Microfluidic experimental study on imbibition-displacement mechanism of tight oil reservoirs using fracture-matrix etched chips

Hydraulic fracturing serves as a primary technique for developing tight sandstone oil reservoirs. Investigating the microscopic seepage mechanism and flow dynamics of residual oil helps guide the exploitation of tight oil reservoirs with high water cut. To explore fluid migration patterns and the imbibition-displacement coupling mechanism during fracturing fluid injection, we conduct visual flooding experiments using microfluidic models based on fracture-matrix laser-etched chips (also referred to as the dual-medium microfluidic models). We analyze the movement of the oil-water interfaces, the stripping of oil droplets, and the microscopic distribution of residual oil under the influence of imbibition-displacement coupling. The results indicate that, following the injection of fracturing fluids into a dual-medium microfluid model, the flow process involved fracture fingering, pore-fissure interactive imbibition, and pore displacement. A lower injection rate corresponded to a stronger dominance of imbibition, resulting in a broader sweep range of oil in dead-end pores near the fractured zone. With an increase in the injection rate, the interactive imbibition weakened, leading to a gradual reduction in sweep range and oil recovery, which was primarily attributable to pore oil displacement. The addition of surfactants enhanced the ability of fracturing fluids to strip oil droplets and residual oil clusters adhering to pore walls. Moreover, fracturing fluids containing surfactants significantly impacted residual oil and promoted its drainage. Consequently, a substantial amount of residual oil was stripped from the pore walls. Following fluid breakthrough, residual oil on pore walls continued to be stripped during the stable displacement stage, significantly enhancing the overall flooding effect. Residual oil remained in the imbibition and flooding processes due to variations in wall roughness and the impact of flow rates and pressure across different pores. Based on its morphology and distribution, residual oil can be categorized into six types: spherical, single-wall-adhered membranous, pore-throat columnar, double-wall-adhered membranous, wall-adhered bent columnar, and inter-wall contiguous types. The columnar and membranous types, among others, are extensively distributed.