Practice and insights on CO₂ pre-enhanced fracturing in deep coal measure gas reservoirs of Jixi Basin

In response to challenges such as low permeability (0.006×10⁻¹⁵−0.066×10⁻¹⁵ m²), high breakdown pressures (up to 52.8 MPa), and difficult in fracture propagation in deep coalbed methane reservoirs of Jixi Basin, a novel CO₂ pre-enhanced fracturing technology is innovatively proposed based on engineering practices from Well Heijidi-2. This technology utilizes the cryogenic effect and chemical dissolution enhancement of liquefied CO₂ to significantly reduce the breakdown pressures of coal matrices, reactivate natural fracture networks, and create multi-scale complex fracture networks. Furthermore, utilizing the adsorption-displacement mechanism of supercritical CO₂, it enhances coalbed methane desorption efficiency. The volumetric expansion during phase transition also boosts reservoir energy, achieving highly efficient fracturing fluid flowback. The implementation employs a combined process of “liquefied CO₂-enhanced fracturing + active water proppant transport for fracture conductivity”. This involves staged injection of 101.4−113.3 tons of liquefied CO₂ per target layer, integrated with ball-sealing staged fracturing (using 50−52 temporary diverting balls) and multi-size quartz sand proppant placement. This methodology constructs a fracture system characterized by “high-conductivity near-wellbore fractures and fully-supported far-field fractures”. Microseismic monitoring revealed that the generated complex fracture networks were oriented east-west and northeast. Individual layers exhibited fracture lengths of 400−431 m, heights of 75−110 m, and network widths of 168−173 m. The cumulative gas production from coal groups C14+C13 and C16 reached 156 131 m³, with a peak daily gas output of 1 145 m³. Field results demonstrate that CO₂ pre-enhanced fracturing overcomes the bottleneck in low-permeability and deep coalbed methane reservoirs (>2 000 m depth) through synergistic mechanisms of “physical breaking-chemical permeability enhancement-energy driven”. This provides both theoretical and technical foundations for efficient coalbed methane development in the Jixi Basin.