Mechanism of Fracture Propagation and Fracturing Parameter Optimization for CO2-Hybrid Fracturing in Multilayered Reservoirs

Summary A carbon dioxide (CO2)-hybrid fracturing approach combines the benefits of CO2 and water-based fracturing techniques, where liquid CO2 is injected to initiates hydraulic fractures (HFs), and is then followed by the injection of proppant-laden water-based fluid to further extend and support the HFs. After being heated by the wellbore and reservoir, liquid CO2 generally transforms into its supercritical state. There is an interface between supercritical CO2 (Sc-CO2) and the water-based fluid, which advances inside the HF. Due to the strong contrast between the two fluid viscosities, the interface dynamics can affect fluid pressure and HF width distributions in two regions occupied by dissimilar fluids. For this study, we developed a fully coupled HF propagation model to deal with two-phase fluid flow in fractures by describing the time-dependent movement of interface with a volume of fluid (VOF) equation. First, we verified the model capability to track interface movement using an analytical solution to the fracture problem with constant inlet pressure. Then, we performed a parametric study to determine the mechanisms affecting HF propagation in layered reservoirs. Numerical results revealed that HF height is significantly smaller than its length at the end of Sc-CO2 fracturing, as the higher-stress layers strongly delay HF vertical growth due to higher-rate leakoff of low-viscosity Sc-CO2 into rock mass and bedding planes (BPs). At this stage, the Sc-CO2 front coincides with the horizontal HF tip, but as fracturing proceeds, the HF propagates vertically into the adjacent higher-stress layers, and horizontal growth is temporarily suppressed until the water-based fluid catches up with the HF tip. The duration of coexistence of two fluids in the HF depends primarily on the pumping rate of water-based fluid. The subsequent fracturing of water-based fluid promotes an increase in HF length and height. For the case studied, the leakoff distance of Sc-CO2 along BPs can reach approximately 56 m, which is about 13 times greater than that of high-viscosity linear gel. The large-scale infiltration of Sc-CO2 elevates the pressure within the BPs, limiting the leakage of linear gel in facilitating HF extension. Obviously, there is an optimization in Sc-CO2 leakoff and HF growth rate by adjusting the pumping rate and amount of liquid CO2, and the optimal operational parameters of CO2-hybrid fracturing play a role. Finally, we performed an optimization study for a vertical shale oil well, and the proposed model and findings offer practical guidance for designing CO2-hybrid fracturing treatments in multilayered reservoirs.