Key Technologies for Large-Scale Acid Fracturing of Ultra-Deep Fault-Karst Carbonate Reservoirs with Ultra-High Temperature for Well S in Shunbei Oilfield

The target formation of Well S in Shunbei Oilfield is a typical ultra-deep fault-karst carbonate reservoir. Due to the complex engineering and geological conditions and wellbore conditions, acid fracturing is confronted with great challenges. In light of above difficulties in reservoir stimulation, a set of compound acid fracturing technologies was proposed by "centralized treatment by backfilling + acid damage to reduce fracture pressure + shallow pipe string + flow rate increase by weighted fracturing fluid + pad fluid fracturing + alternative injection for high conductivity fracture + autogenous acid to connect the far fault-karst". A set of acid fracturing fluid systems was optimized for resistance to ultra-high temperature by tests, included polymer fracturing fluid at 180 ℃, weighted guar gum fracturing fluid at 160 ℃, crosslinking acid at 160 ℃, and autogenous acid. Then, an optimized large-scale acid fracturing treatment plan was made based on recommendations for working fluid scales by numerical simulation. The recommended scale of fracturing fluid was 1 000–1 200 m³ and the scale of acid fluid was 800–1 000 m³. The field test showed a significant decrease in the wellhead pressure with weighted fracturing fluid, which was 7% lower than that with polymer fracturing fluid under the same injection rate. After the large-scale acid fracturing of Well S, the test production of natural gas was 10.45 × 10⁴ m³/d, which made a breakthrough in the exploration of the Shunbei No. 4 fault zone and provided valuable guidance for the large-scale acid fracturing design of similar reservoirs.