酸液预处理对深部裂缝性页岩储层压裂的影响机制

深层裂缝性页岩油气藏具有高温、高应力、高破裂压力以及低渗透的特点,极大提高了现场压裂施工难度,如何有效改善近井筒岩石学性质及降低施工压力是安全高效压裂的关键。从岩石力学性质的影响因素入手,分析了酸化过程中酸与矿物的化学反应,从宏观和微观两方面阐述了酸液预处理改善近井筒岩石性质、降低破裂压力的力学机理。选取川东南地区龙马溪组页岩露头开展真三轴水力压裂物理模拟试验,采用酸液预处理压裂试件的裸眼井段,分析酸岩反应对水力裂缝起裂及扩展规律的影响。试验结果表明,酸液浸泡裸眼井段能够显著降低页岩破裂压力,酸岩反应会引起天然裂缝面性质的变化,容易导致近井筒附近复杂多裂缝的形成;裂缝扩展穿透酸化区时会引起二次憋压,酸化区内的天然裂缝越发育,酸岩反应越剧烈,近井筒裂缝形态越复杂,压裂曲线波动越频繁。室内试验和现场施工结果吻合程度良好,证实了通过酸液预处理提高深部页岩地层改造效果的有效性。

Influencing mechanism of acidification pretreatment on hydraulic fracture for deep fractured shale reservoirs

Due to the characteristics of high temprature, high stress and low permeability deep fractured shale reservoirs, great difficulties are aroused to hydraulic fracture treatment. Therefore, How to improve the rock properties near the wellbore and to reduce the treatment pressure is the key to hydraulic fracturing safely and efficiently. The influencing factors of rock mechanical properties and reaction between hydrochloric acid and shale are analyzed firstly, and then a series of macroscopic and microscopic experiments are conduced, explaining the mechanical mechanism of acidification pretreatment to improve the rock properties near the wellbore and to reduce the decreasing breakdown pressure. Subsequently, true triaxial hydraulic fracturing experiments are performed on the shale specimens collected from Longmaxi in the southeast of Sichuan Basin. The influences of acidification pretreatment on fracture initiation and propagation are investigated by soaking the open-hole section of shale specimens using hydrochloric acid. The experimental results show that the initiation pressure deceases sharply after acidification, and acidification reaction changes the properties of natural fracture surface, resulting in the formation of a complex fracture geometry near the wellbore. Besides, the pump pressure rises for the second time after hydraulic fracture extends through acidification region. The complicated natural fractures near the wellbore promote acid-rock reaction, contributing to forming a complex geometry and fluctuating the fracturing curve frequently. The good match between the indoor experiments and the on-site treatment indicates that it is effective to improve the reconstruction effects of deep shale reservoirs by acidification.