CO2 sequestration wells sealing evaluation model: Jimusar depression, China as an example

Abstract

Suitable geological sites for CO₂ sequestration often contain numerous existing wellbores, which have been identified as high-risk locations for CO₂ leakage. To investigate whether the pressure disturbances caused by CO₂ injection have the potential to drive the propagation of micro-annulus in nearby wells, this paper develops and validates a numerical simulation program, taking the Jimusar Depression in China as a case study. The results indicate that the phenomenon of formation overpressure caused by CO₂ injection occurs not only in the reservoir but also in the caprock. The pressure response of the caprock exhibits a lag, resulting in a dynamic inter-layer pressure difference, with a peak difference observed during the early stages of injection. As the distance from the injection well increases, the peak pressure difference decreases exponentially, with a maximum differential pressure of 1.33 MPa observed at well J172. The pressure difference around the well can drive fluid migration along the cement bonding surface, and excessive pressure differences can lead to changes in the stress state of the surface. When the stress state transitions from compression to tension, and the tensile stress exceeds the bonding strength, micro-annulus may propagate. When the cement curing shrinkage rate is zero, the minimum pressure difference for micro-annulus propagation at the cement-sheath formation interface in well J172 is 9.39 MPa, which is significantly higher than the peak formation pressure difference of 1.33 MPa around the well, indicating a low risk of micro-annulus propagation. The shrinkage of the cement upon curing will considerably lower the critical pressure for micro-annulus propagation. At a cement shrinkage rate of 0.15%, the propagation pressure for micro-annulus decreases to 1.0 MPa, indicating a higher risk of micro-annulus propagation. This study proposes a CO₂ sequestration well sealing assessment program, which confirms that under unfavorable conditions, CO₂ injection may induce the propagation of micro-annulus near the well. This research holds significant implications for the safety of wellbores at CO₂ sequestration sites, as well as for monitoring CO₂ plumes and pressure.