(157d) The Interaction between Small Clusters of Cohesive Particles and Laminar Flow: Coupled Dem/Cfd Approach

The interaction between a flowing liquid and a granular material is encountered in various fields such as processing of particulate solids (granulation, crystallization), geomorphological applications (river bed flows, soil erosion) or oil exploration (reservoir flows, pipeline flows). In oil production, the oil/water flow from the reservoir through horizontal perforations into the main well-bore can lead to the dislocation and entrainment of sand particles and their clusters. Sand production is a severe problem which occurs when the reservoir sandstone fails under the in-situ stress and flow conditions. The sand debris can be then transported by the flowing liquid and cause problems in the downstream operation such as the abrasion of equipment or flow obstruction by sand deposits. The exact mechanism of sand production and transport are still not completely understood at the level of individual grains where capillary forces between grains due to liquid bridges (oil/water wet systems) may play a significant role.

The main objective of this work is the development and the validation of a model which is able to simulate the erosion and entrainment of sand particles under different flow conditions and varying level of grain cohesiveness.

We use the Discrete Element Method for the simulation of particle movement. The effect of the liquid bridges is embodied by cohesive forces between particles using closed-form approximation. Other arbitrary cohesive forces representing e.g. sintering or the effect of binder are incorporated into the model in a similar way. The drag force on individual particles is obtained from the solution of the Navier-Stokes equations for the laminar flow (Reynolds numbers up to ca 100). The DEM and CFD parts of the model are dynamically coupled via the drag force and the changing boundary conditions due to particle movement.

A parametric study for different flow geometries has been carried out. The combination of varying fluid velocity on one hand and changing the parameters that determine the strength of cohesive/capillary forces on the other hand will lead to qualitatively different scenarios. The deposit of particles will: (i) stay at rest and intact, or (ii) stay at rest but deform, or (iii) single particles can be eroded one by one from the cluster surface, or (iv) larger chunks of particles can be moved along the flow, or (v) the entire cluster may be lifted and entrained. Limiting condition under which these scenarios occur will be identified by the simulations and compared with qualitative experimental observations.

A detailed investigation of the interplay between the effect of capillary forces, the rate of the fluid flow and the system geometry is a crucial step in better understanding of the sand production problem. Moreover the same approach can be utilized in various other applications, e.g., granule dissolution and break-up, hard surface cleaning, or fluidization of cohesive particles.