Carbonate rock mechanical properties are important factors affecting the effect of acid fracturing, However, strength characteristics and deformation mechanism of tight carbonate rock under reservoir conditions are unclear currently. Triaxial compression experiment was conducted in order to figure out that. The result shows that under the same experimental condition, brittle deformation occurs in tight dolomite, while plastic deformation appears in tight limestone; in addition, compressive strength and Young's modulus of dolomite surpass limestone. The deformation mechanism indicates that morphological diversity of fractures is caused by the difference between radial and axial fracture strain rate. The difference would be magnified with increase of rock brittleness, causing fracture morphology becomes more complicated. Further studies were carried out through XRD and CT, the results show that compressive strength of tight dolomite intensified with the increase of quartz and dolomite, whereas weakened with the increase of clay and rock porosity; in addition, nature fractures in the rock would weaken its compressive strength significantly. Above researches provide reference for acid fracturing.
China enjoys abundant tight carbonate reservoir (Wei et al., 2017). Acid fracturing is a key technology for exploitation of such reservoir (Mou et al., 2012). Predication of formation breakdown pressure and fracture propagation pattern, which would be affected by strength and deformation features of the rock, is essential for evaluation of acid fracturing's effects (Yang et al., 2018; Yan et al., 2015). However, the strength characteristics and deformation mechanism of tight carbonate rock under reservoir conditions are unclear currently.
Triaxial compression experiment is one of the important methods to study rock mechanical characteristics under reservoir conditions. A large number of scholars have studied the deformation characteristics of the shale, sandstone and marble under triaxial stress. (Arora et al., 2015; Yang et al., 2016; Zhao et al., 2018; Xu et al., 2019). However, there are few studies on tight carbonate rocks under triaxial stress. Wu (2010) has studied the influence of porosity on uniaxial and triaxial compressive strength of dolomite, the result shows that compressive strength of rock decreases exponentially with the increase of porosity. Liang (2019) has studied the influence of fracture angle on uniaxial compressive strength, the result shows that as the angle becomes larger, the rock compressive strength decreases first and then increases; when the angle ranges from 20° to 30°, rock compressive strength is the weakest, and when the angle is 90°, rock compressive strength is the strongest. Mohsen (2019) has studied strength characteristics and deformation mechanism of tight carbonate rock under multi-stage triaxial compression, it is believed that using multistage triaxial test can decrease the cost and time and the obtained results are still reliable due to less variability in the rock properties. According to literature, strength characteristics and deformation mechanism of tight carbonate are unclear. However, clarification of failure deformation mechanism is essential for acid fracturing on carbonate formation.
