Bullheading involves pumping produced fluids back into the formation using a kill-fluid. A key operational parameter is the required bullheading rate which depends on surface pressure, available horsepower, and erosion limits. There is wide variation in current guidelines for bullheading rates, especially for large-diameter wellbores. Therefore, a well-scale bullheading test program was conducted using a 5200-ft-deep vertical well with 9-5/8"x2-7/8" casing/tubing annulus located at LSU test well facility. The tubing was instrumented with 4 downhole pressure gauges and fiber optic DTS/DAS to obtain data on the downhole flow dynamics and determine bullheading efficiencies. In a typical test, a large nitrogen cap placed at the top of the annulus was bullheaded by pumping fluid in annulus with continuous returns taken from the tubing side. Tests were conducted with varying fluid rates (50 to 500 gpm), initial gas-cap size (30-60 bbl), gas pressurization method and kill fluids (water and synthetic base mud). It was observed that the bullheading process involves simultaneous gas compression, gas bubble breakage, gas dispersion, and gas displacement, unlike the typical assumption of bullheading a large gas slug. The breakage of the initial gas slug depended on the surface pressure and the extent of gas-liquid mixing. The minimum water flowrate required for gas bullheading matched to water velocity just above small bubble velocity in water. Increase in water flowrate increased the bullheading efficiency, e.g., bullheading with 350 gpm required <50% water volume compared to 150 gpm water flowrate. Experiments with a highly pressurized initial gas cap and a larger initial gas cap volume resulted in relatively more efficient bullheading due to lower slip velocity resulting from higher average gas-holdup in the gas-swarm. In one test, the gas was bullheaded for some time and then allowed to migrate upward in a shut-in well. It was observed that the gas migration velocity (0.71 ft/sec) was higher than the gas slip velocity during bullheading (0.3-0.6 ft/sec). Contrary to the popular belief, the gas also did not carry its pressure while migrating in a shut in well. The experimental observation of bubbly flow instead of slug flow during bullheading under sufficiently higher surface pressure helped understand the multiphase flow dynamics of bullheading and it can help provide realistic bullheading guidelines based on well conditions.

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Significant discrepancy exists between the gas migration rates observed during the field applications of Pressurized Mud Cap Drilling (PMCD) and the widely used Taylor bubble velocity correlation. This impacts the fluid logistics planning and design of fluid properties for PMCD applications. Pilot-scale experiments and simulations have shown the importance of wellbore length-scale for estimating gas migration velocity (Samdani et al., 2021, 2022). Therefore, an industry-first well-scale study of gas migration in synthetic-based mud (SBM) was performed using a 5200-ft-deep vertical test-well (9-5/8″ × 2-7/8″ casing/tubing) located at Louisiana State University (LSU) well testing facilities. This test well is instrumented with 4 downhole pressure gauges and distributed temperature/acoustics sensing (DTS/DAS) fiber optic cables which were used to track the migrating gas and to determine its velocity. In a typical test, bottomhole pressure (BHP) was maintained, while gas migrated in a shut-in well. Tests were conducted by varying gas injection rate (10-250 gpm), total gas influx size (10-20 bbl), and BHP (2200-4500 psi). Gas migration rates indicated presence of Taylor bubbles at lower pressures (<2000 psi) and relatively smaller cap-bubbles at higher pressures (>2700 psi). The observation of pressure-dependent flow regime transition in a wellbore is one of the significant outcomes of this study. Changes in gas influx rate also influenced the gas migration velocity as it impacts the gas holdup and the rate at which gas can dissolve in comparison with the injection rate, under the prevailing flow regime. As a result, increase in influx rate led to higher gas migration velocity. A numerical model was also developed incorporating the experimentally observed relationship between pressure and transition of flow regime, to translate the test results into useful information and predictions for field PMCD. For example, the impact of reservoir gas solubility on gas migration rates was determined using this model while using the test-results based on nitrogen gas migration. The model results for reservoir gas migration rates in SBM showed a reasonable match with field-PMCD data under similar conditions.

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Numerical studies on pumps emphasize mainly on modelling the interactions between the impeller and the volute to obtain an accurate understanding of the physics involved. However, the importance of modelling leakage paths, which is known to have a significant influence on the flow structure in the pump, necessitates an in-depth analysis. This activity is undertaken in this paper by investigating a specific case of a centrifugal pump. Numerical studies have been conducted on the pump modelled with and without leakages for the design condition. The sliding mesh method is used to obtain single phase pressure pulsations data at some important locations in the volute and the leakage path, and transient Multiple Reference Frame (MRF) modelling is utilized to conduct the cavitation analysis. It is observed that for the case under study, the pressure pulsations pattern and the cavitation behaviour varies significantly due to the inclusion of leakage paths in the analysis.