Formation Sand Liquefaction: A Mechanism for Explaining Fines Migration and Well Sanding

Abstract

To understand the basic relationship between the rising water-cut, well sanding, and migration of fines, the author’s work led him to examine the mechanism of well sanding due to sand liquefaction in cases where the storm chokes in offshore wells were activated and when the production platform separator lines froze due to sudden cold weather. The author found that as the original water-oil contact plane rose above the original plane of contact, it caused a drastic change in water saturation and relative permeability. We show the increased water saturation decreases the resistance of the formation to stress by as much as 50%. This is one of the causes of sand liquefaction at or near the perforation cavities. The parameters affecting the process most are the fine-particle buoyancy, changes in the effective stress and the bulk density of the sand, the pore pressure buildup and changes in porosity near the perforation cavities, the induced transient shear stress resulting from the water-hammer total pressure wave, and the lack of adequate cohesive cement or interlocking contacts between the grains of the unconsolidated sand. The simple model calculations pointed to the conclusion that if the grains of the unconsolidated sand or the fine particles accelerated above the threshold limit of 0.19000g, the chances for fines migration and sand liquefaction were high. Furthermore, if, at the free face of the washed-out perf tunnels or the cavernous cavities of the unconsolidated sand, the total pressure wave magnitude exceeded the shear resistance of the sand, then sand liquefaction was certain to occur. The obvious outcome of this process is nothing but serious formation damage. Based on his models, the author’s recommendation for alleviating this type of damage, includes but is not limited to selective perforation, oriented perforation, gravel packing, frac packing, filter packing, trajectory planning for well reentries, and controlled drawdown.