Feature of fractures induced by hydrofracturing treatment using water and L-CO2 as fracturing fluids in laboratory experiments

Recently, the use of CO2 as a treatment fluid in the petroleum and gas industry as well as enhanced geothermal system projects has attracted the attention of many scholars. In this paper, hydrofracturing experiments were performed on shale and coal specimens employing CO2 and water as fracturing fluids, respectively. The differences of hydraulic and L-CO2 fracturing in the breakdown pressure and fracture features were investigated under different geological and engineering conditions. The experimental results indicate that preexisting flaws in reservoirs have a significant effect on the stimulation mechanism of the hydrofracturing treatment. The non-reactivation of preexisting flaws leads to tensile stimulation. In contrast, the reactivation of preexisting flaws might lead to shear stimulation under a high horizontal stress difference. The shear stimulation would impact fracture propagation, and can even induce the slip of slant fractures, that generates splay cracks at the fracture tips. Because shear slip requires a high horizontal stress difference, which reduces the conductivity of reservoirs. Comparing with hydraulic fracturing, the low viscosity of CO2 is beneficial for shear stimulation.