Study of alternating acid fracturing treatment in carbonate formation based on true tri-axial experiment

Acid fracturing is a useful technique to stimulate carbonate reservoirs. Nowadays, acid leak-off becomes a vital problem during the operation of acid fracturing. In order to encounter this issue, alternating acid fracturing has been developed. However, the fracture propagation and the interaction between hydraulic fractures and natural fractures/caves during alternating acid fracturing are ambiguous. Hence, a series of alternating acid fracturing experiments covering coupling mechanics and chemical interaction were conducted for the first time. In this study, slick water, self-generating acid, cross-linked gel, and gelled acid were used as a fracturing fluid whereas the specimens for this experiment are carbonate outcrop which excavated from Dengsi Formation. The results show that: (1) acid fracturing can reduce fracturing pressure effectively as the acid will etch and weaken the natural fractures to let them easily connecting each other; (2) different pad fluid properties will have different impacts on the acid fracturing performance; (3) the interaction of hydraulic fractures and natural filled caves can be divided into five categories, and two of them had been observed in the experiments. Moreover, a new curvature-calculation algorithm has been developed based on the 3-D scanning technology. The results have a great significance for acid fracturing design and physical simulation experiments.