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 such as retardation and deeper conductivity generation. The thermal stability of foam at high temperatures is one of the main challenges. In this paper, two distinct CO2 foamed HCl acid systems with retarded acid characteristics are developed for high-temperature applications.

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, the corrosion inhibitor package, the foaming agent, and stabilizers. The performance of CO2 foamed acid systems was assessed by several laboratory tests characterized by compatibility, corrosion, and static foaming. The rheological properties of two retarded acid systems foamed with CO2 were measured using Fann 35 at different shear rates.

The control of acid reactivity, fluid loss, and conductivity generation are three essential aspects of acid fracturing that must be addressed for acid stimulation treatments to be successful. Overall acid reactivity, which governs the dissolution of carbonate rock, can be significantly decreased after foaming at the same acid strength and temperature. This work develops two non-viscous base acid systems for foaming applications, one based on a retarding agent and the other on an organic acid, both combined with a strong mineral acid, HCl. The effectiveness of corrosion inhibitors and intensifiers was tested on T-95 type coupons at 300°F. Static foam half-life measurements were done by generating CO2 foam in a liquid column at atmospheric conditions. The static foam half-life was measured at 3-5 min for an optimized foamed acid system with 90% foam quality. Static foaming tests were also performed using blender tests to evaluate on-site quality check. The fracture geometry and the removal of undesired fines from the fracture face are influenced by the viscous characteristics of foamed acid systems. The CO2 foamed viscosity was measured at 47-55 cp at a 511 1/s shear rate using the B1-R1 bob-rotor combination for different retarded acid systems.

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