Microscopic Difference Characteristics And Mechanism Analysis Of Slickwater Damage In Continental Shale Oil Reservoirs

The shale reservoirs of Nenjiang Formation and Qingshankou Formation in Gulong Sag of Songliao Basin have typical characteristics of low porosity and low permeability.As the dessert section of Gulong shale oil,fracturing technology is needed to improve recovery.Slickwater is a commonly used fracturing fluid system in fracturing construction.Studies have shown that it will cause damage to its reservoir performance after entering the shale reservoir,but the damage characteristics and damage mechanism of slickwater on the microstructure of different types of reservoirs are still unclear.The predecessors have made breakthrough progress in the exploration of Gulong shale oil.The existing results show that the shale reservoirs of Nenjiang Formation and Qingshankou Formation show obvious microstructural differences.However,the damage effect of slick water on the shale reservoirs of the above different microstructural types in fracturing development is rarely concerned,which restricts the reservoir quality and development effect.In this study,the microscopic characterization techniques such as nuclear magnetic resonance and CT scanning experiments were comprehensively used,and the damage characteristics of different types of slick water on the microstructure of shale reservoirs in Nenjiang Formation and Qingshankou Formation were analyzed in combination with previous studies.The mechanism of reservoir damage caused by slick water under different microstructure types provides theoretical guidance and case analysis for the evaluation of fracturing effect of shale oil sweet spots with different microstructure types in continental China.Research shows that :1.The shale reservoir of the Nenjiang Formation is mainly characterized by pore type.During the seepage process of slick water,clay minerals are mainly caused to expand,fall off,block pores and macromolecular substances in slick water are retained in the pore throat structure of the reservoir,resulting in different types of slick water.The damage trend of the reservoir is basically consistent,mainly manifested in the obvious damage of pore types such as medium and small pores,so that some of the larger pores will be transformed into relatively small pores,and the connectivity between pores will become worse.The reservoir as a whole shows the characteristics of gradually decreasing pore radius and pore volume;2.The shale reservoir of the Qinshankou Formation is mainly characterized by fracture-pore type.The hydration expansion of the shale reservoir after the slippery water enters the shale reservoir makes the fracture propagation in the reservoir obvious,and even produces new micro-fractures.The pore-based reservoir space of different types of slick water in the Qingshankou Formation reservoir is consistent with its damage trend to the Nenjiang Formation reservoir.The micro and small pores decrease,and the pore-type reservoir space shows a downward trend in reservoir performance;3.Due to the different types of reservoir space in the two sets of strata,there are obvious differences in reservoir damage caused by different types of slick water,and with the increase of the retention time of slick water in the reservoir,the damage to the reservoir will increase.Therefore,in the actual fracturing of the Nenjiang Formation shale reservoir with pore-type characteristics,the goal should be to improve the slick water flowback efficiency and reduce the slick water retention time in the reservoir to control the expansion of clay minerals in the reservoir.In the actual fracturing of the Qingshankou Formation shale reservoir with fracture-pore characteristics,the formula and ratio of the slick water system should be comprehensively considered and optimized in order to achieve effective expansion of the fracture network on the formation scale during the fracturing process.