A microscopic study of sand arches and sand skeletons under hydrodynamic force based on the CFD-DEM model

Sand production is one of the oldest challenges in the oil and gas industry, causing billions of dollars of losses every year. The main aim of the research reported here is to use our validated 3-D numerical model (Song et al., 2020) based on computational fluid dynamics (CFD) coupled with the discrete element method (DEM) to investigate sand arching under hydrodynamic force in oil and gas production wells. The main findings are as follows: The sand arches observed in our 3-D model have complex 3-D backbones. Sand arches with fewer sand grains are more stable and more common than those with more sand grains. Sand arches contain particles that are coarser than the average size of sand in reservoirs. Most associated angles in arches are less than 180 degrees. For a concave arch with an associated angle greater than 180 degrees, a bead hanging from the equator can be compensated by neighboring particles. A concave sand arch exists only when static friction is introduced. The two arch abutments of sand arches bear the maximum contact force. The critical drawdown pressure gradient of the reservoir increases when the frictional coefficient between sand and screen/liner material increases from 0.0 to 0.75. However, simply increasing the frictional coefficient does not enhance the stability of sand arches if the frictional coefficient is greater than 0.75. The sand skeleton generates greater inner contact force when a higher fluid pressure gradient is introduced. If the drag force exceeds the maximum strength of the sand arch, catastrophic sand production occurs without a new sand arch forming. The collapse and reconstruction of sand arches cause the fall and rebound respectively of the mean coordination number. The greater the mass of the sand production, the greater the mean coordination number's maximum retracement. The proposed method is sufficiently robust and efficient for application to the simulation of fluid-particle interactions for a wide variety of problems in granular systems.