After hydraulic fracturing operations in tight oil reservoirs, a substantial volume of fracturing fluid invades the rock matrix and mixes with the crude oil, potentially causing low recovery rates and limited production capacity. To elucidate the factors hindering crude oil after fracturing fluid invasion, this study employed NMR analysis to investigate the pores and throats distribution characteristics of tight oil reservoir rocks. Furthermore, the emulsification behavior between crude oil and fracturing fluid was studied through rheological experiment and microscopic observations of emulsions before and after formation. Finally, natural core samples and a micro-etching model were used to simulate the process of crude oil recovery after fracturing fluid invasion. The research findings reveal that tight oil reservoir rocks in Daqing Oilfield exhibit small pore sizes, narrow throats, and strong heterogeneity. Crude oil and fracturing fluid display a pronounced tendency to form emulsions, with emulsion viscosity reaching up to 3.4 times that of the crude oil itself. Emulsified droplets primarily fall within the size range of 0.5μm to 1.5μm. After fracturing fluid invasion, emulsification with crude oil within pore throats results in the generation of large droplets and the aggregation of smaller droplets during flow. The crude oil recovery process can be divided into three stages: early stage, mass expulsion of fracturing fluid from the invasion zone coinciding with the release of large droplets; middle stage, flow channels being occupied by smaller droplets, with noticeable viscous effects; late stage, pores blockage by aggregated small droplets, resulting in minimal fracturing fluid content in the produced liquid.
Tight oil reservoirs possess abundant resources but typically lack natural productivity (Zhu et al., 2012). With the advancement of horizontal drilling and multi-stage hydraulic fracturing technologies, tight oil has brought substantial economic potential to the petroleum market, making tight oil development an exceedingly crucial domain in today's unconventional oil resource production and the entire petroleum industry (U.S. Energy Information Administration, 2018). However, during tight oil production, individual well production declines rapidly, and achieving stable production becomes challenging. During the hydraulic fracturing process, a considerable volume of fracturing fluid, often exceeding tens of thousands of cubic meters, is injected on-site. However, as reported, typically only 5-50% of the injected fluids can be recovered through flowback (Zelenev and Ellena, 2009). This implies that a significant portion of the fracturing fluid invades the reservoir matrix.
