A novel approach to precise evaluation of carbon dioxide flow behaviour in siltstone under tri-axial drained conditions

Since the secure storage of CO2 in any geological sink is largely dependent on its caprock and reservoir rock flow properties, it is necessary to check the permeability of both in assessing geological reservoirs for CO2 storage. Siltstone is a common rock type in both. On the other hand, the highly complicated thermodynamic properties of CO2 cause its flow behaviour through any rock mass to become highly complex. This intention of this study is to understand this complexity and to propose an accurate technique to evaluate the apparent permeability of CO2 through siltstone under laboratory conditions, considering the possible phase transitions of CO2 inside the rock mass. A series of tri-axial drained experiments on siltstones at room temperature was therefore performed. According to the results, the proposed method accurately predicts permeability through siltstone in tri-axial drained tests, because it more precisely considers the influence of CO2 phase transition on its flow performance, and the permeability in the sample is separately evaluated for the liquid and gas CO2 regions. The new approach shows that the actual CO2 pressure distribution along the sample has a curvilinear shape. Consideration of the possible phase transition between the sample inlet and the outlet is particularly important for liquid CO2 injection in tri-axial drained tests, due to the certainty of the occurrence of phase transition under this liquid inlet and gas outlet condition. According to the test results, the apparent permeability calculated for liquid CO2 injection using the proposed method is around 50% lower than that evaluated using the traditional method. This suggests the importance of the use of more accurate approaches such as that proposed under such situations. In addition, CO2 permeability in siltstone is found to be significantly increased with increasing injection pressure, and increasing the injection pressure from 3 to 6 MPa (in the gas CO2 region) caused the CO2 permeability in the tested siltstone to be increased from 0.00102 to 0.00228 mD at 15 MPa confining pressure, which is related to the related pore structure modification in siltstone.