Self-Generated Organic Acid System for Acid Fracturing in an Ultrahigh-Temperature Carbonate Reservoir

Abstract

As an important part of acid fracturing technology for the carbonate reservoir, the performance of the acid system directly affects the stimulation effect. In view of the current problems of an acid fluid system in an ultrahigh-temperature carbonate reservoir, such as fast acid–rock reaction, short effective action distance, and difficulty maintaining the conductivity of acid etching fractures, an experimental study on the self-generated organic acid system was carried out. The results showed that acetic anhydride and ethyl acetate, which had a strong acid generating ability, were suitable for the parent acid types of self-generated organic acids. Preferably, the peak temperature of 25% acetic anhydride is 160 °C, while the peak temperature of 30% ethyl acetate is 180 °C. The acid–rock reaction kinetics experiment shows that the order of activation energy is ethyl acetate > acetic anhydride > cross-linked acid. The reaction rate is arranged as follows: ethyl acetate < acetic anhydride ≪ cross-linked acid. The self-generated organic acid of ethyl acetate has the largest activation energy, the smallest reaction rate, and the best retarding effect. Acetic anhydride forms a strong nonuniform pitting morphology on the surface of the rock plate, with an initial conductivity of 225.4 μm2·cm. Ethyl acetate forms an uneven pitting morphology, and the initial conductivity is 53.1 μm2·cm. However, the ability of acid etching fracture formed by ethyl acetate to maintain fracture conductivity is stronger than that of acetic anhydride. Ethyl acetate is more suitable for a deep ultrahigh-temperature carbonate reservoir.

Effect of Acid-Injection Mode on Conductivity for Acid-Fracturing Stimulation in Ultra-Deep Tight Carbonate Reservoirs

“…It currently stands as the primary deep-penetrating acid-fracturing technology [9,15,16]. Compared to conventional gelled acid fracturing and pre-pad acid fracturing, multi-stage alternating acid fracturing has the following obvious advantages: first, the introduction of high viscosity fracturing fluid into the fracture reduces the acid leakoff in wormholes and natural fractures [9,15]; second, the non-reactive fracturing fluid or weakly reactive autogenic acid can effectively lower the acid-rock reaction temperature and increase the acid penetration distance [17,18]; third, the viscosity difference between the fracturing fluid and acid facilitates the occurrence of a viscous fingering phenomenon, creating non-uniform etching on the fracture surfaces and thereby improving the conductivity of acid-etched fractures [10].…”

Permeability Enhancement Mechanism of Acidizing in Steam-Assisted Gravity Flooding Wells

“…Previous studies have focused on the effects of the hinderance of low-permeability interlayers on steam chamber expansion and production in the SAGD development of reservoirs. It has been proposed that both supplementing perforations for steam injection above low-permeability interlayers and optimizing well patterns are effective means to ensure productivity when SAGD flooding is applied to the combination of vertical and horizontal wells [35]. There are currently some means to break through low-permeability interlayers.…”