Vertical fracture propagation behavior upon supercritical carbon dioxide fracturing of multiple layers

There are abundant unconventional oil and gas resources in the sandstone–shale interbed of the Yanchang Formation in Ordos Basin. However, it is difficult to predict the growth of supercritical carbon dioxide (Sc-CO2) fracture height in sandstone–shale interbed reservoirs due to the developed interface and strong heterogeneity. To enhance the productivity of sandstone–shale interbed reservoirs, it is necessary to clarify the Sc-CO2 fracture evolution mechanism. In this study, a global embedded zero-thickness cohesive element method is proposed for numerical simulation of Sc-CO2 fracturing of sandstone–shale interbeds, and the effects of stress difference, interface strength, and perforation position on fracture vertical propagation pattern are investigated. The results lead us to three main conclusions. (1) The fracturing model is established using the global embedded cohesive element method combined with the nonlinear constitutive equation, which solves the problem that the cohesive element cannot effectively simulate the random propagation of Sc-CO2 fractures. (2) Under the combined influence of in situ stresses and natural weak surfaces, there are three typical fracture extension patterns: I-shaped, cross-shaped, and single-shaped. In addition, there are six interaction modes between the main fracture and the interlayer interface. Branch fractures and microfractures are easily formed when the main fracture penetrates the sandstone–shale interface. (3) The vertical stress difference is greater than 4 MPa, and the dimensionless comprehensive interface strength is between 0.3 and 0.4, so the simultaneous perforation in shale and sandstone is a favorable factor for the full expansion of fracture height. The results provide theoretical guidance for selecting the Sc-CO2 fracturing horizon of sandstone–shale interbed reservoirs.