CO2 Foamed Acid Rheology for High-Temperature Acid Fracturing

Foamed acid fracturing is gaining importance to maximize flowback recovery and is particularly applicable when reservoir energy is not sufficient to effectively flow back the well. Foamed acid offers additional benefits, such as reduced water consumption and improved acid diversion, in addition to potential features like retardation and deeper conductivity generation. The thermal stability of foam at high temperatures is one of the main challenges. In this paper, the rheology of two CO2 foamed HCl acid systems having retarded acid characteristics was studied in a high-temperature environment. The development of foam viscosity depends on the type of gas, desired foam quality characteristics, the external phase fluid viscosity as a function of the type of acid blend and strength, corrosion inhibitor package, foaming agent, and stabilizers. The performance of CO2 foamed acid systems was assessed by several laboratory tests, characterized by compatibility, corrosion testing and foam rheology. The rheological properties of different qualities of CO2 foams containing two retarded acid systems were measured at a range of temperatures, i.e., 200 and 250°F, and shear rates from 100 to 1600 1/s, using a custom-designed foamed acid etching equipment which has both rheology and acid etching capability. Acid fracturing has three key aspects for successful acid stimulation treatments: controlling acid reactivity, managing fluid loss, and generating conductivity. The overall acid reactivity, which governs the dissolution of carbonate rock, can be significantly decreased after foaming, even with the same acid strength and temperature. This work develops two base non-viscous acid systems for foaming applications: one based on a retarding agent and the other based on an organic acid, both combined with a strong mineral acid, HCl. The Modulated Retarded Acid (MRA) system, which can be formulated with different concentrations of hydrochloric acid and mineral acids, utilizes a gelling agent and exhibits a CO2 foamed viscosity in the range of 80-105 cP at a shear rate of 100 1/s at 200°F. The second system, Low Viscosity Retarded Acid (LVRA), developed with organic acid, exhibits a viscosity in the range of 44-45 cP at a shear rate of 100 1/s at 250°F. The CO2 foamed rheology of both the MRA and LVRA acid systems exhibits stable viscosity, showing potential for use in acid fracturing applications.