Experimental study on a novel CO2-responsive foam fluid for fracturing and its key performance

Hydraulic fracturing is an essential technique for extracting unconventional oils and gases. However, as unconventional oil and gas resources are extensively exploited, the high costs of traditional fracturing fluids have become increasingly apparent. In this study, a high-performance CO 2 -responsive VES-CO 2 foam fracturing fluid was developed by combining CO 2 foam fracturing fluid with viscoelastic surfactant (VES) fracturing fluid, based on responsive surfactants, to establish a recyclable fracturing fluid system stimulated and reduce operational costs. Firstly, a CO 2 -responsive VES-CO 2 foam fracturing fluid was constructed in a scCO 2 environment. Secondly, the stability mechanism of the CO 2 -responsive VES-CO 2 foam system was revealed through analysis of bulk rheological properties and interfacial characteristics. Finally, the performance of the fracturing fluid under high pressures was tested. The study found that under CO 2 stimulation, oleamidopropyl dimethylamine (DOAPA) forms a cationic surfactant, DOAPA-CO 2 , which, with the assistance of sodium benzenesulfonate (NaSbs), generates worm-like micelles . This transformation increases the zero-shear viscosity of the foaming solution from 12 mPa·s to 2869.69 mPa·s. The elastic modulus rises by two orders of magnitude, while the foam drainage half-life extends from 610 s to 3720 s. Encapsulation parameter P (P = 0.38) and TME tests confirmed the formation of worm-like micelles in the DOAPA@NaSbs-CO 2 system. Moreover, interfacial dilational rheology experiments indicated that the presence of worm-like micelles altered the relaxation mode of DOAPA-CO 2 molecules, enhancing the interfacial dilational viscoelasticity of the liquid film, thereby improving foam stability and the sand suspension capability of the liquid film. Performance evaluations showed that as the pressure increased, the proppant settling rate first decreased slowly and then rapidly, mainly because of the phase transition of CO 2 . Within the test range, the proppant settling rate stabilized below 2.4 cm/min, meeting the engineering requirements. By controlling the presence of CO 2 , the fracturing fluid can break its gel, and the foam viscosity can switch between low and high, providing conditions for recycling the fracturing fluid. Additionally, the damage rate of this CO 2 -responsive VES-CO 2 foam fracturing fluid was only 8.08 %, indicating its compatibility with formations and fractures.