Local capillary trapping in carbon sequestration: Parametric study and implications for leakage assessment

Local capillary trapping (LCT) is the trapping of CO2 by local capillary barriers. It occurs during buoyancy-driven migration of bulk phase CO2 within a saline aquifer exhibiting spatially varying properties (permeability and capillary entry pressure). The benefit of LCT, in the context of CO2 sequestration, is that local capillary trapped CO2 is not susceptible to leakage through failed seals. However, it is unclear how the petrophsyical/geological properties and flow dynamics influence LCT. Thus, the objective of this work is to evaluate the degree to which potential local capillary traps are filled and quantify the extent of immobilization persisting after loss of seal integrity. This paper presents a systematic and thorough study of the influential parameters of LCT. Fine-scale capillary pressure fields are generated by using geostatistical permeability realizations and applying the Leverett j-function. Multiple factors are examined, including injection rate, anisotropy, formation dip, aquifer types, residual gas saturation, and capillary hysteresis. Leakage representative of wellbore failure is simulated, and LCT after leakage is evaluated and compared to other trapping mechanisms. The results show that local capillary traps in the near-well region can be fully filled during injection. Moreover, they remain filled after post-injection buoyancy-driven flow ends. The filling efficiency of local capillary traps increases with the decrease in gravity number (ratio of buoyant force over viscous force). As a result, maximizing LCT in carbon sequestration in porous reservoirs may be achievable with the implementation of appropriate injection strategies.