Experimental Study on High Conductivity and Slow Acid Fracturing in Deep Ultra-high Temperature Carbonate Reservoirs

Abstract Acid fracturing is a commonly used method for increasing production and injection and transformation measure for carbonate reservoirs. However, deep carbonate reservoirs generally have high temperature and closure pressure, low maintenance of acid etched fracture conductivity after acid fracturing, and fast rate of production decline. The experimental study investigated the acid rock etching morphology and acid etched fracture conductivity of hydrochloric acid, gelling acid, authigenic acid, polymer-surfactant acid, and chelating acid when injected alone or alternately at 200 °C and optimized the optimal injection displacement and injection time for different acid solution systems. Experimental research has found that polymer-surfactant acid has the best non-uniform etching effect in a 200 °C environment, with the highest maintenance of acid etched crack conductivity. The non-uniform etching effect of chelating acid is poor, but its conductivity is maintained high under high closing pressure, and it can deepen the etching effect along existing etching cracks. The alternating injection of polymer surfactant acid and chelating acid will form grooved etching channels on the rock wall, which still have high conductivity after crack closure. The secondary alternating injection of polymer-surfactant acid and chelating acid can increase the conductivity of acid etched fractures by 60%. The recommended alternate injection of polymer-surfactant acid and chelating acid into acid fracturing can significantly improve the effectiveness of acid fracturing modification. This study provides new research directions and data support for the selection of acid fracturing working fluid systems in ultra-high temperature carbonate reservoirs.