# Performance of a High-Molecular-Weight AM/AA Copolymer in a CO2–Water Polymer Hybrid Fracturing Fluid Under High-Temperature and High-Pressure Conditions

## TL;DR

A new CO2-water fracturing fluid using an AM/AA copolymer performs well under high temperature and pressure, offering good proppant transport and flow stability.

## Contribution

A CO2–water hybrid fracturing fluid using an AM/AA copolymer is developed and tested under high thermo-pressure conditions without specialized CO2 thickeners.

## Key findings

- The hybrid fluid achieves low fluid loss and structural continuity at 30–50% CO2, with optimal performance at 40% CO2.
- The fluid shows controllable proppant-carrying behavior and stable flow responses across a wide temperature range.
- Phase stability is sensitive to thermo-pressure, with uniform dispersion under moderate conditions and phase separation at high temperature and low pressure.

## Abstract

To reduce water consumption and potential formation damage associated with conventional water-based fracturing fluids while improving the proppant-carrying and flow adaptability of CO2-based systems without relying on specialized CO2 thickeners, a CO2–water polymer hybrid fracturing fluid was developed using an AM/AA copolymer (poly(acrylamide-co-acrylic acid), P(AM-co-AA)) as the thickening agent for the aqueous phase. Systematic experimental investigations were conducted under high-temperature and high-pressure conditions. Fluid-loss tests at different CO2 volume fractions show that the CO2–water polymer hybrid fracturing fluid system achieves a favorable balance between low fluid loss and structural continuity within the range of 30–50% CO2, with the most stable fluid-loss behavior observed at 40% CO2. Based on this ratio window, static proppant-carrying experiments indicate controllable settling behavior over a temperature range of 20–80 °C, leading to the selection of 60% polymer-based aqueous phase + 40% CO2 as the optimal mixing ratio. Rheological results demonstrate pronounced shear-thinning behavior across a wide thermo-pressure range, with viscosity decreasing systematically with increasing shear rate and temperature while maintaining continuous and reproducible flow responses. Pipe-flow tests further reveal that flow resistance decreases monotonically with increasing flow velocity and temperature, indicating stable transport characteristics. Phase visualization observations show that the CO2–water polymer hybrid fracturing fluid system exhibits a uniform milky dispersed appearance under moderate temperature or elevated pressure, whereas bubble-dominated structures and spatial phase separation gradually emerge under high-temperature and relatively low-pressure static conditions, highlighting the sensitivity of phase stability to thermo-pressure conditions. True triaxial hydraulic fracturing experiments confirm that the CO2–water polymer hybrid fracturing fluid enables stable fracture initiation and sustained propagation under complex stress conditions. Overall, the results demonstrate that the AM/AA copolymer-based aqueous phase can provide effective viscosity support, proppant-carrying capacity, and flow adaptability for CO2–water polymer hybrid fracturing fluid over a wide thermo-pressure range, confirming the feasibility of this approach without the use of specialized CO2 thickeners.