In carbon capture, utilization, and storage operations, the continuous injection of high-pressure CO₂ into subsurface reservoirs exposes wellbore tubing to high-pressure, low-temperature, and corrosive CO₂-rich environments. This increases the risk of local damage or perforation. If a tubing leak occurs, high-pressure CO₂ is instantly released into the annular space between the tubing and casing, forming a highly transient confined jet. Unlike free jets, the confined jet in an annulus is strongly affected by geometric constraints, especially during the early leakage stage before wall impingement. This early-stage flow is characterized by rapid evolution, strong unsteadiness, and complex mixing, which are challenging to measure directly in field conditions. Therefore, it is necessary to investigate the early development characteristics of confined CO₂ jets induced by tubing leakage under controlled experimental conditions.

A laboratory-scale experimental system utilizing a Z-type schlieren optical configuration was established to visualize the early-stage confined jet formed by CO₂ leakage in a coaxial annular geometry. The actual wellbore structure was simplified into a concentric tubing-casing model, focusing on jet evolution prior to interaction with the casing wall. High-purity CO₂ served as the working fluid. Experiments were conducted under representative operating conditions: a supply pressure of 4 MPa, a leakage orifice diameter of 1 mm, and an ambient temperature of approximately 18 ℃. High-speed schlieren image sequences were acquired at 24 390 frames/s to capture jet initiation and early development. Based on these schlieren images, a plume identification and jet-front displacement extraction method suitable for high-frame-rate image sequences was developed. This method combines background subtraction and adaptive threshold segmentation to extract candidate plume regions. A nozzle connectivity constraint ensures physical consistency between the identified plume and the leakage origin, whereas a temporal consistency veto mechanism suppresses abnormal segmentation results caused by wall-related schlieren interference and transient noise. A calibrated pixel-to-length conversion factor is used to convert the axial displacement of the jet front into physical distance, and the cumulative average propagation velocity is calculated.

The experimental results show that the confined CO₂ jet exhibits pronounced, rapid axial development immediately after leakage initiation. During the early stage, the jet front advances quickly along the annular axis, driven primarily by high initial momentum at the leakage orifice. As the jet develops, geometric confinement becomes increasingly significant, resulting in a gradual reduction in axial propagation rate and the emergence of lateral spreading and recirculation. Quantitative analysis indicates that the cumulative average jet-front velocity increases rapidly during the early stage and then approaches a quasi-stable level of approximately 60 m/s before wall impingement occurs. The calculated velocity’s evolution trend is consistent with the schlieren-observed plume development, confirming the reliability of the proposed identification and measurement method.

A schlieren-based experimental approach, combined with a robust plume identification and jet-front tracking method, is developed to investigate early-stage confined CO₂ jets induced by tubing leakage. Without relying on complex physical modeling assumptions, this method effectively suppresses wall-induced schlieren artifacts and transient disturbances, enabling the stable extraction of plume morphology and jet-front displacement from high-speed image sequences. The experimental results provide valuable insights into the transient evolution characteristics of confined CO₂ jets in annular geometries, thereby offering a reliable experimental and analytical basis for further studies on leakage behavior and flow characterization in CO₂ injection wells.