(677b) Molecular Dynamic Simulation of Clay Particles Swelling

Clay minerals, such as smectites and mixed-layer illites, can expand in volume up to 20 times their original volume through adsorption of layers of water between their unit cells. This is a very important phenomenon as swelling clays reduce formation permeability and porosity by peeling off the pore surfaces and plugging pore throats, not by reducing porosity alone. 

In an attempt to understand the clay swelling process due to shear, we are using MD simulation to study the swelling and shrinkage of clay particles. A slit pore, the space between two parallel surfaces, is used. In order to show the coupling between swelling and shear in liquid-saturated clay pores, we use Montmorillonite structure and SPC/E model of water molecules. First, the consistency of the simulations using thermodynamics and direct routes is validated when dealing separately with swelling and shear. Then, the coupling between swelling and shear is explored by displacing the solid walls in one direction while letting them move freely on the other.

Our results so far indicate that shear can induce swelling and vice versa, due to the confined fluid phase structure. It implies that the response to a variation in the external load can be a combination of volumetric and shear deformations, because of the presence of the fluid.

We are also exploring the behavior of the pore system induced by solid walls moving at a constant velocity. Interestingly, when the wall velocity exceeds the swelling velocity, the instantaneous states of the system are no longer at equilibrium and the averaged pore width slightly increases with increasing shear rate.

We are also studying the effect of NaCl concentration on the swelling process. So far, we have found that concentration of Na affects the swelling process, such that as the concentration increases, the spacing between two layers of clay decreases.