Effect of cap rock thickness and permeability on geological storage of CO2: laboratory test and numerical simulation

Geological storage of CO2 is considered widely as an efficient method of mitigation of greenhouse gas emission. CO2 storage mechanism includes structural trapping, residual gas trapping, solubility trapping and mineral trapping. The shale cap rock acts as a seal for the storage when CO2 accumulates at the top of the reservoir. The injected CO2 may migrate through the cap rock under buoyancy force or pressure build-up which depends on the seal capacity of the cap rock. As a result, the effectiveness of containment of injected CO2 in the reservoir is largely dependent on the migration rate of CO2 through the cap rock. This paper investigates the effects of CO2 leakage through cap rock by a combination of experimental studies and numerical simulation. Firstly, experimental measurements on shale core samples collected from Australian cap rocks were conducted to determine properties, such as capillary pressure, pore size distribution and permeability. Based on the measured cap rock properties, the effect of thickness and permeability of cap rocks on CO2 leakage was studied using a commercial compositional simulator. Experimental results show that the permeabilities of the shale samples measured by transient pulse technique range from 60 to 300 nD; a non-Darcy calibration factor which equals the ratio of the measured permeability divided by 1000, is identified for samples with permeability lower than 1000 nD. Numerical simulation results show that the largest leakage of CO2 through the seal (cap cock) is about 7.0% with seal thickness of 3m and vertical permeability of 90 nD; both shale thickness and permeability affect the CO2 leakage significantly; with a given seal permeability, the leakage rate has a power relationship with shale thickness.