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

[Image: see text] 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.