The convective-diffusive mechanism in CO₂ sequestration in saline aquifers: experimental and numerical simulation study

Carbone Dioxide can be injected into the saline aquifers and securely stored underground if it is dissolved in water. When CO₂ contacts water, it starts to diffuse through the interface and the water layer near to the interface becomes CO₂ saturated and denser. The heavier saturated layer induces a gravitational instability (disturbance) which can result in fluid convection. This natural convection significantly increases the rate of CO₂ dissolution. Despite a decade of research on this area, this complex mechanism is not fully understood. In addition, mathematical modelling of this process by solving the equations of momentum balance coupled with convection-diffusion is challenging. In order to better understand and study the important parameters affecting the instability of the diffusive layer, a series of visual experiments were performed. A vertical, two-dimensional Hele-Shaw setup composed of a flat glass plate and a flat aluminum plate has been designed to investigate the instability of saturated layer. It has been partially filled with brine with salinity of 20 wt% containing pH sensitive dye of litmus powder. CO₂ was introduced into the top of Hele-Shaw cell with the initial pressure of 100 psi. Experimental results clearly shows the onset of instability and the natural convection phenomenon when the start point of CO₂ fingering and its propagation through the water phase were recorded. Experimental data were also used to simulate the process using the conventional and a new proposed model. In this study, a new model has been proposed to be used instead of the conventional one. Conventionally the governing equations including momentum balance equation coupled with convectiondiffusion equation are solved to simulate the disturbances. In addition, in this study, a new approach is proposed. In this model, a pseudo diffusion coefficient is introduced into the unsteady-state diffusion equation based on the second Fick's law. A so-called pseudo diffusion coefficient was defined as a function of time to capture the variable dissolution rate occurring during CO₂ dissolution in brine. The proposed model was consistent with conventional model in prediction of CO₂ dissolution rate. The results of this study can help to better understand the dissolution mechanisms of CO₂ in brine. Moreover the developed model here can be used as a rapid and simple method to predict the CO₂ dissolution behaviour for other systems with different scales and properties. Additionally the idea of pseudo diffusion coefficient can be used in other systems when the convection can be represented by the diffusion.