Quantitative characterization of crack propagation behavior under the action of stage-by-stage fracturing induced by SC-CO2 fluid

Abstract

The use of supercritical carbon dioxide (SC-CO2) for the fracturing of coal bodies has excellent development prospects and practicability. It alters the fissure structure of coal bodies and achieves CO2 geological sequestration. This process, which involves the mutual interaction of coal, fissures, supercritical fluid, and gas, can be subdivided into fluid-induced and phase change-induced fracturing stages. Numerical simulation of the SC-CO2 fracturing was carried out in a stagewise manner to characterize each stage's fracturing performance quantitatively. The effects exerted on the crack propagation behavior by such factors as the ground stress deviation, the permeability coefficient of coal bodies, fracturing fluid injection rate and temperature, was studied in detail for each stage. The crack morphology, length, and width of cracks induced in each stage in proportion to the total crack length and width values were determined and analyzed. The research results show that the crack length ratio of the SC-CO2 fluid-induced fracturing stage to the total crack length was 80–90%, with the remaining 10–20% corresponding to the CO2 phase change-induced fracturing stage. This indicates that cracks propagation primarily occurred at the former stage. Meanwhile, the crack width ratios were nearly the same: 40–55% for the former stage versus 45–60% for the latter stage. Thus, the crack width increased dramatically at both stages of the SC-CO2 fracturing in the coal bodies. The research findings refined the insight into the SC-CO2 fracturing mechanism in coal bodies.