Influence of Sand Production Damage in Unconsolidated Sandstone Reservoirs on Pore Structure Characteristics and Oil Recovery at the Microscopic Scale

Abstract

Unconsolidated sandstone reservoirs have low rock strength and are easily damaged by sand production. To evaluate the effect of the microscopic pore system on the degree of recovery and sand production damage, five typical unconsolidated sandstone core samples were selected. The nuclear magnetic resonance T 2 spectrum of the water-flooding experiments was used to analyze the oil displacement mechanism and sand production damage of the pore throat structure, and the control mechanism of the injection parameters was used to evaluate the recovery and sand production damage degree. The results showed that the recovery of the unconsolidated sandstone core samples was the highest when the injection volume was 6 PV, and the overall pore throat recovery increased from 14.37 to 48.72%. The recovery and sand production damage increased with increasing injection pressure. The overall pore throat recovery was the highest when the pressure was 20 MPa (48.42%), and the sand production damage index was the maximum when the pressure was 25 MPa (19.98%). Under a lower injection pressure, sand production damage mainly originates from loose sand. The main target of loose sand production damage is a smaller pore throat, and the sand production index increases slowly. The recovery increased with the pressure and increased relatively quickly. Under a higher injection pressure, sand production damage mainly comes from loose sand and skeleton sand, causing damage to both smaller and larger pore throats. The sand production damage is positively correlated with injection pressure. The recovery slows down with an increase in pressure or even decreases.

Quantitative Analysis of Crude Oil Mobilization in Microscopic Pores during the Spontaneous Imbibition of CO2-Enhanced Fracturing Fluids in Shale Oil Reservoirs

“…These reservoirs in the Chang 7 Member are distinguished by their multiscale pore-throat architecture, with a widespread development of micro- to nanoscale pores. In these reservoirs, capillary forces are more pronounced than those in conventional ones, thereby enhancing the efficacy of imbibition in oil displacement. − Additionally, the diverse occurrences of fluid storage, the complexity of pore structures, and the high content of clay minerals in shale oil reservoirs make traditional well-logging techniques less effective in assessing key reservoir parameters, such as porosity, permeability, and oil saturation. This has led to an increased focus on research exploring the mechanisms governing the displacement of oil by fracturing fluids through imbibition, with the ultimate goal of optimizing oil recovery rates. − …”