Experimental investigation on the characteristics of acid-etched fractures in acid fracturing by an improved true tri-axial equipment

The propagation characteristics and influencing factors of hydraulic fractures can be analyzed through fracturing experiments. However, there are almost no fracturing experiments in which acid fluids were considered, for the acid resistance of the equipment is always imperfect. In this paper, a physical modeling experimental equipment for acid fracturing was designed and introduced, in which corrosion protection and alternate injection were realized. The carbonate rock for acid fracturing experiment was collected from Sichuan Basin, China, which was tight with a little cavity developed. Acid fracturing experiments, in which HCL, guanidine gum and guanidine gum with an acid fluid were used as the fracturing fluids, were conducted to analyze the characteristic of acid-etched fractures. The results showed that: Acid fluid etched the fracture surface, especially in the near-wellbore area; Acid fluid would open microfractures and pores easily, and leaked into these spaces, which decreased the propagation distance of the primary acid-etched fracture; High viscosity guanidine gum was first injected and then with low viscosity acid fluid, then the latter one was transported to distal area by viscous fingering, which would lead to a better fracturing effect; Due to the acid-rock reaction, the morphology of acid-etched fracture was obviously different from hydraulic fracture, and two kinds of acid-etched fracture morphology were proposed according to the sequence of acid fluid and guanidine gum. In this paper, the characteristics of acid-etched fractures were investigated by four groups of acid fracturing experiments, which could provide a reference to optimize acid fracturing effect.