Assessing the Role of Thermal Stresses in Caprock Integrity During CO2 Storage

Deep saline aquifers provide unbeatable capacities for long term CO₂ storage, provided CO₂ does not leak. Upon CO₂ injection the pressure in the aquifer increases and induces stresses in the caprock. If CO₂ is injected at the temperature different from the ambient temperature in the aquifer, there could appear thermal stresses also. We study the effect of CO₂ injection temperature on the caprock integrity. Using coupled thermo-poromechanical multi-phase simulations [1], we reveal that thermal expansion of the rock significantly enhances the stresses developing as a result of the fluid pressure buildup. Calculations show that when CO₂ is injected at a temperature below the ambient value, the stresses in the caprock above the horizontal injection well become tensile and even exceed the tensile strength after several years of continuous injection [2]. These stresses lead to the caprock fracturing. Once the fractures form, they start to propagate because of the high pressure in the reservoir and may serve as pathways for CO₂ leakage. Based on the results of our simulations using the method from [3] we estimate the rate of fracture propagation. We show that this rate is strongly affected by the value of the permeability in the caprock. Finally, our results show that injection of CO₂ at temperature close to the ambient value in the aquifer significantly reduces the risk of caprock fracturing and, therefore, possible leakage.

1. Preisig, M. and Prevost, J.H. Coupled multi-phase thermo-poromechanical effects. Case study: CO₂ injection at In Salah, Algeria, International Journal of Greenhouse Gas Control, 5, 1055-1064 (2011).

2. Gor, G. Y. and Prevost, J.H. Effects of thermal stresses on caprock integrity during CO₂ storage, International Journal of Greenhouse Gas Control (submitted)

3. Gor, G.Y., Stone, H.A., Prevost, J.H. Fracture propagation driven by fluid outflow from a low-permeability reservoir, Journal of the Mechanics and Physics of Solids (submitted) http://arxiv.org/pdf/1203.4543