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.