RISER GAS BEHAVIOR IN DEEPWATER WELLS –AN EXPERIMENTAL STUDY OF TAYLOR BUBBLE RISING IN A STAGNANT COLUMN OF LIQUID WITH DRILLING MUD PROPERTIES

The severe effects of the free-bubble gas formation in deep water drilling riser is a major concern for offshore Oil and Gas industry. Understanding the comprehensive assessment of risks and consequences of the hydrocarbons entering the drilling riser is very important to control and prevent future well related accidents.

This study designed and conducted a series of experiments with air slug or Taylor bubble rising in a vertical tube and in a stagnant column of different liquids open to atmospheric pressure. The objective was to (a) gain insights on the mechanisms of gas slug rising and expanding near the top of the drilling riser, (b) develop a method to calculate slug rising velocity and slug length as a function of positions using pressure data, and (c) provide experimental data to help calibrate computational models with liquid properties similar to drilling muds.

The bubble rising test apparatus consists of 3 acrylic tubes for a total of 18 ft of height and 6.5 inches of internal diameter. We generated a Taylor bubble by pressurizing a fixed volume of air trapped in the lower 6 ft section of the apparatus. This bubble is then introduced into the bottom of the upper 12 ft section of the column by opening and closing a ball-valve. We record, track, and calculate the bubble rising velocity and bubble length using twelve pressure transducers and cameras placed 1 ft apart and two movable cameras traveling along the height of the riser. We completed tests with Taylor air bubble in three different fluids: (a) water, (b) viscous shear-thinning gel made with 0.75% (by weight) concentration of Xanthan Gum, and (c) slurry of ceramic proppant with Xanthan Gum gel. We selected Xanthan Gum gel and the proppant slurry to emulate the shear thinning fluid properties and effects of solids in drilling mud, respectively.

Experimental results showed, as the bubble rises in the riser tube, the bubble expands due to the gradually decreasing hydrostatic pressure in the column. Air bubble-water tests showed the bubble rising velocity and length expansion were comparable to open literature data. Values interpreted from video cameras and pressure data, measured along the height of the riser tube, were consistent and complementary. Based on air bubble-water results, we established and verified a method to calculate bubble rising velocity and bubble expansion using only pressure data and without video camera data. Air bubble-viscous gel tests calculated slower bubble rising velocity and longer bubble length because of the higher gel viscosity and the thicker liquid film, respectively. Air bubble-slurry tests calculated slower bubble rising velocity and similar bubble length to that of air bubble-water case. We did not find data of Taylor bubble in slurry fluids in the literature for comparison. Results from this experimental program provided new insights on the mechanisms of slug or Taylor bubble movement in drilling riser with comparable drilling mud rheology.