Solid and Fluid Coupling in Discrete Element Method

  • Author / Creator
    Cui, Yifei
  • The Discrete Element Method (DEM) has been developed and used in modelling dry granular materials with or without cohesive bonding between particles. The incorporation of fluid flow in the DEM analysis is difficult since the DEM is a discrete approach to mechanistic analysis while fluid is a continuum. This research is focused on the development of numerical techniques and procedures that provide the coupling of the DEM with the continuum fluid. This thesis provides two coupling methods for (1) the coupling of solid permeability and fluid flow (SPF), and (2) the coupling between solid deformation and pore pressure (SDP) for undrained and semi drained conditions. The SPF coupling accounts for the interaction of fluid flow in a porous medium with solid deformation and solid dislodgement by the flow forces imposed by the pressure gradient and drag forces on solid particles. Deformation of the porous medium alters the pore sizes, hence, changes the permeability of the material. Further, the production of the solid grains alters the porosity and the solid boundaries from which the fluid is produced. The SDP coupling refers to the generation of pore water pressure as a result of solid deformation in the porous medium. The variation of pore sizes results in a temporary increase in pore fluid pressure, which gives rise to the pressure gradient causing pore water diffusion. The dissipation of pore pressure can be analyzed using the SPF approach but the generation of excess pore pressure requires the coupled analysis of the fluid and solid deformation. Any decrease in void space results in excess pore pressure that reduces the effective stresses, hence, changes the amount of deformation. Therefore, the SDP is considered to be a two-way coupling technique. The motivation of this work arises from the desire to analyze sand production in oil wells during the oil recovery from sandstone formations. Large amounts of sand production in a short period may clog up the well, and destabilize the well due to the loss of materials. On the other hand, a controllable amount of sand production may increase the wellbore productivity and reduce the wellbore completion cost. Therefore, understanding the sand production mechanisms and the ability to predict and manage the rate of sand production at the field scale are beneficial. This thesis presents a model for the investigation of sandstone degradation and sand production mechanisms by using the SPF coupling method. The model was used to investigate the effects of in-situ stresses and flow rate on sand production. A linked DEM-fluid flow model for sanding analysis is developed. The model calculates seepage forces and applies them to solid particles in the DEM model. The model accounts for permeability and porosity changes due to sandstone deformation and sand production. The DEM model is verified against poro-elastoplastic analytical solutions. Subsequently, the model is used for sanding simulation from a block-shaped sample under different far-field stress and pressure conditions. The boundary stresses and fluid pressures are varied to study their influence on sandstone degradation and sand production. Another important factor affecting stability of wellbore is the generation and dissipation of excess pore pressure. Excess pore pressure can lead to the loss of shear strength and particles contacts resulting in plastic deformations. The methodology this thesis presents for the incorporation of excess pore pressure in the DEM simulation (SDP) with a new liquid particle element is novel. The liquid particle element has a specific stiffness that enables the calculation of excess pore pressure build up or the dissipation due to pore space deformation. Analytical solutions of conventional soil mechanics problems, such as isotropic compression and consolidated triaxial undrained test, have been used to verify the proposed algorithm quantitatively under undrained condition. The oedometer test and consolidation theory are then used to quantitatively validate and verify the dissipation model. The SDP models are then applied to simulate consolidated-undrained triaxial tests at different levels of porosity and pore pressure. The numerical results show good agreement between the proposed scheme and the laboratory results. The proposed scheme provides an effective method to calculate pore pressure in a porous medium by using the discrete element approach.

  • Subjects / Keywords
  • Graduation date
    Fall 2016
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.