New Experimental Results Show the Application of Fiber Optic to Detect and to Track Gas Position in Marine Risers and Shed Lights on the Gas Migration Phenomenon Inside a Closed Well
Abstract:The main objective of this manuscript is to present and to discuss the results and significant observations gathered during 13 experimental runs conducted in a full-scale test well at Louisiana State University (LSU). The other two objectives of this manuscript are to show the use of distributed fiber optic sensing and downhole pressure sensors data to detect and to track the gas position inside the test well during the experiments; and to discuss experimental and simulated data of the gas migration phenomenon… Show more
Addressing gas migration in a static mud column during the shut-in period is a major concern in Pressurized Mud Cap Drilling (PMCD). Significant discrepancies have been found between the field data and existing correlations for gas migration velocity, since the latter are based on either small-scale experiments or overly simplified assumptions, resulting in overly conservative estimations. To meet the Light Annular Mud (LAM) requirement for managing gas migration and to monitor the transient pressure experienced throughout the PMCD operation, an improved gas migration velocity model was developed by combining the equation of motion (bubble flow) and Taylor-bubble correlation (slug flow). In the bubble flow model, the effects of non-Newtonian fluid properties and drill pipe rotation are considered through a modified drag coefficient (CD) that incorporates the bubble Reynolds number (Reb) and dimensionless shear rate (Sr). The effect of bubble swarm is taken into account through a void fraction (αg) term. The slug flow model is based on a Taylor bubble correlation in terms of Eötvös number (Eo) and inverse viscosity number (Nf). For the first time, the dependence of Taylor bubble velocity on drill pipe rotation has been shown and correlated as a function of Sr. Predictions of the gas migration velocities in PMCD operations are made and successfully compared with the existing models and test-well experimental data. The drift flux model embedded in the new gas migration velocity model was applied to simulate the gas migration in a test well. Good agreement between the model and measured pressure results in the full-scale test-well experiments can be obtained. Its companion work (Liu et al., 2023) provides the design and calculation method of key parameters in bullheading/PMCD operations.
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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