Understanding the Role of Buoyancy on Jet Flows Within Viscoplastic Medium: Insights for Plug and Abandonment of Wells

Plug and abandonment (P&A) operation refers to the final stage of the oil and gas well’s operational life-cycle. The successful implementation of a P&A operation ensures the secure sealing of the well to prevent fluid movement and eliminate leakage between different layers. The key steps of a P&A operation entail three processes: (1) accessing the annulus section, (2) cleaning the target area (i.e., inside and outside the casing), and (3) installing cement plug barriers. The proper cleaning helps to enhance the cement-casing bonding while mitigating the risk of cement contamination. Jet cleaning, known as an efficient technique in the second step of P&A operations, displaces undesirable fluids and materials by injecting a cleaning fluid into the target area. Fluid properties, including density difference between injected and ambient fluids and the fluids’ rheological parameters, stand as key variables that directly influence the cleaning efficiency. Hence, a comprehensive understanding of how these parameters affect the jet flow is crucial for optimizing efficiency in well-cleaning procedures. In this study, we experimentally examine the miscible jet flow dynamics for two distinct scenarios: positively buoyant jets, where momentum and buoyancy act in the same direction, and negatively buoyant jets, where they act in opposite directions. The jet flow is formed by introducing a Newtonian fluid vertically downward, through a circular nozzle, into a transparent tank containing a viscoplastic fluid. We assume a “free jet” condition in our experiments, as the large tank dimensions render the wall effect negligible on the jet behavior. We use high-speed imaging to study the effects of the injection velocity, the density difference, and the rheological parameters (in particular, the yield stress) on the jet flow dynamics. The penetration length (indicating the evolution of the jet length over time) is the primary jet feature addressed in this study, which is investigated in both positively and negatively buoyant jets. Our findings show that the yield stress of the ambient fluid resists the jet evolution, leading to a decrease in the jet penetration length.