Permeability enhancement theory and a complete set of technology for low permeability coal seam by liquid CO2 fracturing

Abstract

Liquid CO2 fracturing in low-permeability coal seams has dual gas-enhanced drainage effects of fracture reconstruction and displacement. It is a permeability enhancement technology that is easy to be used in combination with other technologies and has the potential for large-scale application. This paper proposes a theoretical system and a complete set of technical framework for fracturing and permeability enhancement of low-permeability coal seam with liquid CO2; according to the principle of stress superposition and the maximum tensile stress criterion, a model for determining the initiation pressure of liquid CO2 fracturing was established considering the change of phase state and low viscosity and strong permeability of liquid CO2 during the injection process; taking type I fracture criterion as the discriminant condition of fracture propagation, the quantitative characterizing relationship between fracturing orifice pressure and fracture propagation distance under constant fluid injection flow was established, and the influence law of fluid injection pressure, fracturing fluid viscosity and fracture occurrence on fracture propagation was clarified. On this basis, the four key parameters and their calculation methods to determine the performance of the complete set of equipment for liquid CO2 fracturing and permeability improvement in coal seam were proposed, the first complete set of equipment for underground fracturing and permeability improvement in China was developed, and the construction technology of fracturing and permeability improvement was developed, which was applied in Huainan Mining Area and Binchang Mining Area respectively.

Keywords

• low permeability coal seam
• liquid co2
• permeability enhancement by fracturing
• complete set of equipment
• gas drainage