In recent years, scientists have focused on processes which can be used to reduce greenhouse gas emissions, and CO2 sequestration in deep coal seams and saline aquifers has been identified as a potential method. However, there is a fundamental lack of understanding concerning the flow and mechanical phenomena that occur when CO2 is injected into coal seams. The main objective of this parametric study is to model the effect of CO2 injection pressure on cap rock deformation and investigate the possibility of leakage or back-migration of CO2 to the atmosphere through the cap rock. The COMSOL Multiphysics numerical simulator was used to investigate the effect of CO2 injection on coal and other adjacent rock strata. For the purpose of the model, a 200 m long and 5 m thick coal seam was assumed to be lying below a 200 m long and 3 m thick cap rock layer 1000 m below the surface. According to the model's results, the cap rock deforms considerably just after injection in an upward direction due to the CO2 movement, and the amount of deformation greatly depends on the injecting gas pressure. When the injecting gas pressure is increased from 10.2 MPa to 30 MPa, the cap rock is raised from 0.2 mm to 15 mm during the injecting period. Moreover, CO2 first spreads and is stored in the coal mass and then after about 3-4 years it starts to move towards the cap rock layer. In the second stage of the study, a small rock fracture was inserted between the cap rock and the coal layer and a sudden increase in leakage of CO2 to the cap rock through the fracture was observed. The existence of a rock fracture is highly possible, and may happen naturally in coal seams. This model shows that this kind of fracture greatly increases the risk of back-migration of CO2 into the atmosphere. The model can be used to study general coal mass and cap rock behaviour and the back-movement of CO2 flow to the atmosphere. (C) 2012 Elsevier Ltd. All rights reserved.