Investigation of acid erosion behavior under multi-stage alternating injection acid fracturing in ultra-deep dolomite reservoirs

Acid fracturing in ultra-deep dolomite reservoirs faces significant engineering challenges. This study investigates the Cambrian dolomite in the Tarim Basin through experiments simulating ultra-deep conditions (160°C and a maximum closure pressure of 60 MPa). The results show that acid type and number of injection levels significantly influence the etching morphology and resulting fracture conductivity. Quantitative analysis revealed that alternating between self-generating acid and gelled acid (JRC: 62–84), with etching volumes of 19.30–28.55 cm)3, or crosslinked acid and gelled acid (JRC: 52–92, with etching volumes of 19.50–30.25 cm)3 produced step and groove etching morphologies, which significantly enhanced fracture flow capacity under high-pressure closed conditions, confirming a positive correlation between larger etching volumes, higher joint roughness coefficient (JRC) values, and fracture flow capacity. Further analysis demonstrates that the two-stage alternating injection scheme achieves an optimal balance between acid penetration efficiency and etching heterogeneity. These findings provide a theoretical and experimental basis for optimizing acid-fracturing technology in ultra-deep dolomite reservoirs.