Mechanism of Acid Effect on Fracture Behavior of Marine Shale

In order to reveal the influence mechanism of acid-rock reaction in different bedding directions on fracture properties of Marine shale, SNDB samples were used to obtain the load-displacement relationship and fracture load of samples under three-point bending loading, and the fracture toughness (K_IC) and fracture energy of shale were calculated. Combined with network fracturing requirements, acid injection modes of marine shale reservoirs under different depths and stress states were initially discussed. The results show that: (1) The fracture process of the sample under three-point loading includes the stages of compaction, elastic compression, fracture, and fracture expansion. The bearing capacity of the sample is rapidly lost after fracture, and the shale is brittle. (2) In parallel, vertical, and shear stratification directions, the fracture toughness of shale decreases with the increase of acid concentration, K_IC decreases from 1.12 to 0.44 MPa· m^0.5, and fracture energy decreases from 211.50 to 56.00 J·m^-2. The fracture performance of Marine shale is significantly controlled by bedding orientation and acid concentration. (3) According to the acid filtration mechanism, the acid intrusion mode of parallel bedding shale is fracture intrusion, and the intrusion depth is large; The vertical bedding shale is a porous intrusion with shallow intrusion depth. The shear stratification sample is of mixed type. (4) Shallow shale fracturing needs to inhibit fracture propagation along the horizontal bedding direction, and acid injection is not appropriate; In deep shale fracturing, acid injection is suggested to enhance the horizontal expansion trend of fractures and improve the complexity of fractures. Due to the limitation of vertical fractures in ultra-deep shale fracturing, it is suggested that acid injection should be cautious to promote vertical fracture expansion and realize the expansion of ultra-deep reservoirs.