential function with an asymptote. Uncertainty analysis accounted for dead volume, cylinder readings, and pressure/temperature effects.

Key Results

1. Flow-Rate Dependence of Sweep Efficiency:

   • At the lowest flow rate (0.05 cc/min), microbubble injection increased average CO₂ saturation by approximately 8.9% compared to conventional flooding in the replicated sample (BO5). This was attributed to the temporary blockage of dominant flow channels, forcing CO₂ into smaller pores.
   • However, this benefit diminished and reversed at higher flow rates (0.1–0.5 cc/min). In sequential tests, microbubble injection yielded significantly lower final CO₂ saturations (13–25% lower) than conventional injection. The authors suggest that at higher rates, the hydraulic resistance of the microbubble generation filter may exacerbate preferential flow, negating the sweep benefits.
   • A discrepancy was noted between samples: while BO5 showed MB improvement at 0.05 cc/min, BO3 (tested sequentially) showed MB underperformance even at the same low rate, likely due to cumulative sample alteration and pore structure changes from prior flooding.

2. Permeability Decay and Integrity:

   • Brine permeability in sample BO3 declined significantly over sequential floods, dropping from ~955 mD to ~288 mD.
   • The decay curve indicated that permeability continued to decline during the microbubble stage even after stabilizing during conventional injection.
   • Control tests with nitrogen (Sample BO7) showed no permeability decay, confirming that the damage is driven by CO₂ exposure.
   • The authors attribute the accelerated decay in MB tests to a combination of CO₂ exposure, microbubble generation, filter effects, and cumulative sample alteration, potentially driven by fines mobilization and enhanced mineral reactions due to rapid microbubble dissolution and local acidification.

3. Pressure Drop:

   • Microbubble injections exhibited pressure drops up to three times higher than conventional injections. However, the authors note that this is largely dominated by the low permeability of the generation filter rather than the rock core itself, limiting direct cross-mode comparisons of hydraulic resistance.

Significance and Claims
The paper claims to be the first to systematically quantify flow-rate-dependent microbubble CO₂ behavior and permeability decay in high-permeability sandstone under repeated injection cycles. The study highlights a critical trade-off in microbubble CO₂ injection:

• Potential Gains: At low flow rates, MB injection may improve sweep efficiency and residual trapping.
• Operational Risks: At higher flow rates, MB injection may lead to reduced sweep efficiency and accelerated near-wellbore permeability damage (injectivity loss).

The authors conclude that while microbubble injection offers potential benefits for storage security in the far-field or at low injection rates, its application near the wellbore in high-permeability systems requires careful optimization of flow rates. The results suggest that field-scale strategies must account for the risk of injectivity impairment and the complex interplay between flow rate, sample history, and cumulative rock alteration. The study calls for further research at higher flow rates to better align laboratory findings with practical operational conditions.