Abstract: The sealing ability of cap rock has a critical influence on the storage capacity, economy, and security in CO2 geological storage project. Due to the obvious characteristic of layered heterogeneity of the cap rock, it directly affects the migration and leakage process of CO2 in the cap rock. As a result, the cap rock sealing ability is also affected. Currently, the effect of isotropic heterogeneity on the sealing ability of cap rock was studied in the literature. However, the effect of layered heterogeneity has not been investigated yet. In this study, the homogeneous, non-layered heterogeneous and layered heterogeneous models were separately established by considering the different spatial distribution of porosity, permeability and capillary pressure of cap rock. Under consistent CO2 injection conditions, numerical simulations were conducted using these three models to comparatively study the whole leaking process of CO2 entering, migrating and breakthrough all over the cap rock by using TOUGH2. Results revealed that the distributions of free CO2 saturation in the cap rock agreed with the spatial distributions of porosity and permeability among the models. In contrast with the homogeneous model, non-layered heterogeneity accelerated the CO2 migration, whereas layered heterogeneity slowed down CO2 migration in the cap rock, and increased the period of CO2 breaking through the cap rock. Compared with the homogeneous and non-layered heterogeneous models, layered heterogeneous model greatly decreased the leakage rate and total leakage amount of CO2 after the breakthrough of CO2. At last, limitations on the geological modelling were addressed. Hence, the research results provide helpful guidance for more objective and quantificational evaluation on CO2 migration and leakage through the cap rock.

Key words: CO2 geological sequestration, cap rock, layered heterogeneity, migration, numerical simulation