低渗透油藏水锁机理与助排表面活性剂的优选原则

低渗透油藏水力压裂之后,压裂液进入岩石基质会造成水锁伤害,从而降低基质渗透率。由于低渗透岩石具有孔喉小、毛细管力大、原油动用深度浅等特征,毛细管末端效应会加剧毛细管力滞留侵入压裂液所产生的水锁伤害。明确这种水锁形式的伤害程度与作用时长,有利于正确评估水平井单井产能在返排过程中的恢复程度,并依此合理地优化现场"焖井"处理时机与压裂液中助排剂的使用。利用CT扫描实时监测下的岩心流动实验,明确了低渗透岩石中毛细管末端效应造成的附加水锁伤害,及其伤害程度与作用时长随岩石渗透率与水侵深度的变化规律。该附加伤害从程度和时长上,都大于原油生产对应毛细管数下残余水饱和度对基质渗透率的水锁伤害,其作用时长随岩心物性参数√K/φ的减小而线性增加,随水侵深度的增加而呈幂函数增加趋势。实验结果表明,利用渗吸作用解除水锁伤害对致密储层的效果甚微,需要依靠在压裂液中添加表面活性剂（助排剂）辅助压裂液返排而进行提产。通过对比3种能够实现跨越5个数量级油水界面张力（10^-4~23 mN/m）的表面活性剂体系,揭示了水湿低渗透油藏中助排剂的显著效果,以及油湿低渗透储层中润湿性反转优于降低油水界面张力的提产效果。

Water blocking mechanism of low-permeability reservoirs and screening principle of flowback surfactants

After hydraulic fracturing in low-permeability reservoirs, fracturing fluid entering the rock matrix will cause water blocking damage, thereby reducing the matrix permeability. The low-permeability rock is characteristic of small pore throat, large capillary force, and shallow producing depth of crude oil, which can aggravate the water blocking damage in rock matrix near the fracture face due to the capillary end effect. To determine the damage degree and duration of this type of water blocking, core flow experiments under real-time monitoring by CT scanning is conducted in this study. Results indicate that this form of water blocking delays the recovery of matrix permeability during the flowback, and its duration increases linearly with the decrease of the core physical parameter √K/φ and exponentially increases with the increasing water invasion depth. Results also indicate that shut-in treatments has little effect on mitigating such water blocking in low-permeability reservoirs, and it is necessary to add flowback surfactants to the fracturing fluid to enhance the flowback of fracturing fluid and increase production. By comparing three surfactant systems that can achieve the oil-water interfacial tension increased by five orders of magnitude (10^-4~23 mN/m), the paper clarifies the significant effects of flowback surfactants in water-wet low-permeability reservoirs, and that of wettability-alteration surfactants in oil-wet low-permeability reservoirs.