Experimental Evaluation of Fracture Conductivity for Acid-Fracturing of Ultra-High Temperature Carbonate Reservoir

Summary Acid-fracture conductivity plays a critical role in enhancing oil and gas production from carbonate reservoirs, and laboratory experiments remain the primary approach for its investigation. However, recent discoveries of carbonate reservoirs in China, characterized by depths exceeding 10,000 meters and temperatures ranging from 160°C to 220°C, present significant challenges for current experimental methodologies. The existing testing equipment, with a temperature resistance limit of 160°C, is insufficient to accurately simulate acid etching behavior under such high-temperature conditions. Therefore, a novel ultra-high-temperature API acid-etching system was developed, capable of withstanding up to 240 °C and operating stably at 220 °C, which employed a three-stage heating process involving the acid-tank, pipelines and rock slabs holder. Additionally, acid was circulated within a fully sealed system to capture CO2 and Ca2+, Mg2+ produced during the acid-rock reaction in the fracture. This approach facilitates a more accurate simulation of in-situ acidizing conditions. The acid-fracture morphology was characterized by a 3D laser scanner, and fracture conductivity was quantitatively evaluated. The research results show that the acid-rock reaction rate increases initially exponentially with temperature, then linearly; at 180 °C, it reaches 11.45 times the rate at 90 °C. The "peak-removing" effect of acid-etching on fracture becomes more pronounced with increasing temperature. At 160 °C and above, acid uniformly dissolves surface protrusions, widening and deepening valleys, and forming a uniform single-sided channel at 180°C. However, below 160 °C, acid dissolves protrusions locally, only widening valleys. Under closure pressure, conductivity declines transitioned from linear to exponential with increasing temperature. At 80 MPa, the acid-fracture conductivity at 180°C is only 67% of that observed at 90 °C. This study provides a robust experimental device and methodology for assessing acid-fracture conductivity in ultra-high temperature carbonate reservoirs. And future advancements in acid systems capable of withstanding temperatures above 200 °C are expected to provide deeper insights into acid-etching behavior and fracture conductivity under such extreme conditions.