A Novel Protocol for Evaluating Acid Etching Length and Fracture Conductivity in High-Temperature Carbonate Reservoirs

Acid fracturing remains a critical stimulation method for enhancing well performance in carbonate reservoirs by creating etched fractures that provide conductive pathways for fluid flow. The etched fracture length plays a pivotal role in determining treatment success. However, conventional methods for evaluating acid fracture conductivity, which rely on 7-inch long and 2-inch wide core samples tested using an API conductivity apparatus after acid etching, often fail to capture the full extent of acid etching behavior due to the limited length of conventional test samples. This study introduces a new systematic protocol designed to assess alternative acid systems’ ability to transport live acid and achieve extended etched lengths under high-temperature conditions. The experimental work utilized 4-inch diameter Indiana limestone cores, each 20 inches in length and sawed in half longitudinally, with permeability below 2 mD. Different acid systems were injected at a temperature of 200 °F for prescribed durations, and fracture conductivity was measured across a range of closure stresses. Effluent samples were analyzed for calcium ion concentration using inductively coupled plasma (ICP) to quantify rock dissolution, while computed tomography (CT) scans captured fracture morphology before and after acid injection. A successful procedure was developed which ensured the core sample integrity under closure stress similar to reservoir conditions. This new procedure is not only a more effective method to evaluate acid etching length and conductivity characteristics; it is also an easier and faster procedure compared with the traditional method. The conductivity measured with the new procedure was compared with the results from the traditional procedure with similar conditions. The results of this work demonstrate that the novel protocol provides a robust framework for evaluating acid systems in fracture conductivity studies, addressing the limitations of traditional short-core experiments. These findings contribute to optimized acid fracturing designs tailored for challenging high-temperature reservoirs, enhancing treatment efficiency and reservoir productivity. The experimental observations and measurements can be used to calibrate predictive acid fracture models.